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    1 libpng-manual.txt - A description on how to use and modify libpng
    2 
    3  Copyright (c) 2018-2019 Cosmin Truta
    4  Copyright (c) 1998-2018 Glenn Randers-Pehrson
    5 
    6  This document is released under the libpng license.
    7  For conditions of distribution and use, see the disclaimer
    8  and license in png.h
    9 
   10  Based on:
   11 
   12  libpng version 1.6.36, December 2018, through 1.6.37 - April 2019
   13  Updated and distributed by Cosmin Truta
   14  Copyright (c) 2018-2019 Cosmin Truta
   15 
   16  libpng versions 0.97, January 1998, through 1.6.35 - July 2018
   17  Updated and distributed by Glenn Randers-Pehrson
   18  Copyright (c) 1998-2018 Glenn Randers-Pehrson
   19 
   20  libpng 1.0 beta 6 - version 0.96 - May 28, 1997
   21  Updated and distributed by Andreas Dilger
   22  Copyright (c) 1996, 1997 Andreas Dilger
   23 
   24  libpng 1.0 beta 2 - version 0.88 - January 26, 1996
   25  For conditions of distribution and use, see copyright
   26  notice in png.h. Copyright (c) 1995, 1996 Guy Eric
   27  Schalnat, Group 42, Inc.
   28 
   29  Updated/rewritten per request in the libpng FAQ
   30  Copyright (c) 1995, 1996 Frank J. T. Wojcik
   31  December 18, 1995 & January 20, 1996
   32 
   33  TABLE OF CONTENTS
   34 
   35     I. Introduction
   36    II. Structures
   37   III. Reading
   38    IV. Writing
   39     V. Simplified API
   40    VI. Modifying/Customizing libpng
   41   VII. MNG support
   42  VIII. Changes to Libpng from version 0.88
   43    IX. Changes to Libpng from version 1.0.x to 1.2.x
   44     X. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x
   45    XI. Changes to Libpng from version 1.4.x to 1.5.x
   46   XII. Changes to Libpng from version 1.5.x to 1.6.x
   47  XIII. Detecting libpng
   48   XIV. Source code repository
   49    XV. Coding style
   50 
   51 I. Introduction
   52 
   53 This file describes how to use and modify the PNG reference library
   54 (known as libpng) for your own use.  In addition to this
   55 file, example.c is a good starting point for using the library, as
   56 it is heavily commented and should include everything most people
   57 will need.  We assume that libpng is already installed; see the
   58 INSTALL file for instructions on how to configure and install libpng.
   59 
   60 For examples of libpng usage, see the files "example.c", "pngtest.c",
   61 and the files in the "contrib" directory, all of which are included in
   62 the libpng distribution.
   63 
   64 Libpng was written as a companion to the PNG specification, as a way
   65 of reducing the amount of time and effort it takes to support the PNG
   66 file format in application programs.
   67 
   68 The PNG specification (second edition), November 2003, is available as
   69 a W3C Recommendation and as an ISO Standard (ISO/IEC 15948:2004 (E)) at
   70 <https://www.w3.org/TR/2003/REC-PNG-20031110/>.
   71 The W3C and ISO documents have identical technical content.
   72 
   73 The PNG-1.2 specification is available at
   74 <https://png-mng.sourceforge.io/pub/png/spec/1.2/>.
   75 It is technically equivalent
   76 to the PNG specification (second edition) but has some additional material.
   77 
   78 The PNG-1.0 specification is available as RFC 2083 at
   79 <https://png-mng.sourceforge.io/pub/png/spec/1.0/> and as a
   80 W3C Recommendation at <https://www.w3.org/TR/REC-png-961001>.
   81 
   82 Some additional chunks are described in the special-purpose public chunks
   83 documents at <http://www.libpng.org/pub/png/spec/register/>
   84 
   85 Other information
   86 about PNG, and the latest version of libpng, can be found at the PNG home
   87 page, <http://www.libpng.org/pub/png/>.
   88 
   89 Most users will not have to modify the library significantly; advanced
   90 users may want to modify it more.  All attempts were made to make it as
   91 complete as possible, while keeping the code easy to understand.
   92 Currently, this library only supports C.  Support for other languages
   93 is being considered.
   94 
   95 Libpng has been designed to handle multiple sessions at one time,
   96 to be easily modifiable, to be portable to the vast majority of
   97 machines (ANSI, K&R, 16-, 32-, and 64-bit) available, and to be easy
   98 to use.  The ultimate goal of libpng is to promote the acceptance of
   99 the PNG file format in whatever way possible.  While there is still
  100 work to be done (see the TODO file), libpng should cover the
  101 majority of the needs of its users.
  102 
  103 Libpng uses zlib for its compression and decompression of PNG files.
  104 Further information about zlib, and the latest version of zlib, can
  105 be found at the zlib home page, <https://zlib.net/>.
  106 The zlib compression utility is a general purpose utility that is
  107 useful for more than PNG files, and can be used without libpng.
  108 See the documentation delivered with zlib for more details.
  109 You can usually find the source files for the zlib utility wherever you
  110 find the libpng source files.
  111 
  112 Libpng is thread safe, provided the threads are using different
  113 instances of the structures.  Each thread should have its own
  114 png_struct and png_info instances, and thus its own image.
  115 Libpng does not protect itself against two threads using the
  116 same instance of a structure.
  117 
  118 II. Structures
  119 
  120 There are two main structures that are important to libpng, png_struct
  121 and png_info.  Both are internal structures that are no longer exposed
  122 in the libpng interface (as of libpng 1.5.0).
  123 
  124 The png_info structure is designed to provide information about the
  125 PNG file.  At one time, the fields of png_info were intended to be
  126 directly accessible to the user.  However, this tended to cause problems
  127 with applications using dynamically loaded libraries, and as a result
  128 a set of interface functions for png_info (the png_get_*() and png_set_*()
  129 functions) was developed, and direct access to the png_info fields was
  130 deprecated..
  131 
  132 The png_struct structure is the object used by the library to decode a
  133 single image.  As of 1.5.0 this structure is also not exposed.
  134 
  135 Almost all libpng APIs require a pointer to a png_struct as the first argument.
  136 Many (in particular the png_set and png_get APIs) also require a pointer
  137 to png_info as the second argument.  Some application visible macros
  138 defined in png.h designed for basic data access (reading and writing
  139 integers in the PNG format) don't take a png_info pointer, but it's almost
  140 always safe to assume that a (png_struct*) has to be passed to call an API
  141 function.
  142 
  143 You can have more than one png_info structure associated with an image,
  144 as illustrated in pngtest.c, one for information valid prior to the
  145 IDAT chunks and another (called "end_info" below) for things after them.
  146 
  147 The png.h header file is an invaluable reference for programming with libpng.
  148 And while I'm on the topic, make sure you include the libpng header file:
  149 
  150 #include <png.h>
  151 
  152 and also (as of libpng-1.5.0) the zlib header file, if you need it:
  153 
  154 #include <zlib.h>
  155 
  156 Types
  157 
  158 The png.h header file defines a number of integral types used by the
  159 APIs.  Most of these are fairly obvious; for example types corresponding
  160 to integers of particular sizes and types for passing color values.
  161 
  162 One exception is how non-integral numbers are handled.  For application
  163 convenience most APIs that take such numbers have C (double) arguments;
  164 however, internally PNG, and libpng, use 32 bit signed integers and encode
  165 the value by multiplying by 100,000.  As of libpng 1.5.0 a convenience
  166 macro PNG_FP_1 is defined in png.h along with a type (png_fixed_point)
  167 which is simply (png_int_32).
  168 
  169 All APIs that take (double) arguments also have a matching API that
  170 takes the corresponding fixed point integer arguments.  The fixed point
  171 API has the same name as the floating point one with "_fixed" appended.
  172 The actual range of values permitted in the APIs is frequently less than
  173 the full range of (png_fixed_point) (-21474 to +21474).  When APIs require
  174 a non-negative argument the type is recorded as png_uint_32 above.  Consult
  175 the header file and the text below for more information.
  176 
  177 Special care must be take with sCAL chunk handling because the chunk itself
  178 uses non-integral values encoded as strings containing decimal floating point
  179 numbers.  See the comments in the header file.
  180 
  181 Configuration
  182 
  183 The main header file function declarations are frequently protected by C
  184 preprocessing directives of the form:
  185 
  186     #ifdef PNG_feature_SUPPORTED
  187     declare-function
  188     #endif
  189     ...
  190     #ifdef PNG_feature_SUPPORTED
  191     use-function
  192     #endif
  193 
  194 The library can be built without support for these APIs, although a
  195 standard build will have all implemented APIs.  Application programs
  196 should check the feature macros before using an API for maximum
  197 portability.  From libpng 1.5.0 the feature macros set during the build
  198 of libpng are recorded in the header file "pnglibconf.h" and this file
  199 is always included by png.h.
  200 
  201 If you don't need to change the library configuration from the default, skip to
  202 the next section ("Reading").
  203 
  204 Notice that some of the makefiles in the 'scripts' directory and (in 1.5.0) all
  205 of the build project files in the 'projects' directory simply copy
  206 scripts/pnglibconf.h.prebuilt to pnglibconf.h.  This means that these build
  207 systems do not permit easy auto-configuration of the library - they only
  208 support the default configuration.
  209 
  210 The easiest way to make minor changes to the libpng configuration when
  211 auto-configuration is supported is to add definitions to the command line
  212 using (typically) CPPFLAGS.  For example:
  213 
  214 CPPFLAGS=-DPNG_NO_FLOATING_ARITHMETIC
  215 
  216 will change the internal libpng math implementation for gamma correction and
  217 other arithmetic calculations to fixed point, avoiding the need for fast
  218 floating point support.  The result can be seen in the generated pnglibconf.h -
  219 make sure it contains the changed feature macro setting.
  220 
  221 If you need to make more extensive configuration changes - more than one or two
  222 feature macro settings - you can either add -DPNG_USER_CONFIG to the build
  223 command line and put a list of feature macro settings in pngusr.h or you can set
  224 DFA_XTRA (a makefile variable) to a file containing the same information in the
  225 form of 'option' settings.
  226 
  227 A. Changing pnglibconf.h
  228 
  229 A variety of methods exist to build libpng.  Not all of these support
  230 reconfiguration of pnglibconf.h.  To reconfigure pnglibconf.h it must either be
  231 rebuilt from scripts/pnglibconf.dfa using awk or it must be edited by hand.
  232 
  233 Hand editing is achieved by copying scripts/pnglibconf.h.prebuilt to
  234 pnglibconf.h and changing the lines defining the supported features, paying
  235 very close attention to the 'option' information in scripts/pnglibconf.dfa
  236 that describes those features and their requirements.  This is easy to get
  237 wrong.
  238 
  239 B. Configuration using DFA_XTRA
  240 
  241 Rebuilding from pnglibconf.dfa is easy if a functioning 'awk', or a later
  242 variant such as 'nawk' or 'gawk', is available.  The configure build will
  243 automatically find an appropriate awk and build pnglibconf.h.
  244 The scripts/pnglibconf.mak file contains a set of make rules for doing the
  245 same thing if configure is not used, and many of the makefiles in the scripts
  246 directory use this approach.
  247 
  248 When rebuilding simply write a new file containing changed options and set
  249 DFA_XTRA to the name of this file.  This causes the build to append the new file
  250 to the end of scripts/pnglibconf.dfa.  The pngusr.dfa file should contain lines
  251 of the following forms:
  252 
  253 everything = off
  254 
  255 This turns all optional features off.  Include it at the start of pngusr.dfa to
  256 make it easier to build a minimal configuration.  You will need to turn at least
  257 some features on afterward to enable either reading or writing code, or both.
  258 
  259 option feature on
  260 option feature off
  261 
  262 Enable or disable a single feature.  This will automatically enable other
  263 features required by a feature that is turned on or disable other features that
  264 require a feature which is turned off.  Conflicting settings will cause an error
  265 message to be emitted by awk.
  266 
  267 setting feature default value
  268 
  269 Changes the default value of setting 'feature' to 'value'.  There are a small
  270 number of settings listed at the top of pnglibconf.h, they are documented in the
  271 source code.  Most of these values have performance implications for the library
  272 but most of them have no visible effect on the API.  Some can also be overridden
  273 from the API.
  274 
  275 This method of building a customized pnglibconf.h is illustrated in
  276 contrib/pngminim/*.  See the "$(PNGCONF):" target in the makefile and
  277 pngusr.dfa in these directories.
  278 
  279 C. Configuration using PNG_USER_CONFIG
  280 
  281 If -DPNG_USER_CONFIG is added to the CPPFLAGS when pnglibconf.h is built,
  282 the file pngusr.h will automatically be included before the options in
  283 scripts/pnglibconf.dfa are processed.  Your pngusr.h file should contain only
  284 macro definitions turning features on or off or setting settings.
  285 
  286 Apart from the global setting "everything = off" all the options listed above
  287 can be set using macros in pngusr.h:
  288 
  289 #define PNG_feature_SUPPORTED
  290 
  291 is equivalent to:
  292 
  293 option feature on
  294 
  295 #define PNG_NO_feature
  296 
  297 is equivalent to:
  298 
  299 option feature off
  300 
  301 #define PNG_feature value
  302 
  303 is equivalent to:
  304 
  305 setting feature default value
  306 
  307 Notice that in both cases, pngusr.dfa and pngusr.h, the contents of the
  308 pngusr file you supply override the contents of scripts/pnglibconf.dfa
  309 
  310 If confusing or incomprehensible behavior results it is possible to
  311 examine the intermediate file pnglibconf.dfn to find the full set of
  312 dependency information for each setting and option.  Simply locate the
  313 feature in the file and read the C comments that precede it.
  314 
  315 This method is also illustrated in the contrib/pngminim/* makefiles and
  316 pngusr.h.
  317 
  318 III. Reading
  319 
  320 We'll now walk you through the possible functions to call when reading
  321 in a PNG file sequentially, briefly explaining the syntax and purpose
  322 of each one.  See example.c and png.h for more detail.  While
  323 progressive reading is covered in the next section, you will still
  324 need some of the functions discussed in this section to read a PNG
  325 file.
  326 
  327 Setup
  328 
  329 You will want to do the I/O initialization(*) before you get into libpng,
  330 so if it doesn't work, you don't have much to undo.  Of course, you
  331 will also want to insure that you are, in fact, dealing with a PNG
  332 file.  Libpng provides a simple check to see if a file is a PNG file.
  333 To use it, pass in the first 1 to 8 bytes of the file to the function
  334 png_sig_cmp(), and it will return 0 (false) if the bytes match the
  335 corresponding bytes of the PNG signature, or nonzero (true) otherwise.
  336 Of course, the more bytes you pass in, the greater the accuracy of the
  337 prediction.
  338 
  339 If you are intending to keep the file pointer open for use in libpng,
  340 you must ensure you don't read more than 8 bytes from the beginning
  341 of the file, and you also have to make a call to png_set_sig_bytes()
  342 with the number of bytes you read from the beginning.  Libpng will
  343 then only check the bytes (if any) that your program didn't read.
  344 
  345 (*): If you are not using the standard I/O functions, you will need
  346 to replace them with custom functions.  See the discussion under
  347 Customizing libpng.
  348 
  349     FILE *fp = fopen(file_name, "rb");
  350     if (!fp)
  351     {
  352        return ERROR;
  353     }
  354 
  355     if (fread(header, 1, number, fp) != number)
  356     {
  357        return ERROR;
  358     }
  359 
  360     is_png = !png_sig_cmp(header, 0, number);
  361     if (!is_png)
  362     {
  363        return NOT_PNG;
  364     }
  365 
  366 Next, png_struct and png_info need to be allocated and initialized.  In
  367 order to ensure that the size of these structures is correct even with a
  368 dynamically linked libpng, there are functions to initialize and
  369 allocate the structures.  We also pass the library version, optional
  370 pointers to error handling functions, and a pointer to a data struct for
  371 use by the error functions, if necessary (the pointer and functions can
  372 be NULL if the default error handlers are to be used).  See the section
  373 on Changes to Libpng below regarding the old initialization functions.
  374 The structure allocation functions quietly return NULL if they fail to
  375 create the structure, so your application should check for that.
  376 
  377     png_structp png_ptr = png_create_read_struct
  378         (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
  379         user_error_fn, user_warning_fn);
  380 
  381     if (!png_ptr)
  382        return ERROR;
  383 
  384     png_infop info_ptr = png_create_info_struct(png_ptr);
  385 
  386     if (!info_ptr)
  387     {
  388        png_destroy_read_struct(&png_ptr,
  389            (png_infopp)NULL, (png_infopp)NULL);
  390        return ERROR;
  391     }
  392 
  393 If you want to use your own memory allocation routines,
  394 use a libpng that was built with PNG_USER_MEM_SUPPORTED defined, and use
  395 png_create_read_struct_2() instead of png_create_read_struct():
  396 
  397     png_structp png_ptr = png_create_read_struct_2
  398         (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
  399         user_error_fn, user_warning_fn, (png_voidp)
  400         user_mem_ptr, user_malloc_fn, user_free_fn);
  401 
  402 The error handling routines passed to png_create_read_struct()
  403 and the memory alloc/free routines passed to png_create_struct_2()
  404 are only necessary if you are not using the libpng supplied error
  405 handling and memory alloc/free functions.
  406 
  407 When libpng encounters an error, it expects to longjmp back
  408 to your routine.  Therefore, you will need to call setjmp and pass
  409 your png_jmpbuf(png_ptr).  If you read the file from different
  410 routines, you will need to update the longjmp buffer every time you enter
  411 a new routine that will call a png_*() function.
  412 
  413 See your documentation of setjmp/longjmp for your compiler for more
  414 information on setjmp/longjmp.  See the discussion on libpng error
  415 handling in the Customizing Libpng section below for more information
  416 on the libpng error handling.  If an error occurs, and libpng longjmp's
  417 back to your setjmp, you will want to call png_destroy_read_struct() to
  418 free any memory.
  419 
  420     if (setjmp(png_jmpbuf(png_ptr)))
  421     {
  422        png_destroy_read_struct(&png_ptr, &info_ptr,
  423            &end_info);
  424        fclose(fp);
  425        return ERROR;
  426     }
  427 
  428 Pass (png_infopp)NULL instead of &end_info if you didn't create
  429 an end_info structure.
  430 
  431 If you would rather avoid the complexity of setjmp/longjmp issues,
  432 you can compile libpng with PNG_NO_SETJMP, in which case
  433 errors will result in a call to PNG_ABORT() which defaults to abort().
  434 
  435 You can #define PNG_ABORT() to a function that does something
  436 more useful than abort(), as long as your function does not
  437 return.
  438 
  439 Now you need to set up the input code.  The default for libpng is to
  440 use the C function fread().  If you use this, you will need to pass a
  441 valid FILE * in the function png_init_io().  Be sure that the file is
  442 opened in binary mode.  If you wish to handle reading data in another
  443 way, you need not call the png_init_io() function, but you must then
  444 implement the libpng I/O methods discussed in the Customizing Libpng
  445 section below.
  446 
  447     png_init_io(png_ptr, fp);
  448 
  449 If you had previously opened the file and read any of the signature from
  450 the beginning in order to see if this was a PNG file, you need to let
  451 libpng know that there are some bytes missing from the start of the file.
  452 
  453     png_set_sig_bytes(png_ptr, number);
  454 
  455 You can change the zlib compression buffer size to be used while
  456 reading compressed data with
  457 
  458     png_set_compression_buffer_size(png_ptr, buffer_size);
  459 
  460 where the default size is 8192 bytes.  Note that the buffer size
  461 is changed immediately and the buffer is reallocated immediately,
  462 instead of setting a flag to be acted upon later.
  463 
  464 If you want CRC errors to be handled in a different manner than
  465 the default, use
  466 
  467     png_set_crc_action(png_ptr, crit_action, ancil_action);
  468 
  469 The values for png_set_crc_action() say how libpng is to handle CRC errors in
  470 ancillary and critical chunks, and whether to use the data contained
  471 therein. Starting with libpng-1.6.26, this also governs how an ADLER32 error
  472 is handled while reading the IDAT chunk. Note that it is impossible to
  473 "discard" data in a critical chunk.
  474 
  475 Choices for (int) crit_action are
  476    PNG_CRC_DEFAULT      0  error/quit
  477    PNG_CRC_ERROR_QUIT   1  error/quit
  478    PNG_CRC_WARN_USE     3  warn/use data
  479    PNG_CRC_QUIET_USE    4  quiet/use data
  480    PNG_CRC_NO_CHANGE    5  use the current value
  481 
  482 Choices for (int) ancil_action are
  483    PNG_CRC_DEFAULT      0  error/quit
  484    PNG_CRC_ERROR_QUIT   1  error/quit
  485    PNG_CRC_WARN_DISCARD 2  warn/discard data
  486    PNG_CRC_WARN_USE     3  warn/use data
  487    PNG_CRC_QUIET_USE    4  quiet/use data
  488    PNG_CRC_NO_CHANGE    5  use the current value
  489 
  490 When the setting for crit_action is PNG_CRC_QUIET_USE, the CRC and ADLER32
  491 checksums are not only ignored, but they are not evaluated.
  492 
  493 Setting up callback code
  494 
  495 You can set up a callback function to handle any unknown chunks in the
  496 input stream. You must supply the function
  497 
  498     read_chunk_callback(png_structp png_ptr,
  499          png_unknown_chunkp chunk);
  500     {
  501        /* The unknown chunk structure contains your
  502           chunk data, along with similar data for any other
  503           unknown chunks: */
  504 
  505            png_byte name[5];
  506            png_byte *data;
  507            size_t size;
  508 
  509        /* Note that libpng has already taken care of
  510           the CRC handling */
  511 
  512        /* put your code here.  Search for your chunk in the
  513           unknown chunk structure, process it, and return one
  514           of the following: */
  515 
  516        return -n; /* chunk had an error */
  517        return 0; /* did not recognize */
  518        return n; /* success */
  519     }
  520 
  521 (You can give your function another name that you like instead of
  522 "read_chunk_callback")
  523 
  524 To inform libpng about your function, use
  525 
  526     png_set_read_user_chunk_fn(png_ptr, user_chunk_ptr,
  527         read_chunk_callback);
  528 
  529 This names not only the callback function, but also a user pointer that
  530 you can retrieve with
  531 
  532     png_get_user_chunk_ptr(png_ptr);
  533 
  534 If you call the png_set_read_user_chunk_fn() function, then all unknown
  535 chunks which the callback does not handle will be saved when read.  You can
  536 cause them to be discarded by returning '1' ("handled") instead of '0'.  This
  537 behavior will change in libpng 1.7 and the default handling set by the
  538 png_set_keep_unknown_chunks() function, described below, will be used when the
  539 callback returns 0.  If you want the existing behavior you should set the global
  540 default to PNG_HANDLE_CHUNK_IF_SAFE now; this is compatible with all current
  541 versions of libpng and with 1.7.  Libpng 1.6 issues a warning if you keep the
  542 default, or PNG_HANDLE_CHUNK_NEVER, and the callback returns 0.
  543 
  544 At this point, you can set up a callback function that will be
  545 called after each row has been read, which you can use to control
  546 a progress meter or the like.  It's demonstrated in pngtest.c.
  547 You must supply a function
  548 
  549     void read_row_callback(png_structp png_ptr,
  550        png_uint_32 row, int pass);
  551     {
  552       /* put your code here */
  553     }
  554 
  555 (You can give it another name that you like instead of "read_row_callback")
  556 
  557 To inform libpng about your function, use
  558 
  559     png_set_read_status_fn(png_ptr, read_row_callback);
  560 
  561 When this function is called the row has already been completely processed and
  562 the 'row' and 'pass' refer to the next row to be handled.  For the
  563 non-interlaced case the row that was just handled is simply one less than the
  564 passed in row number, and pass will always be 0.  For the interlaced case the
  565 same applies unless the row value is 0, in which case the row just handled was
  566 the last one from one of the preceding passes.  Because interlacing may skip a
  567 pass you cannot be sure that the preceding pass is just 'pass-1'; if you really
  568 need to know what the last pass is record (row,pass) from the callback and use
  569 the last recorded value each time.
  570 
  571 As with the user transform you can find the output row using the
  572 PNG_ROW_FROM_PASS_ROW macro.
  573 
  574 Unknown-chunk handling
  575 
  576 Now you get to set the way the library processes unknown chunks in the
  577 input PNG stream. Both known and unknown chunks will be read.  Normal
  578 behavior is that known chunks will be parsed into information in
  579 various info_ptr members while unknown chunks will be discarded. This
  580 behavior can be wasteful if your application will never use some known
  581 chunk types. To change this, you can call:
  582 
  583     png_set_keep_unknown_chunks(png_ptr, keep,
  584         chunk_list, num_chunks);
  585 
  586     keep       - 0: default unknown chunk handling
  587                  1: ignore; do not keep
  588                  2: keep only if safe-to-copy
  589                  3: keep even if unsafe-to-copy
  590 
  591                You can use these definitions:
  592                  PNG_HANDLE_CHUNK_AS_DEFAULT   0
  593                  PNG_HANDLE_CHUNK_NEVER        1
  594                  PNG_HANDLE_CHUNK_IF_SAFE      2
  595                  PNG_HANDLE_CHUNK_ALWAYS       3
  596 
  597     chunk_list - list of chunks affected (a byte string,
  598                  five bytes per chunk, NULL or '\0' if
  599                  num_chunks is positive; ignored if
  600                  numchunks <= 0).
  601 
  602     num_chunks - number of chunks affected; if 0, all
  603                  unknown chunks are affected.  If positive,
  604                  only the chunks in the list are affected,
  605                  and if negative all unknown chunks and
  606                  all known chunks except for the IHDR,
  607                  PLTE, tRNS, IDAT, and IEND chunks are
  608                  affected.
  609 
  610 Unknown chunks declared in this way will be saved as raw data onto a
  611 list of png_unknown_chunk structures.  If a chunk that is normally
  612 known to libpng is named in the list, it will be handled as unknown,
  613 according to the "keep" directive.  If a chunk is named in successive
  614 instances of png_set_keep_unknown_chunks(), the final instance will
  615 take precedence.  The IHDR and IEND chunks should not be named in
  616 chunk_list; if they are, libpng will process them normally anyway.
  617 If you know that your application will never make use of some particular
  618 chunks, use PNG_HANDLE_CHUNK_NEVER (or 1) as demonstrated below.
  619 
  620 Here is an example of the usage of png_set_keep_unknown_chunks(),
  621 where the private "vpAg" chunk will later be processed by a user chunk
  622 callback function:
  623 
  624     png_byte vpAg[5]={118, 112,  65, 103, (png_byte) '\0'};
  625 
  626     #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
  627       png_byte unused_chunks[]=
  628       {
  629         104,  73,  83,  84, (png_byte) '\0',   /* hIST */
  630         105,  84,  88, 116, (png_byte) '\0',   /* iTXt */
  631         112,  67,  65,  76, (png_byte) '\0',   /* pCAL */
  632         115,  67,  65,  76, (png_byte) '\0',   /* sCAL */
  633         115,  80,  76,  84, (png_byte) '\0',   /* sPLT */
  634         116,  73,  77,  69, (png_byte) '\0',   /* tIME */
  635       };
  636     #endif
  637 
  638     ...
  639 
  640     #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
  641       /* ignore all unknown chunks
  642        * (use global setting "2" for libpng16 and earlier):
  643        */
  644       png_set_keep_unknown_chunks(read_ptr, 2, NULL, 0);
  645 
  646       /* except for vpAg: */
  647       png_set_keep_unknown_chunks(read_ptr, 2, vpAg, 1);
  648 
  649       /* also ignore unused known chunks: */
  650       png_set_keep_unknown_chunks(read_ptr, 1, unused_chunks,
  651          (int)(sizeof unused_chunks)/5);
  652     #endif
  653 
  654 User limits
  655 
  656 The PNG specification allows the width and height of an image to be as
  657 large as 2^31-1 (0x7fffffff), or about 2.147 billion rows and columns.
  658 For safety, libpng imposes a default limit of 1 million rows and columns.
  659 Larger images will be rejected immediately with a png_error() call. If
  660 you wish to change these limits, you can use
  661 
  662    png_set_user_limits(png_ptr, width_max, height_max);
  663 
  664 to set your own limits (libpng may reject some very wide images
  665 anyway because of potential buffer overflow conditions).
  666 
  667 You should put this statement after you create the PNG structure and
  668 before calling png_read_info(), png_read_png(), or png_process_data().
  669 
  670 When writing a PNG datastream, put this statement before calling
  671 png_write_info() or png_write_png().
  672 
  673 If you need to retrieve the limits that are being applied, use
  674 
  675    width_max = png_get_user_width_max(png_ptr);
  676    height_max = png_get_user_height_max(png_ptr);
  677 
  678 The PNG specification sets no limit on the number of ancillary chunks
  679 allowed in a PNG datastream.  By default, libpng imposes a limit of
  680 a total of 1000 sPLT, tEXt, iTXt, zTXt, and unknown chunks to be stored.
  681 If you have set up both info_ptr and end_info_ptr, the limit applies
  682 separately to each.  You can change the limit on the total number of such
  683 chunks that will be stored, with
  684 
  685    png_set_chunk_cache_max(png_ptr, user_chunk_cache_max);
  686 
  687 where 0x7fffffffL means unlimited.  You can retrieve this limit with
  688 
  689    chunk_cache_max = png_get_chunk_cache_max(png_ptr);
  690 
  691 Libpng imposes a limit of 8 Megabytes (8,000,000 bytes) on the amount of
  692 memory that any chunk other than IDAT can occupy, originally or when
  693 decompressed (prior to libpng-1.6.32 the limit was only applied to compressed
  694 chunks after decompression). You can change this limit with
  695 
  696    png_set_chunk_malloc_max(png_ptr, user_chunk_malloc_max);
  697 
  698 and you can retrieve the limit with
  699 
  700    chunk_malloc_max = png_get_chunk_malloc_max(png_ptr);
  701 
  702 Any chunks that would cause either of these limits to be exceeded will
  703 be ignored.
  704 
  705 Information about your system
  706 
  707 If you intend to display the PNG or to incorporate it in other image data you
  708 need to tell libpng information about your display or drawing surface so that
  709 libpng can convert the values in the image to match the display.
  710 
  711 From libpng-1.5.4 this information can be set before reading the PNG file
  712 header.  In earlier versions png_set_gamma() existed but behaved incorrectly if
  713 called before the PNG file header had been read and png_set_alpha_mode() did not
  714 exist.
  715 
  716 If you need to support versions prior to libpng-1.5.4 test the version number
  717 as illustrated below using "PNG_LIBPNG_VER >= 10504" and follow the procedures
  718 described in the appropriate manual page.
  719 
  720 You give libpng the encoding expected by your system expressed as a 'gamma'
  721 value.  You can also specify a default encoding for the PNG file in
  722 case the required information is missing from the file.  By default libpng
  723 assumes that the PNG data matches your system, to keep this default call:
  724 
  725    png_set_gamma(png_ptr, screen_gamma, output_gamma);
  726 
  727 or you can use the fixed point equivalent:
  728 
  729    png_set_gamma_fixed(png_ptr, PNG_FP_1*screen_gamma,
  730       PNG_FP_1*output_gamma);
  731 
  732 If you don't know the gamma for your system it is probably 2.2 - a good
  733 approximation to the IEC standard for display systems (sRGB).  If images are
  734 too contrasty or washed out you got the value wrong - check your system
  735 documentation!
  736 
  737 Many systems permit the system gamma to be changed via a lookup table in the
  738 display driver, a few systems, including older Macs, change the response by
  739 default.  As of 1.5.4 three special values are available to handle common
  740 situations:
  741 
  742    PNG_DEFAULT_sRGB: Indicates that the system conforms to the
  743                      IEC 61966-2-1 standard.  This matches almost
  744                      all systems.
  745    PNG_GAMMA_MAC_18: Indicates that the system is an older
  746                      (pre Mac OS 10.6) Apple Macintosh system with
  747                      the default settings.
  748    PNG_GAMMA_LINEAR: Just the fixed point value for 1.0 - indicates
  749                      that the system expects data with no gamma
  750                      encoding.
  751 
  752 You would use the linear (unencoded) value if you need to process the pixel
  753 values further because this avoids the need to decode and re-encode each
  754 component value whenever arithmetic is performed.  A lot of graphics software
  755 uses linear values for this reason, often with higher precision component values
  756 to preserve overall accuracy.
  757 
  758 
  759 The output_gamma value expresses how to decode the output values, not how
  760 they are encoded.  The values used correspond to the normal numbers used to
  761 describe the overall gamma of a computer display system; for example 2.2 for
  762 an sRGB conformant system.  The values are scaled by 100000 in the _fixed
  763 version of the API (so 220000 for sRGB.)
  764 
  765 The inverse of the value is always used to provide a default for the PNG file
  766 encoding if it has no gAMA chunk and if png_set_gamma() has not been called
  767 to override the PNG gamma information.
  768 
  769 When the ALPHA_OPTIMIZED mode is selected the output gamma is used to encode
  770 opaque pixels however pixels with lower alpha values are not encoded,
  771 regardless of the output gamma setting.
  772 
  773 When the standard Porter Duff handling is requested with mode 1 the output
  774 encoding is set to be linear and the output_gamma value is only relevant
  775 as a default for input data that has no gamma information.  The linear output
  776 encoding will be overridden if png_set_gamma() is called - the results may be
  777 highly unexpected!
  778 
  779 The following numbers are derived from the sRGB standard and the research
  780 behind it.  sRGB is defined to be approximated by a PNG gAMA chunk value of
  781 0.45455 (1/2.2) for PNG.  The value implicitly includes any viewing
  782 correction required to take account of any differences in the color
  783 environment of the original scene and the intended display environment; the
  784 value expresses how to *decode* the image for display, not how the original
  785 data was *encoded*.
  786 
  787 sRGB provides a peg for the PNG standard by defining a viewing environment.
  788 sRGB itself, and earlier TV standards, actually use a more complex transform
  789 (a linear portion then a gamma 2.4 power law) than PNG can express.  (PNG is
  790 limited to simple power laws.)  By saying that an image for direct display on
  791 an sRGB conformant system should be stored with a gAMA chunk value of 45455
  792 (11.3.3.2 and 11.3.3.5 of the ISO PNG specification) the PNG specification
  793 makes it possible to derive values for other display systems and
  794 environments.
  795 
  796 The Mac value is deduced from the sRGB based on an assumption that the actual
  797 extra viewing correction used in early Mac display systems was implemented as
  798 a power 1.45 lookup table.
  799 
  800 Any system where a programmable lookup table is used or where the behavior of
  801 the final display device characteristics can be changed requires system
  802 specific code to obtain the current characteristic.  However this can be
  803 difficult and most PNG gamma correction only requires an approximate value.
  804 
  805 By default, if png_set_alpha_mode() is not called, libpng assumes that all
  806 values are unencoded, linear, values and that the output device also has a
  807 linear characteristic.  This is only very rarely correct - it is invariably
  808 better to call png_set_alpha_mode() with PNG_DEFAULT_sRGB than rely on the
  809 default if you don't know what the right answer is!
  810 
  811 The special value PNG_GAMMA_MAC_18 indicates an older Mac system (pre Mac OS
  812 10.6) which used a correction table to implement a somewhat lower gamma on an
  813 otherwise sRGB system.
  814 
  815 Both these values are reserved (not simple gamma values) in order to allow
  816 more precise correction internally in the future.
  817 
  818 NOTE: the values can be passed to either the fixed or floating
  819 point APIs, but the floating point API will also accept floating point
  820 values.
  821 
  822 The second thing you may need to tell libpng about is how your system handles
  823 alpha channel information.  Some, but not all, PNG files contain an alpha
  824 channel.  To display these files correctly you need to compose the data onto a
  825 suitable background, as described in the PNG specification.
  826 
  827 Libpng only supports composing onto a single color (using png_set_background;
  828 see below).  Otherwise you must do the composition yourself and, in this case,
  829 you may need to call png_set_alpha_mode:
  830 
  831    #if PNG_LIBPNG_VER >= 10504
  832       png_set_alpha_mode(png_ptr, mode, screen_gamma);
  833    #else
  834       png_set_gamma(png_ptr, screen_gamma, 1.0/screen_gamma);
  835    #endif
  836 
  837 The screen_gamma value is the same as the argument to png_set_gamma; however,
  838 how it affects the output depends on the mode.  png_set_alpha_mode() sets the
  839 file gamma default to 1/screen_gamma, so normally you don't need to call
  840 png_set_gamma.  If you need different defaults call png_set_gamma() before
  841 png_set_alpha_mode() - if you call it after it will override the settings made
  842 by png_set_alpha_mode().
  843 
  844 The mode is as follows:
  845 
  846     PNG_ALPHA_PNG: The data is encoded according to the PNG
  847 specification.  Red, green and blue, or gray, components are
  848 gamma encoded color values and are not premultiplied by the
  849 alpha value.  The alpha value is a linear measure of the
  850 contribution of the pixel to the corresponding final output pixel.
  851 
  852 You should normally use this format if you intend to perform
  853 color correction on the color values; most, maybe all, color
  854 correction software has no handling for the alpha channel and,
  855 anyway, the math to handle pre-multiplied component values is
  856 unnecessarily complex.
  857 
  858 Before you do any arithmetic on the component values you need
  859 to remove the gamma encoding and multiply out the alpha
  860 channel.  See the PNG specification for more detail.  It is
  861 important to note that when an image with an alpha channel is
  862 scaled, linear encoded, pre-multiplied component values must
  863 be used!
  864 
  865 The remaining modes assume you don't need to do any further color correction or
  866 that if you do, your color correction software knows all about alpha (it
  867 probably doesn't!).  They 'associate' the alpha with the color information by
  868 storing color channel values that have been scaled by the alpha.  The
  869 advantage is that the color channels can be resampled (the image can be
  870 scaled) in this form.  The disadvantage is that normal practice is to store
  871 linear, not (gamma) encoded, values and this requires 16-bit channels for
  872 still images rather than the 8-bit channels that are just about sufficient if
  873 gamma encoding is used.  In addition all non-transparent pixel values,
  874 including completely opaque ones, must be gamma encoded to produce the final
  875 image.  These are the 'STANDARD', 'ASSOCIATED' or 'PREMULTIPLIED' modes
  876 described below (the latter being the two common names for associated alpha
  877 color channels). Note that PNG files always contain non-associated color
  878 channels; png_set_alpha_mode() with one of the modes causes the decoder to
  879 convert the pixels to an associated form before returning them to your
  880 application. 
  881 
  882 Since it is not necessary to perform arithmetic on opaque color values so
  883 long as they are not to be resampled and are in the final color space it is
  884 possible to optimize the handling of alpha by storing the opaque pixels in
  885 the PNG format (adjusted for the output color space) while storing partially
  886 opaque pixels in the standard, linear, format.  The accuracy required for
  887 standard alpha composition is relatively low, because the pixels are
  888 isolated, therefore typically the accuracy loss in storing 8-bit linear
  889 values is acceptable.  (This is not true if the alpha channel is used to
  890 simulate transparency over large areas - use 16 bits or the PNG mode in
  891 this case!)  This is the 'OPTIMIZED' mode.  For this mode a pixel is
  892 treated as opaque only if the alpha value is equal to the maximum value.
  893 
  894     PNG_ALPHA_STANDARD:  The data libpng produces is encoded in the
  895 standard way assumed by most correctly written graphics software.
  896 The gamma encoding will be removed by libpng and the
  897 linear component values will be pre-multiplied by the
  898 alpha channel.
  899 
  900 With this format the final image must be re-encoded to
  901 match the display gamma before the image is displayed.
  902 If your system doesn't do that, yet still seems to
  903 perform arithmetic on the pixels without decoding them,
  904 it is broken - check out the modes below.
  905 
  906 With PNG_ALPHA_STANDARD libpng always produces linear
  907 component values, whatever screen_gamma you supply.  The
  908 screen_gamma value is, however, used as a default for
  909 the file gamma if the PNG file has no gamma information.
  910 
  911 If you call png_set_gamma() after png_set_alpha_mode() you
  912 will override the linear encoding.  Instead the
  913 pre-multiplied pixel values will be gamma encoded but
  914 the alpha channel will still be linear.  This may
  915 actually match the requirements of some broken software,
  916 but it is unlikely.
  917 
  918 While linear 8-bit data is often used it has
  919 insufficient precision for any image with a reasonable
  920 dynamic range.  To avoid problems, and if your software
  921 supports it, use png_set_expand_16() to force all
  922 components to 16 bits.
  923 
  924     PNG_ALPHA_OPTIMIZED: This mode is the same as PNG_ALPHA_STANDARD
  925 except that completely opaque pixels are gamma encoded according to
  926 the screen_gamma value.  Pixels with alpha less than 1.0
  927 will still have linear components.
  928 
  929 Use this format if you have control over your
  930 compositing software and so don't do other arithmetic
  931 (such as scaling) on the data you get from libpng.  Your
  932 compositing software can simply copy opaque pixels to
  933 the output but still has linear values for the
  934 non-opaque pixels.
  935 
  936 In normal compositing, where the alpha channel encodes
  937 partial pixel coverage (as opposed to broad area
  938 translucency), the inaccuracies of the 8-bit
  939 representation of non-opaque pixels are irrelevant.
  940 
  941 You can also try this format if your software is broken;
  942 it might look better.
  943 
  944     PNG_ALPHA_BROKEN: This is PNG_ALPHA_STANDARD; however, all component
  945 values, including the alpha channel are gamma encoded.  This is
  946 broken because, in practice, no implementation that uses this choice
  947 correctly undoes the encoding before handling alpha composition.  Use this
  948 choice only if other serious errors in the software or hardware you use
  949 mandate it.  In most cases of broken software or hardware the bug in the
  950 final display manifests as a subtle halo around composited parts of the
  951 image.  You may not even perceive this as a halo; the composited part of
  952 the image may simply appear separate from the background, as though it had
  953 been cut out of paper and pasted on afterward.
  954 
  955 If you don't have to deal with bugs in software or hardware, or if you can fix
  956 them, there are three recommended ways of using png_set_alpha_mode():
  957 
  958    png_set_alpha_mode(png_ptr, PNG_ALPHA_PNG,
  959        screen_gamma);
  960 
  961 You can do color correction on the result (libpng does not currently
  962 support color correction internally).  When you handle the alpha channel
  963 you need to undo the gamma encoding and multiply out the alpha.
  964 
  965    png_set_alpha_mode(png_ptr, PNG_ALPHA_STANDARD,
  966        screen_gamma);
  967    png_set_expand_16(png_ptr);
  968 
  969 If you are using the high level interface, don't call png_set_expand_16();
  970 instead pass PNG_TRANSFORM_EXPAND_16 to the interface.
  971 
  972 With this mode you can't do color correction, but you can do arithmetic,
  973 including composition and scaling, on the data without further processing.
  974 
  975    png_set_alpha_mode(png_ptr, PNG_ALPHA_OPTIMIZED,
  976        screen_gamma);
  977 
  978 You can avoid the expansion to 16-bit components with this mode, but you
  979 lose the ability to scale the image or perform other linear arithmetic.
  980 All you can do is compose the result onto a matching output.  Since this
  981 mode is libpng-specific you also need to write your own composition
  982 software.
  983 
  984 The following are examples of calls to png_set_alpha_mode to achieve the
  985 required overall gamma correction and, where necessary, alpha
  986 premultiplication.
  987 
  988     png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB);
  989 
  990 Choices for the alpha_mode are
  991 
  992     PNG_ALPHA_PNG           0 /* according to the PNG standard */
  993     PNG_ALPHA_STANDARD      1 /* according to Porter/Duff */
  994     PNG_ALPHA_ASSOCIATED    1 /* as above; this is the normal practice */
  995     PNG_ALPHA_PREMULTIPLIED 1 /* as above */
  996     PNG_ALPHA_OPTIMIZED     2 /* 'PNG' for opaque pixels, else 'STANDARD' */
  997     PNG_ALPHA_BROKEN        3 /* the alpha channel is gamma encoded */
  998 
  999 PNG_ALPHA_PNG is the default libpng handling of the alpha channel. It is not
 1000 pre-multiplied into the color components. In addition the call states
 1001 that the output is for a sRGB system and causes all PNG files without gAMA
 1002 chunks to be assumed to be encoded using sRGB.
 1003 
 1004     png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC);
 1005 
 1006 In this case the output is assumed to be something like an sRGB conformant
 1007 display preceded by a power-law lookup table of power 1.45.  This is how
 1008 early Mac systems behaved.
 1009 
 1010     png_set_alpha_mode(pp, PNG_ALPHA_STANDARD, PNG_GAMMA_LINEAR);
 1011 
 1012 This is the classic Jim Blinn approach and will work in academic
 1013 environments where everything is done by the book.  It has the shortcoming
 1014 of assuming that input PNG data with no gamma information is linear - this
 1015 is unlikely to be correct unless the PNG files were generated locally.
 1016 Most of the time the output precision will be so low as to show
 1017 significant banding in dark areas of the image.
 1018 
 1019     png_set_expand_16(pp);
 1020     png_set_alpha_mode(pp, PNG_ALPHA_STANDARD, PNG_DEFAULT_sRGB);
 1021 
 1022 This is a somewhat more realistic Jim Blinn inspired approach.  PNG files
 1023 are assumed to have the sRGB encoding if not marked with a gamma value and
 1024 the output is always 16 bits per component.  This permits accurate scaling
 1025 and processing of the data.  If you know that your input PNG files were
 1026 generated locally you might need to replace PNG_DEFAULT_sRGB with the
 1027 correct value for your system.
 1028 
 1029     png_set_alpha_mode(pp, PNG_ALPHA_OPTIMIZED, PNG_DEFAULT_sRGB);
 1030 
 1031 If you just need to composite the PNG image onto an existing background
 1032 and if you control the code that does this you can use the optimization
 1033 setting.  In this case you just copy completely opaque pixels to the
 1034 output.  For pixels that are not completely transparent (you just skip
 1035 those) you do the composition math using png_composite or png_composite_16
 1036 below then encode the resultant 8-bit or 16-bit values to match the output
 1037 encoding.
 1038 
 1039     Other cases
 1040 
 1041 If neither the PNG nor the standard linear encoding work for you because
 1042 of the software or hardware you use then you have a big problem.  The PNG
 1043 case will probably result in halos around the image.  The linear encoding
 1044 will probably result in a washed out, too bright, image (it's actually too
 1045 contrasty.)  Try the ALPHA_OPTIMIZED mode above - this will probably
 1046 substantially reduce the halos.  Alternatively try:
 1047 
 1048     png_set_alpha_mode(pp, PNG_ALPHA_BROKEN, PNG_DEFAULT_sRGB);
 1049 
 1050 This option will also reduce the halos, but there will be slight dark
 1051 halos round the opaque parts of the image where the background is light.
 1052 In the OPTIMIZED mode the halos will be light halos where the background
 1053 is dark.  Take your pick - the halos are unavoidable unless you can get
 1054 your hardware/software fixed!  (The OPTIMIZED approach is slightly
 1055 faster.)
 1056 
 1057 When the default gamma of PNG files doesn't match the output gamma.
 1058 If you have PNG files with no gamma information png_set_alpha_mode allows
 1059 you to provide a default gamma, but it also sets the output gamma to the
 1060 matching value.  If you know your PNG files have a gamma that doesn't
 1061 match the output you can take advantage of the fact that
 1062 png_set_alpha_mode always sets the output gamma but only sets the PNG
 1063 default if it is not already set:
 1064 
 1065     png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB);
 1066     png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC);
 1067 
 1068 The first call sets both the default and the output gamma values, the
 1069 second call overrides the output gamma without changing the default.  This
 1070 is easier than achieving the same effect with png_set_gamma.  You must use
 1071 PNG_ALPHA_PNG for the first call - internal checking in png_set_alpha will
 1072 fire if more than one call to png_set_alpha_mode and png_set_background is
 1073 made in the same read operation, however multiple calls with PNG_ALPHA_PNG
 1074 are ignored.
 1075 
 1076 If you don't need, or can't handle, the alpha channel you can call
 1077 png_set_background() to remove it by compositing against a fixed color.  Don't
 1078 call png_set_strip_alpha() to do this - it will leave spurious pixel values in
 1079 transparent parts of this image.
 1080 
 1081    png_set_background(png_ptr, &background_color,
 1082        PNG_BACKGROUND_GAMMA_SCREEN, 0, 1);
 1083 
 1084 The background_color is an RGB or grayscale value according to the data format
 1085 libpng will produce for you.  Because you don't yet know the format of the PNG
 1086 file, if you call png_set_background at this point you must arrange for the
 1087 format produced by libpng to always have 8-bit or 16-bit components and then
 1088 store the color as an 8-bit or 16-bit color as appropriate.  The color contains
 1089 separate gray and RGB component values, so you can let libpng produce gray or
 1090 RGB output according to the input format, but low bit depth grayscale images
 1091 must always be converted to at least 8-bit format.  (Even though low bit depth
 1092 grayscale images can't have an alpha channel they can have a transparent
 1093 color!)
 1094 
 1095 You set the transforms you need later, either as flags to the high level
 1096 interface or libpng API calls for the low level interface.  For reference the
 1097 settings and API calls required are:
 1098 
 1099 8-bit values:
 1100    PNG_TRANSFORM_SCALE_16 | PNG_EXPAND
 1101    png_set_expand(png_ptr); png_set_scale_16(png_ptr);
 1102 
 1103    If you must get exactly the same inaccurate results
 1104    produced by default in versions prior to libpng-1.5.4,
 1105    use PNG_TRANSFORM_STRIP_16 and png_set_strip_16(png_ptr)
 1106    instead.
 1107 
 1108 16-bit values:
 1109    PNG_TRANSFORM_EXPAND_16
 1110    png_set_expand_16(png_ptr);
 1111 
 1112 In either case palette image data will be expanded to RGB.  If you just want
 1113 color data you can add PNG_TRANSFORM_GRAY_TO_RGB or png_set_gray_to_rgb(png_ptr)
 1114 to the list.
 1115 
 1116 Calling png_set_background before the PNG file header is read will not work
 1117 prior to libpng-1.5.4.  Because the failure may result in unexpected warnings or
 1118 errors it is therefore much safer to call png_set_background after the head has
 1119 been read.  Unfortunately this means that prior to libpng-1.5.4 it cannot be
 1120 used with the high level interface.
 1121 
 1122 The high-level read interface
 1123 
 1124 At this point there are two ways to proceed; through the high-level
 1125 read interface, or through a sequence of low-level read operations.
 1126 You can use the high-level interface if (a) you are willing to read
 1127 the entire image into memory, and (b) the input transformations
 1128 you want to do are limited to the following set:
 1129 
 1130     PNG_TRANSFORM_IDENTITY      No transformation
 1131     PNG_TRANSFORM_SCALE_16      Strip 16-bit samples to
 1132                                 8-bit accurately
 1133     PNG_TRANSFORM_STRIP_16      Chop 16-bit samples to
 1134                                 8-bit less accurately
 1135     PNG_TRANSFORM_STRIP_ALPHA   Discard the alpha channel
 1136     PNG_TRANSFORM_PACKING       Expand 1, 2 and 4-bit
 1137                                 samples to bytes
 1138     PNG_TRANSFORM_PACKSWAP      Change order of packed
 1139                                 pixels to LSB first
 1140     PNG_TRANSFORM_EXPAND        Perform set_expand()
 1141     PNG_TRANSFORM_INVERT_MONO   Invert monochrome images
 1142     PNG_TRANSFORM_SHIFT         Normalize pixels to the
 1143                                 sBIT depth
 1144     PNG_TRANSFORM_BGR           Flip RGB to BGR, RGBA
 1145                                 to BGRA
 1146     PNG_TRANSFORM_SWAP_ALPHA    Flip RGBA to ARGB or GA
 1147                                 to AG
 1148     PNG_TRANSFORM_INVERT_ALPHA  Change alpha from opacity
 1149                                 to transparency
 1150     PNG_TRANSFORM_SWAP_ENDIAN   Byte-swap 16-bit samples
 1151     PNG_TRANSFORM_GRAY_TO_RGB   Expand grayscale samples
 1152                                 to RGB (or GA to RGBA)
 1153     PNG_TRANSFORM_EXPAND_16     Expand samples to 16 bits
 1154 
 1155 (This excludes setting a background color, doing gamma transformation,
 1156 quantizing, and setting filler.)  If this is the case, simply do this:
 1157 
 1158     png_read_png(png_ptr, info_ptr, png_transforms, NULL)
 1159 
 1160 where png_transforms is an integer containing the bitwise OR of some
 1161 set of transformation flags.  This call is equivalent to png_read_info(),
 1162 followed the set of transformations indicated by the transform mask,
 1163 then png_read_image(), and finally png_read_end().
 1164 
 1165 (The final parameter of this call is not yet used.  Someday it might point
 1166 to transformation parameters required by some future input transform.)
 1167 
 1168 You must use png_transforms and not call any png_set_transform() functions
 1169 when you use png_read_png().
 1170 
 1171 After you have called png_read_png(), you can retrieve the image data
 1172 with
 1173 
 1174    row_pointers = png_get_rows(png_ptr, info_ptr);
 1175 
 1176 where row_pointers is an array of pointers to the pixel data for each row:
 1177 
 1178    png_bytep row_pointers[height];
 1179 
 1180 If you know your image size and pixel size ahead of time, you can allocate
 1181 row_pointers prior to calling png_read_png() with
 1182 
 1183    if (height > PNG_UINT_32_MAX/(sizeof (png_byte)))
 1184       png_error (png_ptr,
 1185           "Image is too tall to process in memory");
 1186 
 1187    if (width > PNG_UINT_32_MAX/pixel_size)
 1188       png_error (png_ptr,
 1189           "Image is too wide to process in memory");
 1190 
 1191    row_pointers = png_malloc(png_ptr,
 1192        height*(sizeof (png_bytep)));
 1193 
 1194    for (int i=0; i<height, i++)
 1195       row_pointers[i]=NULL;  /* security precaution */
 1196 
 1197    for (int i=0; i<height, i++)
 1198       row_pointers[i]=png_malloc(png_ptr,
 1199           width*pixel_size);
 1200 
 1201    png_set_rows(png_ptr, info_ptr, &row_pointers);
 1202 
 1203 Alternatively you could allocate your image in one big block and define
 1204 row_pointers[i] to point into the proper places in your block, but first
 1205 be sure that your platform is able to allocate such a large buffer:
 1206 
 1207    /* Guard against integer overflow */
 1208    if (height > PNG_SIZE_MAX/(width*pixel_size)) {
 1209         png_error(png_ptr,"image_data buffer would be too large");
 1210    }
 1211 
 1212    png_bytep buffer=png_malloc(png_ptr,height*width*pixel_size);
 1213 
 1214    for (int i=0; i<height, i++)
 1215       row_pointers[i]=buffer+i*width*pixel_size;
 1216 
 1217    png_set_rows(png_ptr, info_ptr, &row_pointers);
 1218 
 1219 If you use png_set_rows(), the application is responsible for freeing
 1220 row_pointers (and row_pointers[i], if they were separately allocated).
 1221 
 1222 If you don't allocate row_pointers ahead of time, png_read_png() will
 1223 do it, and it'll be free'ed by libpng when you call png_destroy_*().
 1224 
 1225 The low-level read interface
 1226 
 1227 If you are going the low-level route, you are now ready to read all
 1228 the file information up to the actual image data.  You do this with a
 1229 call to png_read_info().
 1230 
 1231     png_read_info(png_ptr, info_ptr);
 1232 
 1233 This will process all chunks up to but not including the image data.
 1234 
 1235 This also copies some of the data from the PNG file into the decode structure
 1236 for use in later transformations.  Important information copied in is:
 1237 
 1238 1) The PNG file gamma from the gAMA chunk.  This overwrites the default value
 1239 provided by an earlier call to png_set_gamma or png_set_alpha_mode.
 1240 
 1241 2) Prior to libpng-1.5.4 the background color from a bKGd chunk.  This
 1242 damages the information provided by an earlier call to png_set_background
 1243 resulting in unexpected behavior.  Libpng-1.5.4 no longer does this.
 1244 
 1245 3) The number of significant bits in each component value.  Libpng uses this to
 1246 optimize gamma handling by reducing the internal lookup table sizes.
 1247 
 1248 4) The transparent color information from a tRNS chunk.  This can be modified by
 1249 a later call to png_set_tRNS.
 1250 
 1251 Querying the info structure
 1252 
 1253 Functions are used to get the information from the info_ptr once it
 1254 has been read.  Note that these fields may not be completely filled
 1255 in until png_read_end() has read the chunk data following the image.
 1256 
 1257     png_get_IHDR(png_ptr, info_ptr, &width, &height,
 1258        &bit_depth, &color_type, &interlace_type,
 1259        &compression_type, &filter_method);
 1260 
 1261     width          - holds the width of the image
 1262                      in pixels (up to 2^31).
 1263 
 1264     height         - holds the height of the image
 1265                      in pixels (up to 2^31).
 1266 
 1267     bit_depth      - holds the bit depth of one of the
 1268                      image channels.  (valid values are
 1269                      1, 2, 4, 8, 16 and depend also on
 1270                      the color_type.  See also
 1271                      significant bits (sBIT) below).
 1272 
 1273     color_type     - describes which color/alpha channels
 1274                          are present.
 1275                      PNG_COLOR_TYPE_GRAY
 1276                         (bit depths 1, 2, 4, 8, 16)
 1277                      PNG_COLOR_TYPE_GRAY_ALPHA
 1278                         (bit depths 8, 16)
 1279                      PNG_COLOR_TYPE_PALETTE
 1280                         (bit depths 1, 2, 4, 8)
 1281                      PNG_COLOR_TYPE_RGB
 1282                         (bit_depths 8, 16)
 1283                      PNG_COLOR_TYPE_RGB_ALPHA
 1284                         (bit_depths 8, 16)
 1285 
 1286                      PNG_COLOR_MASK_PALETTE
 1287                      PNG_COLOR_MASK_COLOR
 1288                      PNG_COLOR_MASK_ALPHA
 1289 
 1290     interlace_type - (PNG_INTERLACE_NONE or
 1291                      PNG_INTERLACE_ADAM7)
 1292 
 1293     compression_type - (must be PNG_COMPRESSION_TYPE_BASE
 1294                      for PNG 1.0)
 1295 
 1296     filter_method  - (must be PNG_FILTER_TYPE_BASE
 1297                      for PNG 1.0, and can also be
 1298                      PNG_INTRAPIXEL_DIFFERENCING if
 1299                      the PNG datastream is embedded in
 1300                      a MNG-1.0 datastream)
 1301 
 1302     Any of width, height, color_type, bit_depth,
 1303     interlace_type, compression_type, or filter_method can
 1304     be NULL if you are not interested in their values.
 1305 
 1306     Note that png_get_IHDR() returns 32-bit data into
 1307     the application's width and height variables.
 1308     This is an unsafe situation if these are not png_uint_32
 1309     variables.  In such situations, the
 1310     png_get_image_width() and png_get_image_height()
 1311     functions described below are safer.
 1312 
 1313     width            = png_get_image_width(png_ptr,
 1314                          info_ptr);
 1315 
 1316     height           = png_get_image_height(png_ptr,
 1317                          info_ptr);
 1318 
 1319     bit_depth        = png_get_bit_depth(png_ptr,
 1320                          info_ptr);
 1321 
 1322     color_type       = png_get_color_type(png_ptr,
 1323                          info_ptr);
 1324 
 1325     interlace_type   = png_get_interlace_type(png_ptr,
 1326                          info_ptr);
 1327 
 1328     compression_type = png_get_compression_type(png_ptr,
 1329                          info_ptr);
 1330 
 1331     filter_method    = png_get_filter_type(png_ptr,
 1332                          info_ptr);
 1333 
 1334     channels = png_get_channels(png_ptr, info_ptr);
 1335 
 1336     channels       - number of channels of info for the
 1337                      color type (valid values are 1 (GRAY,
 1338                      PALETTE), 2 (GRAY_ALPHA), 3 (RGB),
 1339                      4 (RGB_ALPHA or RGB + filler byte))
 1340 
 1341     rowbytes = png_get_rowbytes(png_ptr, info_ptr);
 1342 
 1343     rowbytes       - number of bytes needed to hold a row
 1344                      This value, the bit_depth, color_type,
 1345                      and the number of channels can change
 1346                      if you use transforms such as
 1347                      png_set_expand(). See
 1348                      png_read_update_info(), below.
 1349 
 1350     signature = png_get_signature(png_ptr, info_ptr);
 1351 
 1352     signature      - holds the signature read from the
 1353                      file (if any).  The data is kept in
 1354                      the same offset it would be if the
 1355                      whole signature were read (i.e. if an
 1356                      application had already read in 4
 1357                      bytes of signature before starting
 1358                      libpng, the remaining 4 bytes would
 1359                      be in signature[4] through signature[7]
 1360                      (see png_set_sig_bytes())).
 1361 
 1362 These are also important, but their validity depends on whether the chunk
 1363 has been read.  The png_get_valid(png_ptr, info_ptr, PNG_INFO_<chunk>) and
 1364 png_get_<chunk>(png_ptr, info_ptr, ...) functions return non-zero if the
 1365 data has been read, or zero if it is missing.  The parameters to the
 1366 png_get_<chunk> are set directly if they are simple data types, or a
 1367 pointer into the info_ptr is returned for any complex types.
 1368 
 1369 The colorspace data from gAMA, cHRM, sRGB, iCCP, and sBIT chunks
 1370 is simply returned to give the application information about how the
 1371 image was encoded.  Libpng itself only does transformations using the file
 1372 gamma when combining semitransparent pixels with the background color, and,
 1373 since libpng-1.6.0, when converting between 8-bit sRGB and 16-bit linear pixels
 1374 within the simplified API.  Libpng also uses the file gamma when converting
 1375 RGB to gray, beginning with libpng-1.0.5, if the application calls
 1376 png_set_rgb_to_gray()).
 1377 
 1378     png_get_PLTE(png_ptr, info_ptr, &palette,
 1379                      &num_palette);
 1380 
 1381     palette        - the palette for the file
 1382                      (array of png_color)
 1383 
 1384     num_palette    - number of entries in the palette
 1385 
 1386     png_get_gAMA(png_ptr, info_ptr, &file_gamma);
 1387     png_get_gAMA_fixed(png_ptr, info_ptr, &int_file_gamma);
 1388 
 1389     file_gamma     - the gamma at which the file is
 1390                      written (PNG_INFO_gAMA)
 1391 
 1392     int_file_gamma - 100,000 times the gamma at which the
 1393                      file is written
 1394 
 1395     png_get_cHRM(png_ptr, info_ptr,  &white_x, &white_y, &red_x,
 1396                      &red_y, &green_x, &green_y, &blue_x, &blue_y)
 1397     png_get_cHRM_XYZ(png_ptr, info_ptr, &red_X, &red_Y, &red_Z,
 1398                      &green_X, &green_Y, &green_Z, &blue_X, &blue_Y,
 1399                      &blue_Z)
 1400     png_get_cHRM_fixed(png_ptr, info_ptr, &int_white_x,
 1401                      &int_white_y, &int_red_x, &int_red_y,
 1402                      &int_green_x, &int_green_y, &int_blue_x,
 1403                      &int_blue_y)
 1404     png_get_cHRM_XYZ_fixed(png_ptr, info_ptr, &int_red_X, &int_red_Y,
 1405                      &int_red_Z, &int_green_X, &int_green_Y,
 1406                      &int_green_Z, &int_blue_X, &int_blue_Y,
 1407                      &int_blue_Z)
 1408 
 1409     {white,red,green,blue}_{x,y}
 1410                      A color space encoding specified using the
 1411                      chromaticities of the end points and the
 1412                      white point. (PNG_INFO_cHRM)
 1413 
 1414     {red,green,blue}_{X,Y,Z}
 1415                      A color space encoding specified using the
 1416                      encoding end points - the CIE tristimulus
 1417                      specification of the intended color of the red,
 1418                      green and blue channels in the PNG RGB data.
 1419                      The white point is simply the sum of the three
 1420                      end points. (PNG_INFO_cHRM)
 1421 
 1422     png_get_sRGB(png_ptr, info_ptr, &srgb_intent);
 1423 
 1424     srgb_intent -    the rendering intent (PNG_INFO_sRGB)
 1425                      The presence of the sRGB chunk
 1426                      means that the pixel data is in the
 1427                      sRGB color space.  This chunk also
 1428                      implies specific values of gAMA and
 1429                      cHRM.
 1430 
 1431     png_get_iCCP(png_ptr, info_ptr, &name,
 1432        &compression_type, &profile, &proflen);
 1433 
 1434     name             - The profile name.
 1435 
 1436     compression_type - The compression type; always
 1437                        PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
 1438                        You may give NULL to this argument to
 1439                        ignore it.
 1440 
 1441     profile          - International Color Consortium color
 1442                        profile data. May contain NULs.
 1443 
 1444     proflen          - length of profile data in bytes.
 1445 
 1446     png_get_sBIT(png_ptr, info_ptr, &sig_bit);
 1447 
 1448     sig_bit        - the number of significant bits for
 1449                      (PNG_INFO_sBIT) each of the gray,
 1450                      red, green, and blue channels,
 1451                      whichever are appropriate for the
 1452                      given color type (png_color_16)
 1453 
 1454     png_get_tRNS(png_ptr, info_ptr, &trans_alpha,
 1455                      &num_trans, &trans_color);
 1456 
 1457     trans_alpha    - array of alpha (transparency)
 1458                      entries for palette (PNG_INFO_tRNS)
 1459 
 1460     num_trans      - number of transparent entries
 1461                      (PNG_INFO_tRNS)
 1462 
 1463     trans_color    - graylevel or color sample values of
 1464                      the single transparent color for
 1465                      non-paletted images (PNG_INFO_tRNS)
 1466 
 1467     png_get_eXIf_1(png_ptr, info_ptr, &num_exif, &exif);
 1468                      (PNG_INFO_eXIf)
 1469 
 1470     exif           - Exif profile (array of png_byte)
 1471 
 1472     png_get_hIST(png_ptr, info_ptr, &hist);
 1473                      (PNG_INFO_hIST)
 1474 
 1475     hist           - histogram of palette (array of
 1476                      png_uint_16)
 1477 
 1478     png_get_tIME(png_ptr, info_ptr, &mod_time);
 1479 
 1480     mod_time       - time image was last modified
 1481                     (PNG_VALID_tIME)
 1482 
 1483     png_get_bKGD(png_ptr, info_ptr, &background);
 1484 
 1485     background     - background color (of type
 1486                      png_color_16p) (PNG_VALID_bKGD)
 1487                      valid 16-bit red, green and blue
 1488                      values, regardless of color_type
 1489 
 1490     num_comments   = png_get_text(png_ptr, info_ptr,
 1491                      &text_ptr, &num_text);
 1492 
 1493     num_comments   - number of comments
 1494 
 1495     text_ptr       - array of png_text holding image
 1496                      comments
 1497 
 1498     text_ptr[i].compression - type of compression used
 1499                  on "text" PNG_TEXT_COMPRESSION_NONE
 1500                            PNG_TEXT_COMPRESSION_zTXt
 1501                            PNG_ITXT_COMPRESSION_NONE
 1502                            PNG_ITXT_COMPRESSION_zTXt
 1503 
 1504     text_ptr[i].key   - keyword for comment.  Must contain
 1505                          1-79 characters.
 1506 
 1507     text_ptr[i].text  - text comments for current
 1508                          keyword.  Can be empty.
 1509 
 1510     text_ptr[i].text_length - length of text string,
 1511                  after decompression, 0 for iTXt
 1512 
 1513     text_ptr[i].itxt_length - length of itxt string,
 1514                  after decompression, 0 for tEXt/zTXt
 1515 
 1516     text_ptr[i].lang  - language of comment (empty
 1517                          string for unknown).
 1518 
 1519     text_ptr[i].lang_key  - keyword in UTF-8
 1520                          (empty string for unknown).
 1521 
 1522     Note that the itxt_length, lang, and lang_key
 1523     members of the text_ptr structure only exist when the
 1524     library is built with iTXt chunk support.  Prior to
 1525     libpng-1.4.0 the library was built by default without
 1526     iTXt support. Also note that when iTXt is supported,
 1527     they contain NULL pointers when the "compression"
 1528     field contains PNG_TEXT_COMPRESSION_NONE or
 1529     PNG_TEXT_COMPRESSION_zTXt.
 1530 
 1531     num_text       - number of comments (same as
 1532                      num_comments; you can put NULL here
 1533                      to avoid the duplication)
 1534 
 1535     Note while png_set_text() will accept text, language,
 1536     and translated keywords that can be NULL pointers, the
 1537     structure returned by png_get_text will always contain
 1538     regular zero-terminated C strings.  They might be
 1539     empty strings but they will never be NULL pointers.
 1540 
 1541     num_spalettes = png_get_sPLT(png_ptr, info_ptr,
 1542        &palette_ptr);
 1543 
 1544     num_spalettes  - number of sPLT chunks read.
 1545 
 1546     palette_ptr    - array of palette structures holding
 1547                      contents of one or more sPLT chunks
 1548                      read.
 1549 
 1550     png_get_oFFs(png_ptr, info_ptr, &offset_x, &offset_y,
 1551        &unit_type);
 1552 
 1553     offset_x       - positive offset from the left edge
 1554                      of the screen (can be negative)
 1555 
 1556     offset_y       - positive offset from the top edge
 1557                      of the screen (can be negative)
 1558 
 1559     unit_type      - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER
 1560 
 1561     png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y,
 1562        &unit_type);
 1563 
 1564     res_x          - pixels/unit physical resolution in
 1565                      x direction
 1566 
 1567     res_y          - pixels/unit physical resolution in
 1568                      x direction
 1569 
 1570     unit_type      - PNG_RESOLUTION_UNKNOWN,
 1571                      PNG_RESOLUTION_METER
 1572 
 1573     png_get_sCAL(png_ptr, info_ptr, &unit, &width,
 1574        &height)
 1575 
 1576     unit        - physical scale units (an integer)
 1577 
 1578     width       - width of a pixel in physical scale units
 1579 
 1580     height      - height of a pixel in physical scale units
 1581                  (width and height are doubles)
 1582 
 1583     png_get_sCAL_s(png_ptr, info_ptr, &unit, &width,
 1584        &height)
 1585 
 1586     unit        - physical scale units (an integer)
 1587 
 1588     width       - width of a pixel in physical scale units
 1589                   (expressed as a string)
 1590 
 1591     height      - height of a pixel in physical scale units
 1592                  (width and height are strings like "2.54")
 1593 
 1594     num_unknown_chunks = png_get_unknown_chunks(png_ptr,
 1595        info_ptr, &unknowns)
 1596 
 1597     unknowns          - array of png_unknown_chunk
 1598                         structures holding unknown chunks
 1599 
 1600     unknowns[i].name  - name of unknown chunk
 1601 
 1602     unknowns[i].data  - data of unknown chunk
 1603 
 1604     unknowns[i].size  - size of unknown chunk's data
 1605 
 1606     unknowns[i].location - position of chunk in file
 1607 
 1608     The value of "i" corresponds to the order in which the
 1609     chunks were read from the PNG file or inserted with the
 1610     png_set_unknown_chunks() function.
 1611 
 1612     The value of "location" is a bitwise "or" of
 1613 
 1614          PNG_HAVE_IHDR  (0x01)
 1615          PNG_HAVE_PLTE  (0x02)
 1616          PNG_AFTER_IDAT (0x08)
 1617 
 1618 The data from the pHYs chunk can be retrieved in several convenient
 1619 forms:
 1620 
 1621     res_x = png_get_x_pixels_per_meter(png_ptr,
 1622        info_ptr)
 1623 
 1624     res_y = png_get_y_pixels_per_meter(png_ptr,
 1625        info_ptr)
 1626 
 1627     res_x_and_y = png_get_pixels_per_meter(png_ptr,
 1628        info_ptr)
 1629 
 1630     res_x = png_get_x_pixels_per_inch(png_ptr,
 1631        info_ptr)
 1632 
 1633     res_y = png_get_y_pixels_per_inch(png_ptr,
 1634        info_ptr)
 1635 
 1636     res_x_and_y = png_get_pixels_per_inch(png_ptr,
 1637        info_ptr)
 1638 
 1639     aspect_ratio = png_get_pixel_aspect_ratio(png_ptr,
 1640        info_ptr)
 1641 
 1642     Each of these returns 0 [signifying "unknown"] if
 1643        the data is not present or if res_x is 0;
 1644        res_x_and_y is 0 if res_x != res_y
 1645 
 1646     Note that because of the way the resolutions are
 1647        stored internally, the inch conversions won't
 1648        come out to exactly even number.  For example,
 1649        72 dpi is stored as 0.28346 pixels/meter, and
 1650        when this is retrieved it is 71.9988 dpi, so
 1651        be sure to round the returned value appropriately
 1652        if you want to display a reasonable-looking result.
 1653 
 1654 The data from the oFFs chunk can be retrieved in several convenient
 1655 forms:
 1656 
 1657     x_offset = png_get_x_offset_microns(png_ptr, info_ptr);
 1658 
 1659     y_offset = png_get_y_offset_microns(png_ptr, info_ptr);
 1660 
 1661     x_offset = png_get_x_offset_inches(png_ptr, info_ptr);
 1662 
 1663     y_offset = png_get_y_offset_inches(png_ptr, info_ptr);
 1664 
 1665     Each of these returns 0 [signifying "unknown" if both
 1666        x and y are 0] if the data is not present or if the
 1667        chunk is present but the unit is the pixel.  The
 1668        remark about inexact inch conversions applies here
 1669        as well, because a value in inches can't always be
 1670        converted to microns and back without some loss
 1671        of precision.
 1672 
 1673 For more information, see the
 1674 PNG specification for chunk contents.  Be careful with trusting
 1675 rowbytes, as some of the transformations could increase the space
 1676 needed to hold a row (expand, filler, gray_to_rgb, etc.).
 1677 See png_read_update_info(), below.
 1678 
 1679 A quick word about text_ptr and num_text.  PNG stores comments in
 1680 keyword/text pairs, one pair per chunk, with no limit on the number
 1681 of text chunks, and a 2^31 byte limit on their size.  While there are
 1682 suggested keywords, there is no requirement to restrict the use to these
 1683 strings.  It is strongly suggested that keywords and text be sensible
 1684 to humans (that's the point), so don't use abbreviations.  Non-printing
 1685 symbols are not allowed.  See the PNG specification for more details.
 1686 There is also no requirement to have text after the keyword.
 1687 
 1688 Keywords should be limited to 79 Latin-1 characters without leading or
 1689 trailing spaces, but non-consecutive spaces are allowed within the
 1690 keyword.  It is possible to have the same keyword any number of times.
 1691 The text_ptr is an array of png_text structures, each holding a
 1692 pointer to a language string, a pointer to a keyword and a pointer to
 1693 a text string.  The text string, language code, and translated
 1694 keyword may be empty or NULL pointers.  The keyword/text
 1695 pairs are put into the array in the order that they are received.
 1696 However, some or all of the text chunks may be after the image, so, to
 1697 make sure you have read all the text chunks, don't mess with these
 1698 until after you read the stuff after the image.  This will be
 1699 mentioned again below in the discussion that goes with png_read_end().
 1700 
 1701 Input transformations
 1702 
 1703 After you've read the header information, you can set up the library
 1704 to handle any special transformations of the image data.  The various
 1705 ways to transform the data will be described in the order that they
 1706 should occur.  This is important, as some of these change the color
 1707 type and/or bit depth of the data, and some others only work on
 1708 certain color types and bit depths.
 1709 
 1710 Transformations you request are ignored if they don't have any meaning for a
 1711 particular input data format.  However some transformations can have an effect
 1712 as a result of a previous transformation.  If you specify a contradictory set of
 1713 transformations, for example both adding and removing the alpha channel, you
 1714 cannot predict the final result.
 1715 
 1716 The color used for the transparency values should be supplied in the same
 1717 format/depth as the current image data.  It is stored in the same format/depth
 1718 as the image data in a tRNS chunk, so this is what libpng expects for this data.
 1719 
 1720 The color used for the background value depends on the need_expand argument as
 1721 described below.
 1722 
 1723 Data will be decoded into the supplied row buffers packed into bytes
 1724 unless the library has been told to transform it into another format.
 1725 For example, 4 bit/pixel paletted or grayscale data will be returned
 1726 2 pixels/byte with the leftmost pixel in the high-order bits of the byte,
 1727 unless png_set_packing() is called.  8-bit RGB data will be stored
 1728 in RGB RGB RGB format unless png_set_filler() or png_set_add_alpha()
 1729 is called to insert filler bytes, either before or after each RGB triplet.
 1730 
 1731 16-bit RGB data will be returned RRGGBB RRGGBB, with the most significant
 1732 byte of the color value first, unless png_set_scale_16() is called to
 1733 transform it to regular RGB RGB triplets, or png_set_filler() or
 1734 png_set_add alpha() is called to insert two filler bytes, either before
 1735 or after each RRGGBB triplet.  Similarly, 8-bit or 16-bit grayscale data can
 1736 be modified with png_set_filler(), png_set_add_alpha(), png_set_strip_16(),
 1737 or png_set_scale_16().
 1738 
 1739 The following code transforms grayscale images of less than 8 to 8 bits,
 1740 changes paletted images to RGB, and adds a full alpha channel if there is
 1741 transparency information in a tRNS chunk.  This is most useful on
 1742 grayscale images with bit depths of 2 or 4 or if there is a multiple-image
 1743 viewing application that wishes to treat all images in the same way.
 1744 
 1745     if (color_type == PNG_COLOR_TYPE_PALETTE)
 1746         png_set_palette_to_rgb(png_ptr);
 1747 
 1748     if (png_get_valid(png_ptr, info_ptr,
 1749         PNG_INFO_tRNS)) png_set_tRNS_to_alpha(png_ptr);
 1750 
 1751     if (color_type == PNG_COLOR_TYPE_GRAY &&
 1752         bit_depth < 8) png_set_expand_gray_1_2_4_to_8(png_ptr);
 1753 
 1754 The first two functions are actually aliases for png_set_expand(), added
 1755 in libpng version 1.0.4, with the function names expanded to improve code
 1756 readability.  In some future version they may actually do different
 1757 things.
 1758 
 1759 As of libpng version 1.2.9, png_set_expand_gray_1_2_4_to_8() was
 1760 added.  It expands the sample depth without changing tRNS to alpha.
 1761 
 1762 As of libpng version 1.5.2, png_set_expand_16() was added.  It behaves as
 1763 png_set_expand(); however, the resultant channels have 16 bits rather than 8.
 1764 Use this when the output color or gray channels are made linear to avoid fairly
 1765 severe accuracy loss.
 1766 
 1767    if (bit_depth < 16)
 1768       png_set_expand_16(png_ptr);
 1769 
 1770 PNG can have files with 16 bits per channel.  If you only can handle
 1771 8 bits per channel, this will strip the pixels down to 8-bit.
 1772 
 1773     if (bit_depth == 16)
 1774 #if PNG_LIBPNG_VER >= 10504
 1775        png_set_scale_16(png_ptr);
 1776 #else
 1777        png_set_strip_16(png_ptr);
 1778 #endif
 1779 
 1780 (The more accurate "png_set_scale_16()" API became available in libpng version
 1781 1.5.4).
 1782 
 1783 If you need to process the alpha channel on the image separately from the image
 1784 data (for example if you convert it to a bitmap mask) it is possible to have
 1785 libpng strip the channel leaving just RGB or gray data:
 1786 
 1787     if (color_type & PNG_COLOR_MASK_ALPHA)
 1788        png_set_strip_alpha(png_ptr);
 1789 
 1790 If you strip the alpha channel you need to find some other way of dealing with
 1791 the information.  If, instead, you want to convert the image to an opaque
 1792 version with no alpha channel use png_set_background; see below.
 1793 
 1794 As of libpng version 1.5.2, almost all useful expansions are supported, the
 1795 major ommissions are conversion of grayscale to indexed images (which can be
 1796 done trivially in the application) and conversion of indexed to grayscale (which
 1797 can be done by a trivial manipulation of the palette.)
 1798 
 1799 In the following table, the 01 means grayscale with depth<8, 31 means
 1800 indexed with depth<8, other numerals represent the color type, "T" means
 1801 the tRNS chunk is present, A means an alpha channel is present, and O
 1802 means tRNS or alpha is present but all pixels in the image are opaque.
 1803 
 1804   FROM  01  31   0  0T  0O   2  2T  2O   3  3T  3O  4A  4O  6A  6O
 1805    TO
 1806    01    -  [G]  -   -   -   -   -   -   -   -   -   -   -   -   -
 1807    31   [Q]  Q  [Q] [Q] [Q]  Q   Q   Q   Q   Q   Q  [Q] [Q]  Q   Q
 1808     0    1   G   +   .   .   G   G   G   G   G   G   B   B  GB  GB
 1809    0T    lt  Gt  t   +   .   Gt  G   G   Gt  G   G   Bt  Bt GBt GBt
 1810    0O    lt  Gt  t   .   +   Gt  Gt  G   Gt  Gt  G   Bt  Bt GBt GBt
 1811     2    C   P   C   C   C   +   .   .   C   -   -  CB  CB   B   B
 1812    2T    Ct  -   Ct  C   C   t   +   t   -   -   -  CBt CBt  Bt  Bt
 1813    2O    Ct  -   Ct  C   C   t   t   +   -   -   -  CBt CBt  Bt  Bt
 1814     3   [Q]  p  [Q] [Q] [Q]  Q   Q   Q   +   .   .  [Q] [Q]  Q   Q
 1815    3T   [Qt] p  [Qt][Q] [Q]  Qt  Qt  Qt  t   +   t  [Qt][Qt] Qt  Qt
 1816    3O   [Qt] p  [Qt][Q] [Q]  Qt  Qt  Qt  t   t   +  [Qt][Qt] Qt  Qt
 1817    4A    lA  G   A   T   T   GA  GT  GT  GA  GT  GT  +   BA  G  GBA
 1818    4O    lA GBA  A   T   T   GA  GT  GT  GA  GT  GT  BA  +  GBA  G
 1819    6A    CA  PA  CA  C   C   A   T  tT   PA  P   P   C  CBA  +   BA
 1820    6O    CA PBA  CA  C   C   A  tT   T   PA  P   P  CBA  C   BA  +
 1821 
 1822 Within the matrix,
 1823      "+" identifies entries where 'from' and 'to' are the same.
 1824      "-" means the transformation is not supported.
 1825      "." means nothing is necessary (a tRNS chunk can just be ignored).
 1826      "t" means the transformation is obtained by png_set_tRNS.
 1827      "A" means the transformation is obtained by png_set_add_alpha().
 1828      "X" means the transformation is obtained by png_set_expand().
 1829      "1" means the transformation is obtained by
 1830          png_set_expand_gray_1_2_4_to_8() (and by png_set_expand()
 1831          if there is no transparency in the original or the final
 1832          format).
 1833      "C" means the transformation is obtained by png_set_gray_to_rgb().
 1834      "G" means the transformation is obtained by png_set_rgb_to_gray().
 1835      "P" means the transformation is obtained by
 1836          png_set_expand_palette_to_rgb().
 1837      "p" means the transformation is obtained by png_set_packing().
 1838      "Q" means the transformation is obtained by png_set_quantize().
 1839      "T" means the transformation is obtained by
 1840          png_set_tRNS_to_alpha().
 1841      "B" means the transformation is obtained by
 1842          png_set_background(), or png_strip_alpha().
 1843 
 1844 When an entry has multiple transforms listed all are required to cause the
 1845 right overall transformation.  When two transforms are separated by a comma
 1846 either will do the job.  When transforms are enclosed in [] the transform should
 1847 do the job but this is currently unimplemented - a different format will result
 1848 if the suggested transformations are used.
 1849 
 1850 In PNG files, the alpha channel in an image
 1851 is the level of opacity.  If you need the alpha channel in an image to
 1852 be the level of transparency instead of opacity, you can invert the
 1853 alpha channel (or the tRNS chunk data) after it's read, so that 0 is
 1854 fully opaque and 255 (in 8-bit or paletted images) or 65535 (in 16-bit
 1855 images) is fully transparent, with
 1856 
 1857     png_set_invert_alpha(png_ptr);
 1858 
 1859 PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as
 1860 they can, resulting in, for example, 8 pixels per byte for 1 bit
 1861 files.  This code expands to 1 pixel per byte without changing the
 1862 values of the pixels:
 1863 
 1864     if (bit_depth < 8)
 1865        png_set_packing(png_ptr);
 1866 
 1867 PNG files have possible bit depths of 1, 2, 4, 8, and 16.  All pixels
 1868 stored in a PNG image have been "scaled" or "shifted" up to the next
 1869 higher possible bit depth (e.g. from 5 bits/sample in the range [0,31]
 1870 to 8 bits/sample in the range [0, 255]).  However, it is also possible
 1871 to convert the PNG pixel data back to the original bit depth of the
 1872 image.  This call reduces the pixels back down to the original bit depth:
 1873 
 1874     png_color_8p sig_bit;
 1875 
 1876     if (png_get_sBIT(png_ptr, info_ptr, &sig_bit))
 1877        png_set_shift(png_ptr, sig_bit);
 1878 
 1879 PNG files store 3-color pixels in red, green, blue order.  This code
 1880 changes the storage of the pixels to blue, green, red:
 1881 
 1882     if (color_type == PNG_COLOR_TYPE_RGB ||
 1883         color_type == PNG_COLOR_TYPE_RGB_ALPHA)
 1884        png_set_bgr(png_ptr);
 1885 
 1886 PNG files store RGB pixels packed into 3 or 6 bytes. This code expands them
 1887 into 4 or 8 bytes for windowing systems that need them in this format:
 1888 
 1889     if (color_type == PNG_COLOR_TYPE_RGB)
 1890        png_set_filler(png_ptr, filler, PNG_FILLER_BEFORE);
 1891 
 1892 where "filler" is the 8-bit or 16-bit number to fill with, and the location
 1893 is either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether
 1894 you want the filler before the RGB or after. When filling an 8-bit pixel,
 1895 the least significant 8 bits of the number are used, if a 16-bit number is
 1896 supplied.  This transformation does not affect images that already have full
 1897 alpha channels.  To add an opaque alpha channel, use filler=0xffff and
 1898 PNG_FILLER_AFTER which will generate RGBA pixels.
 1899 
 1900 Note that png_set_filler() does not change the color type.  If you want
 1901 to do that, you can add a true alpha channel with
 1902 
 1903     if (color_type == PNG_COLOR_TYPE_RGB ||
 1904        color_type == PNG_COLOR_TYPE_GRAY)
 1905        png_set_add_alpha(png_ptr, filler, PNG_FILLER_AFTER);
 1906 
 1907 where "filler" contains the alpha value to assign to each pixel.
 1908 The png_set_add_alpha() function was added in libpng-1.2.7.
 1909 
 1910 If you are reading an image with an alpha channel, and you need the
 1911 data as ARGB instead of the normal PNG format RGBA:
 1912 
 1913     if (color_type == PNG_COLOR_TYPE_RGB_ALPHA)
 1914        png_set_swap_alpha(png_ptr);
 1915 
 1916 For some uses, you may want a grayscale image to be represented as
 1917 RGB.  This code will do that conversion:
 1918 
 1919     if (color_type == PNG_COLOR_TYPE_GRAY ||
 1920         color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
 1921        png_set_gray_to_rgb(png_ptr);
 1922 
 1923 Conversely, you can convert an RGB or RGBA image to grayscale or grayscale
 1924 with alpha.
 1925 
 1926     if (color_type == PNG_COLOR_TYPE_RGB ||
 1927         color_type == PNG_COLOR_TYPE_RGB_ALPHA)
 1928        png_set_rgb_to_gray(png_ptr, error_action,
 1929           double red_weight, double green_weight);
 1930 
 1931     error_action = 1: silently do the conversion
 1932 
 1933     error_action = 2: issue a warning if the original
 1934                       image has any pixel where
 1935                       red != green or red != blue
 1936 
 1937     error_action = 3: issue an error and abort the
 1938                       conversion if the original
 1939                       image has any pixel where
 1940                       red != green or red != blue
 1941 
 1942     red_weight:       weight of red component
 1943 
 1944     green_weight:     weight of green component
 1945                       If either weight is negative, default
 1946                       weights are used.
 1947 
 1948 In the corresponding fixed point API the red_weight and green_weight values are
 1949 simply scaled by 100,000:
 1950 
 1951     png_set_rgb_to_gray(png_ptr, error_action,
 1952        png_fixed_point red_weight,
 1953        png_fixed_point green_weight);
 1954 
 1955 If you have set error_action = 1 or 2, you can
 1956 later check whether the image really was gray, after processing
 1957 the image rows, with the png_get_rgb_to_gray_status(png_ptr) function.
 1958 It will return a png_byte that is zero if the image was gray or
 1959 1 if there were any non-gray pixels.  Background and sBIT data
 1960 will be silently converted to grayscale, using the green channel
 1961 data for sBIT, regardless of the error_action setting.
 1962 
 1963 The default values come from the PNG file cHRM chunk if present; otherwise, the
 1964 defaults correspond to the ITU-R recommendation 709, and also the sRGB color
 1965 space, as recommended in the Charles Poynton's Colour FAQ,
 1966 Copyright (c) 2006-11-28 Charles Poynton, in section 9:
 1967 
 1968 <http://www.poynton.com/notes/colour_and_gamma/ColorFAQ.html#RTFToC9>
 1969 
 1970     Y = 0.2126 * R + 0.7152 * G + 0.0722 * B
 1971 
 1972 Previous versions of this document, 1998 through 2002, recommended a slightly
 1973 different formula:
 1974 
 1975     Y = 0.212671 * R + 0.715160 * G + 0.072169 * B
 1976 
 1977 Libpng uses an integer approximation:
 1978 
 1979     Y = (6968 * R + 23434 * G + 2366 * B)/32768
 1980 
 1981 The calculation is done in a linear colorspace, if the image gamma
 1982 can be determined.
 1983 
 1984 The png_set_background() function has been described already; it tells libpng to
 1985 composite images with alpha or simple transparency against the supplied
 1986 background color.  For compatibility with versions of libpng earlier than
 1987 libpng-1.5.4 it is recommended that you call the function after reading the file
 1988 header, even if you don't want to use the color in a bKGD chunk, if one exists.
 1989 
 1990 If the PNG file contains a bKGD chunk (PNG_INFO_bKGD valid),
 1991 you may use this color, or supply another color more suitable for
 1992 the current display (e.g., the background color from a web page).  You
 1993 need to tell libpng how the color is represented, both the format of the
 1994 component values in the color (the number of bits) and the gamma encoding of the
 1995 color.  The function takes two arguments, background_gamma_mode and need_expand
 1996 to convey this information; however, only two combinations are likely to be
 1997 useful:
 1998 
 1999     png_color_16 my_background;
 2000     png_color_16p image_background;
 2001 
 2002     if (png_get_bKGD(png_ptr, info_ptr, &image_background))
 2003        png_set_background(png_ptr, image_background,
 2004            PNG_BACKGROUND_GAMMA_FILE, 1/*needs to be expanded*/, 1);
 2005     else
 2006        png_set_background(png_ptr, &my_background,
 2007            PNG_BACKGROUND_GAMMA_SCREEN, 0/*do not expand*/, 1);
 2008 
 2009 The second call was described above - my_background is in the format of the
 2010 final, display, output produced by libpng.  Because you now know the format of
 2011 the PNG it is possible to avoid the need to choose either 8-bit or 16-bit
 2012 output and to retain palette images (the palette colors will be modified
 2013 appropriately and the tRNS chunk removed.)  However, if you are doing this,
 2014 take great care not to ask for transformations without checking first that
 2015 they apply!
 2016 
 2017 In the first call the background color has the original bit depth and color type
 2018 of the PNG file.  So, for palette images the color is supplied as a palette
 2019 index and for low bit greyscale images the color is a reduced bit value in
 2020 image_background->gray.
 2021 
 2022 If you didn't call png_set_gamma() before reading the file header, for example
 2023 if you need your code to remain compatible with older versions of libpng prior
 2024 to libpng-1.5.4, this is the place to call it.
 2025 
 2026 Do not call it if you called png_set_alpha_mode(); doing so will damage the
 2027 settings put in place by png_set_alpha_mode().  (If png_set_alpha_mode() is
 2028 supported then you can certainly do png_set_gamma() before reading the PNG
 2029 header.)
 2030 
 2031 This API unconditionally sets the screen and file gamma values, so it will
 2032 override the value in the PNG file unless it is called before the PNG file
 2033 reading starts.  For this reason you must always call it with the PNG file
 2034 value when you call it in this position:
 2035 
 2036    if (png_get_gAMA(png_ptr, info_ptr, &file_gamma))
 2037       png_set_gamma(png_ptr, screen_gamma, file_gamma);
 2038 
 2039    else
 2040       png_set_gamma(png_ptr, screen_gamma, 0.45455);
 2041 
 2042 If you need to reduce an RGB file to a paletted file, or if a paletted
 2043 file has more entries than will fit on your screen, png_set_quantize()
 2044 will do that.  Note that this is a simple match quantization that merely
 2045 finds the closest color available.  This should work fairly well with
 2046 optimized palettes, but fairly badly with linear color cubes.  If you
 2047 pass a palette that is larger than maximum_colors, the file will
 2048 reduce the number of colors in the palette so it will fit into
 2049 maximum_colors.  If there is a histogram, libpng will use it to make
 2050 more intelligent choices when reducing the palette.  If there is no
 2051 histogram, it may not do as good a job.
 2052 
 2053    if (color_type & PNG_COLOR_MASK_COLOR)
 2054    {
 2055       if (png_get_valid(png_ptr, info_ptr,
 2056           PNG_INFO_PLTE))
 2057       {
 2058          png_uint_16p histogram = NULL;
 2059 
 2060          png_get_hIST(png_ptr, info_ptr,
 2061              &histogram);
 2062          png_set_quantize(png_ptr, palette, num_palette,
 2063             max_screen_colors, histogram, 1);
 2064       }
 2065 
 2066       else
 2067       {
 2068          png_color std_color_cube[MAX_SCREEN_COLORS] =
 2069             { ... colors ... };
 2070 
 2071          png_set_quantize(png_ptr, std_color_cube,
 2072             MAX_SCREEN_COLORS, MAX_SCREEN_COLORS,
 2073             NULL,0);
 2074       }
 2075    }
 2076 
 2077 PNG files describe monochrome as black being zero and white being one.
 2078 The following code will reverse this (make black be one and white be
 2079 zero):
 2080 
 2081    if (bit_depth == 1 && color_type == PNG_COLOR_TYPE_GRAY)
 2082       png_set_invert_mono(png_ptr);
 2083 
 2084 This function can also be used to invert grayscale and gray-alpha images:
 2085 
 2086    if (color_type == PNG_COLOR_TYPE_GRAY ||
 2087        color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
 2088       png_set_invert_mono(png_ptr);
 2089 
 2090 PNG files store 16-bit pixels in network byte order (big-endian,
 2091 ie. most significant bits first).  This code changes the storage to the
 2092 other way (little-endian, i.e. least significant bits first, the
 2093 way PCs store them):
 2094 
 2095     if (bit_depth == 16)
 2096        png_set_swap(png_ptr);
 2097 
 2098 If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you
 2099 need to change the order the pixels are packed into bytes, you can use:
 2100 
 2101     if (bit_depth < 8)
 2102        png_set_packswap(png_ptr);
 2103 
 2104 Finally, you can write your own transformation function if none of
 2105 the existing ones meets your needs.  This is done by setting a callback
 2106 with
 2107 
 2108     png_set_read_user_transform_fn(png_ptr,
 2109         read_transform_fn);
 2110 
 2111 You must supply the function
 2112 
 2113     void read_transform_fn(png_structp png_ptr, png_row_infop
 2114         row_info, png_bytep data)
 2115 
 2116 See pngtest.c for a working example.  Your function will be called
 2117 after all of the other transformations have been processed.  Take care with
 2118 interlaced images if you do the interlace yourself - the width of the row is the
 2119 width in 'row_info', not the overall image width.
 2120 
 2121 If supported, libpng provides two information routines that you can use to find
 2122 where you are in processing the image:
 2123 
 2124    png_get_current_pass_number(png_structp png_ptr);
 2125    png_get_current_row_number(png_structp png_ptr);
 2126 
 2127 Don't try using these outside a transform callback - firstly they are only
 2128 supported if user transforms are supported, secondly they may well return
 2129 unexpected results unless the row is actually being processed at the moment they
 2130 are called.
 2131 
 2132 With interlaced
 2133 images the value returned is the row in the input sub-image image.  Use
 2134 PNG_ROW_FROM_PASS_ROW(row, pass) and PNG_COL_FROM_PASS_COL(col, pass) to
 2135 find the output pixel (x,y) given an interlaced sub-image pixel (row,col,pass).
 2136 
 2137 The discussion of interlace handling above contains more information on how to
 2138 use these values.
 2139 
 2140 You can also set up a pointer to a user structure for use by your
 2141 callback function, and you can inform libpng that your transform
 2142 function will change the number of channels or bit depth with the
 2143 function
 2144 
 2145     png_set_user_transform_info(png_ptr, user_ptr,
 2146         user_depth, user_channels);
 2147 
 2148 The user's application, not libpng, is responsible for allocating and
 2149 freeing any memory required for the user structure.
 2150 
 2151 You can retrieve the pointer via the function
 2152 png_get_user_transform_ptr().  For example:
 2153 
 2154     voidp read_user_transform_ptr =
 2155         png_get_user_transform_ptr(png_ptr);
 2156 
 2157 The last thing to handle is interlacing; this is covered in detail below,
 2158 but you must call the function here if you want libpng to handle expansion
 2159 of the interlaced image.
 2160 
 2161     number_of_passes = png_set_interlace_handling(png_ptr);
 2162 
 2163 After setting the transformations, libpng can update your png_info
 2164 structure to reflect any transformations you've requested with this
 2165 call.
 2166 
 2167     png_read_update_info(png_ptr, info_ptr);
 2168 
 2169 This is most useful to update the info structure's rowbytes
 2170 field so you can use it to allocate your image memory.  This function
 2171 will also update your palette with the correct screen_gamma and
 2172 background if these have been given with the calls above.  You may
 2173 only call png_read_update_info() once with a particular info_ptr.
 2174 
 2175 After you call png_read_update_info(), you can allocate any
 2176 memory you need to hold the image.  The row data is simply
 2177 raw byte data for all forms of images.  As the actual allocation
 2178 varies among applications, no example will be given.  If you
 2179 are allocating one large chunk, you will need to build an
 2180 array of pointers to each row, as it will be needed for some
 2181 of the functions below.
 2182 
 2183 Be sure that your platform can allocate the buffer that you'll need.
 2184 libpng internally checks for oversize width, but you'll need to
 2185 do your own check for number_of_rows*width*pixel_size if you are using
 2186 a multiple-row buffer:
 2187 
 2188    /* Guard against integer overflow */
 2189    if (number_of_rows > PNG_SIZE_MAX/(width*pixel_size)) {
 2190         png_error(png_ptr,"image_data buffer would be too large");
 2191    }
 2192 
 2193 Remember: Before you call png_read_update_info(), the png_get_*()
 2194 functions return the values corresponding to the original PNG image.
 2195 After you call png_read_update_info the values refer to the image
 2196 that libpng will output.  Consequently you must call all the png_set_
 2197 functions before you call png_read_update_info().  This is particularly
 2198 important for png_set_interlace_handling() - if you are going to call
 2199 png_read_update_info() you must call png_set_interlace_handling() before
 2200 it unless you want to receive interlaced output.
 2201 
 2202 Reading image data
 2203 
 2204 After you've allocated memory, you can read the image data.
 2205 The simplest way to do this is in one function call.  If you are
 2206 allocating enough memory to hold the whole image, you can just
 2207 call png_read_image() and libpng will read in all the image data
 2208 and put it in the memory area supplied.  You will need to pass in
 2209 an array of pointers to each row.
 2210 
 2211 This function automatically handles interlacing, so you don't
 2212 need to call png_set_interlace_handling() (unless you call
 2213 png_read_update_info()) or call this function multiple times, or any
 2214 of that other stuff necessary with png_read_rows().
 2215 
 2216    png_read_image(png_ptr, row_pointers);
 2217 
 2218 where row_pointers is:
 2219 
 2220    png_bytep row_pointers[height];
 2221 
 2222 You can point to void or char or whatever you use for pixels.
 2223 
 2224 If you don't want to read in the whole image at once, you can
 2225 use png_read_rows() instead.  If there is no interlacing (check
 2226 interlace_type == PNG_INTERLACE_NONE), this is simple:
 2227 
 2228     png_read_rows(png_ptr, row_pointers, NULL,
 2229         number_of_rows);
 2230 
 2231 where row_pointers is the same as in the png_read_image() call.
 2232 
 2233 If you are doing this just one row at a time, you can do this with
 2234 a single row_pointer instead of an array of row_pointers:
 2235 
 2236     png_bytep row_pointer = row;
 2237     png_read_row(png_ptr, row_pointer, NULL);
 2238 
 2239 If the file is interlaced (interlace_type != 0 in the IHDR chunk), things
 2240 get somewhat harder.  The only current (PNG Specification version 1.2)
 2241 interlacing type for PNG is (interlace_type == PNG_INTERLACE_ADAM7);
 2242 a somewhat complicated 2D interlace scheme, known as Adam7, that
 2243 breaks down an image into seven smaller images of varying size, based
 2244 on an 8x8 grid.  This number is defined (from libpng 1.5) as
 2245 PNG_INTERLACE_ADAM7_PASSES in png.h
 2246 
 2247 libpng can fill out those images or it can give them to you "as is".
 2248 It is almost always better to have libpng handle the interlacing for you.
 2249 If you want the images filled out, there are two ways to do that.  The one
 2250 mentioned in the PNG specification is to expand each pixel to cover
 2251 those pixels that have not been read yet (the "rectangle" method).
 2252 This results in a blocky image for the first pass, which gradually
 2253 smooths out as more pixels are read.  The other method is the "sparkle"
 2254 method, where pixels are drawn only in their final locations, with the
 2255 rest of the image remaining whatever colors they were initialized to
 2256 before the start of the read.  The first method usually looks better,
 2257 but tends to be slower, as there are more pixels to put in the rows.
 2258 
 2259 If, as is likely, you want libpng to expand the images, call this before
 2260 calling png_start_read_image() or png_read_update_info():
 2261 
 2262     if (interlace_type == PNG_INTERLACE_ADAM7)
 2263        number_of_passes
 2264            = png_set_interlace_handling(png_ptr);
 2265 
 2266 This will return the number of passes needed.  Currently, this is seven,
 2267 but may change if another interlace type is added.  This function can be
 2268 called even if the file is not interlaced, where it will return one pass.
 2269 You then need to read the whole image 'number_of_passes' times.  Each time
 2270 will distribute the pixels from the current pass to the correct place in
 2271 the output image, so you need to supply the same rows to png_read_rows in
 2272 each pass.
 2273 
 2274 If you are not going to display the image after each pass, but are
 2275 going to wait until the entire image is read in, use the sparkle
 2276 effect.  This effect is faster and the end result of either method
 2277 is exactly the same.  If you are planning on displaying the image
 2278 after each pass, the "rectangle" effect is generally considered the
 2279 better looking one.
 2280 
 2281 If you only want the "sparkle" effect, just call png_read_row() or
 2282 png_read_rows() as
 2283 normal, with the third parameter NULL.  Make sure you make pass over
 2284 the image number_of_passes times, and you don't change the data in the
 2285 rows between calls.  You can change the locations of the data, just
 2286 not the data.  Each pass only writes the pixels appropriate for that
 2287 pass, and assumes the data from previous passes is still valid.
 2288 
 2289     png_read_rows(png_ptr, row_pointers, NULL,
 2290         number_of_rows);
 2291     or
 2292     png_read_row(png_ptr, row_pointers, NULL);
 2293 
 2294 If you only want the first effect (the rectangles), do the same as
 2295 before except pass the row buffer in the third parameter, and leave
 2296 the second parameter NULL.
 2297 
 2298     png_read_rows(png_ptr, NULL, row_pointers,
 2299         number_of_rows);
 2300     or
 2301     png_read_row(png_ptr, NULL, row_pointers);
 2302 
 2303 If you don't want libpng to handle the interlacing details, just call
 2304 png_read_rows() PNG_INTERLACE_ADAM7_PASSES times to read in all the images.
 2305 Each of the images is a valid image by itself; however, you will almost
 2306 certainly need to distribute the pixels from each sub-image to the
 2307 correct place.  This is where everything gets very tricky.
 2308 
 2309 If you want to retrieve the separate images you must pass the correct
 2310 number of rows to each successive call of png_read_rows().  The calculation
 2311 gets pretty complicated for small images, where some sub-images may
 2312 not even exist because either their width or height ends up zero.
 2313 libpng provides two macros to help you in 1.5 and later versions:
 2314 
 2315    png_uint_32 width = PNG_PASS_COLS(image_width, pass_number);
 2316    png_uint_32 height = PNG_PASS_ROWS(image_height, pass_number);
 2317 
 2318 Respectively these tell you the width and height of the sub-image
 2319 corresponding to the numbered pass.  'pass' is in in the range 0 to 6 -
 2320 this can be confusing because the specification refers to the same passes
 2321 as 1 to 7!  Be careful, you must check both the width and height before
 2322 calling png_read_rows() and not call it for that pass if either is zero.
 2323 
 2324 You can, of course, read each sub-image row by row.  If you want to
 2325 produce optimal code to make a pixel-by-pixel transformation of an
 2326 interlaced image this is the best approach; read each row of each pass,
 2327 transform it, and write it out to a new interlaced image.
 2328 
 2329 If you want to de-interlace the image yourself libpng provides further
 2330 macros to help that tell you where to place the pixels in the output image.
 2331 Because the interlacing scheme is rectangular - sub-image pixels are always
 2332 arranged on a rectangular grid - all you need to know for each pass is the
 2333 starting column and row in the output image of the first pixel plus the
 2334 spacing between each pixel.  As of libpng 1.5 there are four macros to
 2335 retrieve this information:
 2336 
 2337    png_uint_32 x = PNG_PASS_START_COL(pass);
 2338    png_uint_32 y = PNG_PASS_START_ROW(pass);
 2339    png_uint_32 xStep = 1U << PNG_PASS_COL_SHIFT(pass);
 2340    png_uint_32 yStep = 1U << PNG_PASS_ROW_SHIFT(pass);
 2341 
 2342 These allow you to write the obvious loop:
 2343 
 2344    png_uint_32 input_y = 0;
 2345    png_uint_32 output_y = PNG_PASS_START_ROW(pass);
 2346 
 2347    while (output_y < output_image_height)
 2348    {
 2349       png_uint_32 input_x = 0;
 2350       png_uint_32 output_x = PNG_PASS_START_COL(pass);
 2351 
 2352       while (output_x < output_image_width)
 2353       {
 2354          image[output_y][output_x] =
 2355              subimage[pass][input_y][input_x++];
 2356 
 2357          output_x += xStep;
 2358       }
 2359 
 2360       ++input_y;
 2361       output_y += yStep;
 2362    }
 2363 
 2364 Notice that the steps between successive output rows and columns are
 2365 returned as shifts.  This is possible because the pixels in the subimages
 2366 are always a power of 2 apart - 1, 2, 4 or 8 pixels - in the original
 2367 image.  In practice you may need to directly calculate the output coordinate
 2368 given an input coordinate.  libpng provides two further macros for this
 2369 purpose:
 2370 
 2371    png_uint_32 output_x = PNG_COL_FROM_PASS_COL(input_x, pass);
 2372    png_uint_32 output_y = PNG_ROW_FROM_PASS_ROW(input_y, pass);
 2373 
 2374 Finally a pair of macros are provided to tell you if a particular image
 2375 row or column appears in a given pass:
 2376 
 2377    int col_in_pass = PNG_COL_IN_INTERLACE_PASS(output_x, pass);
 2378    int row_in_pass = PNG_ROW_IN_INTERLACE_PASS(output_y, pass);
 2379 
 2380 Bear in mind that you will probably also need to check the width and height
 2381 of the pass in addition to the above to be sure the pass even exists!
 2382 
 2383 With any luck you are convinced by now that you don't want to do your own
 2384 interlace handling.  In reality normally the only good reason for doing this
 2385 is if you are processing PNG files on a pixel-by-pixel basis and don't want
 2386 to load the whole file into memory when it is interlaced.
 2387 
 2388 libpng includes a test program, pngvalid, that illustrates reading and
 2389 writing of interlaced images.  If you can't get interlacing to work in your
 2390 code and don't want to leave it to libpng (the recommended approach), see
 2391 how pngvalid.c does it.
 2392 
 2393 Finishing a sequential read
 2394 
 2395 After you are finished reading the image through the
 2396 low-level interface, you can finish reading the file.
 2397 
 2398 If you want to use a different crc action for handling CRC errors in
 2399 chunks after the image data, you can call png_set_crc_action()
 2400 again at this point.
 2401 
 2402 If you are interested in comments or time, which may be stored either
 2403 before or after the image data, you should pass the separate png_info
 2404 struct if you want to keep the comments from before and after the image
 2405 separate.
 2406 
 2407     png_infop end_info = png_create_info_struct(png_ptr);
 2408 
 2409     if (!end_info)
 2410     {
 2411        png_destroy_read_struct(&png_ptr, &info_ptr,
 2412            (png_infopp)NULL);
 2413        return ERROR;
 2414     }
 2415 
 2416    png_read_end(png_ptr, end_info);
 2417 
 2418 If you are not interested, you should still call png_read_end()
 2419 but you can pass NULL, avoiding the need to create an end_info structure.
 2420 If you do this, libpng will not process any chunks after IDAT other than
 2421 skipping over them and perhaps (depending on whether you have called
 2422 png_set_crc_action) checking their CRCs while looking for the IEND chunk.
 2423 
 2424    png_read_end(png_ptr, (png_infop)NULL);
 2425 
 2426 If you don't call png_read_end(), then your file pointer will be
 2427 left pointing to the first chunk after the last IDAT, which is probably
 2428 not what you want if you expect to read something beyond the end of
 2429 the PNG datastream.
 2430 
 2431 When you are done, you can free all memory allocated by libpng like this:
 2432 
 2433    png_destroy_read_struct(&png_ptr, &info_ptr,
 2434        &end_info);
 2435 
 2436 or, if you didn't create an end_info structure,
 2437 
 2438    png_destroy_read_struct(&png_ptr, &info_ptr,
 2439        (png_infopp)NULL);
 2440 
 2441 It is also possible to individually free the info_ptr members that
 2442 point to libpng-allocated storage with the following function:
 2443 
 2444     png_free_data(png_ptr, info_ptr, mask, seq)
 2445 
 2446     mask - identifies data to be freed, a mask
 2447            containing the bitwise OR of one or
 2448            more of
 2449              PNG_FREE_PLTE, PNG_FREE_TRNS,
 2450              PNG_FREE_HIST, PNG_FREE_ICCP,
 2451              PNG_FREE_PCAL, PNG_FREE_ROWS,
 2452              PNG_FREE_SCAL, PNG_FREE_SPLT,
 2453              PNG_FREE_TEXT, PNG_FREE_UNKN,
 2454            or simply PNG_FREE_ALL
 2455 
 2456     seq  - sequence number of item to be freed
 2457            (-1 for all items)
 2458 
 2459 This function may be safely called when the relevant storage has
 2460 already been freed, or has not yet been allocated, or was allocated
 2461 by the user and not by libpng,  and will in those cases do nothing.
 2462 The "seq" parameter is ignored if only one item of the selected data
 2463 type, such as PLTE, is allowed.  If "seq" is not -1, and multiple items
 2464 are allowed for the data type identified in the mask, such as text or
 2465 sPLT, only the n'th item in the structure is freed, where n is "seq".
 2466 
 2467 The default behavior is only to free data that was allocated internally
 2468 by libpng.  This can be changed, so that libpng will not free the data,
 2469 or so that it will free data that was allocated by the user with png_malloc()
 2470 or png_calloc() and passed in via a png_set_*() function, with
 2471 
 2472     png_data_freer(png_ptr, info_ptr, freer, mask)
 2473 
 2474     freer  - one of
 2475                PNG_DESTROY_WILL_FREE_DATA
 2476                PNG_SET_WILL_FREE_DATA
 2477                PNG_USER_WILL_FREE_DATA
 2478 
 2479     mask   - which data elements are affected
 2480              same choices as in png_free_data()
 2481 
 2482 This function only affects data that has already been allocated.
 2483 You can call this function after reading the PNG data but before calling
 2484 any png_set_*() functions, to control whether the user or the png_set_*()
 2485 function is responsible for freeing any existing data that might be present,
 2486 and again after the png_set_*() functions to control whether the user
 2487 or png_destroy_*() is supposed to free the data.  When the user assumes
 2488 responsibility for libpng-allocated data, the application must use
 2489 png_free() to free it, and when the user transfers responsibility to libpng
 2490 for data that the user has allocated, the user must have used png_malloc()
 2491 or png_calloc() to allocate it.
 2492 
 2493 If you allocated your row_pointers in a single block, as suggested above in
 2494 the description of the high level read interface, you must not transfer
 2495 responsibility for freeing it to the png_set_rows or png_read_destroy function,
 2496 because they would also try to free the individual row_pointers[i].
 2497 
 2498 If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword
 2499 separately, do not transfer responsibility for freeing text_ptr to libpng,
 2500 because when libpng fills a png_text structure it combines these members with
 2501 the key member, and png_free_data() will free only text_ptr.key.  Similarly,
 2502 if you transfer responsibility for free'ing text_ptr from libpng to your
 2503 application, your application must not separately free those members.
 2504 
 2505 The png_free_data() function will turn off the "valid" flag for anything
 2506 it frees.  If you need to turn the flag off for a chunk that was freed by
 2507 your application instead of by libpng, you can use
 2508 
 2509     png_set_invalid(png_ptr, info_ptr, mask);
 2510 
 2511     mask - identifies the chunks to be made invalid,
 2512            containing the bitwise OR of one or
 2513            more of
 2514              PNG_INFO_gAMA, PNG_INFO_sBIT,
 2515              PNG_INFO_cHRM, PNG_INFO_PLTE,
 2516              PNG_INFO_tRNS, PNG_INFO_bKGD,
 2517              PNG_INFO_eXIf,
 2518              PNG_INFO_hIST, PNG_INFO_pHYs,
 2519              PNG_INFO_oFFs, PNG_INFO_tIME,
 2520              PNG_INFO_pCAL, PNG_INFO_sRGB,
 2521              PNG_INFO_iCCP, PNG_INFO_sPLT,
 2522              PNG_INFO_sCAL, PNG_INFO_IDAT
 2523 
 2524 For a more compact example of reading a PNG image, see the file example.c.
 2525 
 2526 Reading PNG files progressively
 2527 
 2528 The progressive reader is slightly different from the non-progressive
 2529 reader.  Instead of calling png_read_info(), png_read_rows(), and
 2530 png_read_end(), you make one call to png_process_data(), which calls
 2531 callbacks when it has the info, a row, or the end of the image.  You
 2532 set up these callbacks with png_set_progressive_read_fn().  You don't
 2533 have to worry about the input/output functions of libpng, as you are
 2534 giving the library the data directly in png_process_data().  I will
 2535 assume that you have read the section on reading PNG files above,
 2536 so I will only highlight the differences (although I will show
 2537 all of the code).
 2538 
 2539 png_structp png_ptr;
 2540 png_infop info_ptr;
 2541 
 2542  /*  An example code fragment of how you would
 2543      initialize the progressive reader in your
 2544      application. */
 2545  int
 2546  initialize_png_reader()
 2547  {
 2548     png_ptr = png_create_read_struct
 2549         (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
 2550          user_error_fn, user_warning_fn);
 2551 
 2552     if (!png_ptr)
 2553         return ERROR;
 2554 
 2555     info_ptr = png_create_info_struct(png_ptr);
 2556 
 2557     if (!info_ptr)
 2558     {
 2559        png_destroy_read_struct(&png_ptr,
 2560           (png_infopp)NULL, (png_infopp)NULL);
 2561        return ERROR;
 2562     }
 2563 
 2564     if (setjmp(png_jmpbuf(png_ptr)))
 2565     {
 2566        png_destroy_read_struct(&png_ptr, &info_ptr,
 2567           (png_infopp)NULL);
 2568        return ERROR;
 2569     }
 2570 
 2571     /* This one's new.  You can provide functions
 2572        to be called when the header info is valid,
 2573        when each row is completed, and when the image
 2574        is finished.  If you aren't using all functions,
 2575        you can specify NULL parameters.  Even when all
 2576        three functions are NULL, you need to call
 2577        png_set_progressive_read_fn().  You can use
 2578        any struct as the user_ptr (cast to a void pointer
 2579        for the function call), and retrieve the pointer
 2580        from inside the callbacks using the function
 2581 
 2582           png_get_progressive_ptr(png_ptr);
 2583 
 2584        which will return a void pointer, which you have
 2585        to cast appropriately.
 2586      */
 2587     png_set_progressive_read_fn(png_ptr, (void *)user_ptr,
 2588         info_callback, row_callback, end_callback);
 2589 
 2590     return 0;
 2591  }
 2592 
 2593  /* A code fragment that you call as you receive blocks
 2594    of data */
 2595  int
 2596  process_data(png_bytep buffer, png_uint_32 length)
 2597  {
 2598     if (setjmp(png_jmpbuf(png_ptr)))
 2599     {
 2600        png_destroy_read_struct(&png_ptr, &info_ptr,
 2601            (png_infopp)NULL);
 2602        return ERROR;
 2603     }
 2604 
 2605     /* This one's new also.  Simply give it a chunk
 2606        of data from the file stream (in order, of
 2607        course).  On machines with segmented memory
 2608        models machines, don't give it any more than
 2609        64K.  The library seems to run fine with sizes
 2610        of 4K. Although you can give it much less if
 2611        necessary (I assume you can give it chunks of
 2612        1 byte, I haven't tried less than 256 bytes
 2613        yet).  When this function returns, you may
 2614        want to display any rows that were generated
 2615        in the row callback if you don't already do
 2616        so there.
 2617      */
 2618     png_process_data(png_ptr, info_ptr, buffer, length);
 2619 
 2620     /* At this point you can call png_process_data_skip if
 2621        you want to handle data the library will skip yourself;
 2622        it simply returns the number of bytes to skip (and stops
 2623        libpng skipping that number of bytes on the next
 2624        png_process_data call).
 2625     return 0;
 2626  }
 2627 
 2628  /* This function is called (as set by
 2629     png_set_progressive_read_fn() above) when enough data
 2630     has been supplied so all of the header has been
 2631     read.
 2632  */
 2633  void
 2634  info_callback(png_structp png_ptr, png_infop info)
 2635  {
 2636     /* Do any setup here, including setting any of
 2637        the transformations mentioned in the Reading
 2638        PNG files section.  For now, you _must_ call
 2639        either png_start_read_image() or
 2640        png_read_update_info() after all the
 2641        transformations are set (even if you don't set
 2642        any).  You may start getting rows before
 2643        png_process_data() returns, so this is your
 2644        last chance to prepare for that.
 2645 
 2646        This is where you turn on interlace handling,
 2647        assuming you don't want to do it yourself.
 2648 
 2649        If you need to you can stop the processing of
 2650        your original input data at this point by calling
 2651        png_process_data_pause.  This returns the number
 2652        of unprocessed bytes from the last png_process_data
 2653        call - it is up to you to ensure that the next call
 2654        sees these bytes again.  If you don't want to bother
 2655        with this you can get libpng to cache the unread
 2656        bytes by setting the 'save' parameter (see png.h) but
 2657        then libpng will have to copy the data internally.
 2658      */
 2659  }
 2660 
 2661  /* This function is called when each row of image
 2662     data is complete */
 2663  void
 2664  row_callback(png_structp png_ptr, png_bytep new_row,
 2665     png_uint_32 row_num, int pass)
 2666  {
 2667     /* If the image is interlaced, and you turned
 2668        on the interlace handler, this function will
 2669        be called for every row in every pass.  Some
 2670        of these rows will not be changed from the
 2671        previous pass.  When the row is not changed,
 2672        the new_row variable will be NULL.  The rows
 2673        and passes are called in order, so you don't
 2674        really need the row_num and pass, but I'm
 2675        supplying them because it may make your life
 2676        easier.
 2677 
 2678        If you did not turn on interlace handling then
 2679        the callback is called for each row of each
 2680        sub-image when the image is interlaced.  In this
 2681        case 'row_num' is the row in the sub-image, not
 2682        the row in the output image as it is in all other
 2683        cases.
 2684 
 2685        For the non-NULL rows of interlaced images when
 2686        you have switched on libpng interlace handling,
 2687        you must call png_progressive_combine_row()
 2688        passing in the row and the old row.  You can
 2689        call this function for NULL rows (it will just
 2690        return) and for non-interlaced images (it just
 2691        does the memcpy for you) if it will make the
 2692        code easier.  Thus, you can just do this for
 2693        all cases if you switch on interlace handling;
 2694      */
 2695 
 2696         png_progressive_combine_row(png_ptr, old_row,
 2697           new_row);
 2698 
 2699     /* where old_row is what was displayed
 2700        previously for the row.  Note that the first
 2701        pass (pass == 0, really) will completely cover
 2702        the old row, so the rows do not have to be
 2703        initialized.  After the first pass (and only
 2704        for interlaced images), you will have to pass
 2705        the current row, and the function will combine
 2706        the old row and the new row.
 2707 
 2708        You can also call png_process_data_pause in this
 2709        callback - see above.
 2710     */
 2711  }
 2712 
 2713  void
 2714  end_callback(png_structp png_ptr, png_infop info)
 2715  {
 2716     /* This function is called after the whole image
 2717        has been read, including any chunks after the
 2718        image (up to and including the IEND).  You
 2719        will usually have the same info chunk as you
 2720        had in the header, although some data may have
 2721        been added to the comments and time fields.
 2722 
 2723        Most people won't do much here, perhaps setting
 2724        a flag that marks the image as finished.
 2725      */
 2726  }
 2727 
 2728 
 2729 
 2730 IV. Writing
 2731 
 2732 Much of this is very similar to reading.  However, everything of
 2733 importance is repeated here, so you won't have to constantly look
 2734 back up in the reading section to understand writing.
 2735 
 2736 Setup
 2737 
 2738 You will want to do the I/O initialization before you get into libpng,
 2739 so if it doesn't work, you don't have anything to undo. If you are not
 2740 using the standard I/O functions, you will need to replace them with
 2741 custom writing functions.  See the discussion under Customizing libpng.
 2742 
 2743     FILE *fp = fopen(file_name, "wb");
 2744 
 2745     if (!fp)
 2746        return ERROR;
 2747 
 2748 Next, png_struct and png_info need to be allocated and initialized.
 2749 As these can be both relatively large, you may not want to store these
 2750 on the stack, unless you have stack space to spare.  Of course, you
 2751 will want to check if they return NULL.  If you are also reading,
 2752 you won't want to name your read structure and your write structure
 2753 both "png_ptr"; you can call them anything you like, such as
 2754 "read_ptr" and "write_ptr".  Look at pngtest.c, for example.
 2755 
 2756     png_structp png_ptr = png_create_write_struct
 2757        (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
 2758         user_error_fn, user_warning_fn);
 2759 
 2760     if (!png_ptr)
 2761        return ERROR;
 2762 
 2763     png_infop info_ptr = png_create_info_struct(png_ptr);
 2764     if (!info_ptr)
 2765     {
 2766        png_destroy_write_struct(&png_ptr,
 2767            (png_infopp)NULL);
 2768        return ERROR;
 2769     }
 2770 
 2771 If you want to use your own memory allocation routines,
 2772 define PNG_USER_MEM_SUPPORTED and use
 2773 png_create_write_struct_2() instead of png_create_write_struct():
 2774 
 2775     png_structp png_ptr = png_create_write_struct_2
 2776        (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
 2777         user_error_fn, user_warning_fn, (png_voidp)
 2778         user_mem_ptr, user_malloc_fn, user_free_fn);
 2779 
 2780 After you have these structures, you will need to set up the
 2781 error handling.  When libpng encounters an error, it expects to
 2782 longjmp() back to your routine.  Therefore, you will need to call
 2783 setjmp() and pass the png_jmpbuf(png_ptr).  If you
 2784 write the file from different routines, you will need to update
 2785 the png_jmpbuf(png_ptr) every time you enter a new routine that will
 2786 call a png_*() function.  See your documentation of setjmp/longjmp
 2787 for your compiler for more information on setjmp/longjmp.  See
 2788 the discussion on libpng error handling in the Customizing Libpng
 2789 section below for more information on the libpng error handling.
 2790 
 2791     if (setjmp(png_jmpbuf(png_ptr)))
 2792     {
 2793     png_destroy_write_struct(&png_ptr, &info_ptr);
 2794        fclose(fp);
 2795        return ERROR;
 2796     }
 2797     ...
 2798     return;
 2799 
 2800 If you would rather avoid the complexity of setjmp/longjmp issues,
 2801 you can compile libpng with PNG_NO_SETJMP, in which case
 2802 errors will result in a call to PNG_ABORT() which defaults to abort().
 2803 
 2804 You can #define PNG_ABORT() to a function that does something
 2805 more useful than abort(), as long as your function does not
 2806 return.
 2807 
 2808 Checking for invalid palette index on write was added at libpng
 2809 1.5.10.  If a pixel contains an invalid (out-of-range) index libpng issues
 2810 a benign error.  This is enabled by default because this condition is an
 2811 error according to the PNG specification, Clause 11.3.2, but the error can
 2812 be ignored in each png_ptr with
 2813 
 2814    png_set_check_for_invalid_index(png_ptr, 0);
 2815 
 2816 If the error is ignored, or if png_benign_error() treats it as a warning,
 2817 any invalid pixels are written as-is by the encoder, resulting in an
 2818 invalid PNG datastream as output.  In this case the application is
 2819 responsible for ensuring that the pixel indexes are in range when it writes
 2820 a PLTE chunk with fewer entries than the bit depth would allow.
 2821 
 2822 Now you need to set up the output code.  The default for libpng is to
 2823 use the C function fwrite().  If you use this, you will need to pass a
 2824 valid FILE * in the function png_init_io().  Be sure that the file is
 2825 opened in binary mode.  Again, if you wish to handle writing data in
 2826 another way, see the discussion on libpng I/O handling in the Customizing
 2827 Libpng section below.
 2828 
 2829     png_init_io(png_ptr, fp);
 2830 
 2831 If you are embedding your PNG into a datastream such as MNG, and don't
 2832 want libpng to write the 8-byte signature, or if you have already
 2833 written the signature in your application, use
 2834 
 2835     png_set_sig_bytes(png_ptr, 8);
 2836 
 2837 to inform libpng that it should not write a signature.
 2838 
 2839 Write callbacks
 2840 
 2841 At this point, you can set up a callback function that will be
 2842 called after each row has been written, which you can use to control
 2843 a progress meter or the like.  It's demonstrated in pngtest.c.
 2844 You must supply a function
 2845 
 2846     void write_row_callback(png_structp png_ptr, png_uint_32 row,
 2847        int pass);
 2848     {
 2849       /* put your code here */
 2850     }
 2851 
 2852 (You can give it another name that you like instead of "write_row_callback")
 2853 
 2854 To inform libpng about your function, use
 2855 
 2856     png_set_write_status_fn(png_ptr, write_row_callback);
 2857 
 2858 When this function is called the row has already been completely processed and
 2859 it has also been written out.  The 'row' and 'pass' refer to the next row to be
 2860 handled.  For the
 2861 non-interlaced case the row that was just handled is simply one less than the
 2862 passed in row number, and pass will always be 0.  For the interlaced case the
 2863 same applies unless the row value is 0, in which case the row just handled was
 2864 the last one from one of the preceding passes.  Because interlacing may skip a
 2865 pass you cannot be sure that the preceding pass is just 'pass-1', if you really
 2866 need to know what the last pass is record (row,pass) from the callback and use
 2867 the last recorded value each time.
 2868 
 2869 As with the user transform you can find the output row using the
 2870 PNG_ROW_FROM_PASS_ROW macro.
 2871 
 2872 You now have the option of modifying how the compression library will
 2873 run.  The following functions are mainly for testing, but may be useful
 2874 in some cases, like if you need to write PNG files extremely fast and
 2875 are willing to give up some compression, or if you want to get the
 2876 maximum possible compression at the expense of slower writing.  If you
 2877 have no special needs in this area, let the library do what it wants by
 2878 not calling this function at all, as it has been tuned to deliver a good
 2879 speed/compression ratio. The second parameter to png_set_filter() is
 2880 the filter method, for which the only valid values are 0 (as of the
 2881 July 1999 PNG specification, version 1.2) or 64 (if you are writing
 2882 a PNG datastream that is to be embedded in a MNG datastream).  The third
 2883 parameter is a flag that indicates which filter type(s) are to be tested
 2884 for each scanline.  See the PNG specification for details on the specific
 2885 filter types.
 2886 
 2887 
 2888     /* turn on or off filtering, and/or choose
 2889        specific filters.  You can use either a single
 2890        PNG_FILTER_VALUE_NAME or the bitwise OR of one
 2891        or more PNG_FILTER_NAME masks.
 2892      */
 2893     png_set_filter(png_ptr, 0,
 2894        PNG_FILTER_NONE  | PNG_FILTER_VALUE_NONE |
 2895        PNG_FILTER_SUB   | PNG_FILTER_VALUE_SUB  |
 2896        PNG_FILTER_UP    | PNG_FILTER_VALUE_UP   |
 2897        PNG_FILTER_AVG   | PNG_FILTER_VALUE_AVG  |
 2898        PNG_FILTER_PAETH | PNG_FILTER_VALUE_PAETH|
 2899        PNG_ALL_FILTERS  | PNG_FAST_FILTERS);
 2900 
 2901 If an application wants to start and stop using particular filters during
 2902 compression, it should start out with all of the filters (to ensure that
 2903 the previous row of pixels will be stored in case it's needed later),
 2904 and then add and remove them after the start of compression.
 2905 
 2906 If you are writing a PNG datastream that is to be embedded in a MNG
 2907 datastream, the second parameter can be either 0 or 64.
 2908 
 2909 The png_set_compression_*() functions interface to the zlib compression
 2910 library, and should mostly be ignored unless you really know what you are
 2911 doing.  The only generally useful call is png_set_compression_level()
 2912 which changes how much time zlib spends on trying to compress the image
 2913 data.  See the Compression Library (zlib.h and algorithm.txt, distributed
 2914 with zlib) for details on the compression levels.
 2915 
 2916     #include zlib.h
 2917 
 2918     /* Set the zlib compression level */
 2919     png_set_compression_level(png_ptr,
 2920         Z_BEST_COMPRESSION);
 2921 
 2922     /* Set other zlib parameters for compressing IDAT */
 2923     png_set_compression_mem_level(png_ptr, 8);
 2924     png_set_compression_strategy(png_ptr,
 2925         Z_DEFAULT_STRATEGY);
 2926     png_set_compression_window_bits(png_ptr, 15);
 2927     png_set_compression_method(png_ptr, 8);
 2928     png_set_compression_buffer_size(png_ptr, 8192)
 2929 
 2930     /* Set zlib parameters for text compression
 2931      * If you don't call these, the parameters
 2932      * fall back on those defined for IDAT chunks
 2933      */
 2934     png_set_text_compression_mem_level(png_ptr, 8);
 2935     png_set_text_compression_strategy(png_ptr,
 2936         Z_DEFAULT_STRATEGY);
 2937     png_set_text_compression_window_bits(png_ptr, 15);
 2938     png_set_text_compression_method(png_ptr, 8);
 2939 
 2940 Setting the contents of info for output
 2941 
 2942 You now need to fill in the png_info structure with all the data you
 2943 wish to write before the actual image.  Note that the only thing you
 2944 are allowed to write after the image is the text chunks and the time
 2945 chunk (as of PNG Specification 1.2, anyway).  See png_write_end() and
 2946 the latest PNG specification for more information on that.  If you
 2947 wish to write them before the image, fill them in now, and flag that
 2948 data as being valid.  If you want to wait until after the data, don't
 2949 fill them until png_write_end().  For all the fields in png_info and
 2950 their data types, see png.h.  For explanations of what the fields
 2951 contain, see the PNG specification.
 2952 
 2953 Some of the more important parts of the png_info are:
 2954 
 2955     png_set_IHDR(png_ptr, info_ptr, width, height,
 2956        bit_depth, color_type, interlace_type,
 2957        compression_type, filter_method)
 2958 
 2959     width          - holds the width of the image
 2960                      in pixels (up to 2^31).
 2961 
 2962     height         - holds the height of the image
 2963                      in pixels (up to 2^31).
 2964 
 2965     bit_depth      - holds the bit depth of one of the
 2966                      image channels.
 2967                      (valid values are 1, 2, 4, 8, 16
 2968                      and depend also on the
 2969                      color_type.  See also significant
 2970                      bits (sBIT) below).
 2971 
 2972     color_type     - describes which color/alpha
 2973                      channels are present.
 2974                      PNG_COLOR_TYPE_GRAY
 2975                         (bit depths 1, 2, 4, 8, 16)
 2976                      PNG_COLOR_TYPE_GRAY_ALPHA
 2977                         (bit depths 8, 16)
 2978                      PNG_COLOR_TYPE_PALETTE
 2979                         (bit depths 1, 2, 4, 8)
 2980                      PNG_COLOR_TYPE_RGB
 2981                         (bit_depths 8, 16)
 2982                      PNG_COLOR_TYPE_RGB_ALPHA
 2983                         (bit_depths 8, 16)
 2984 
 2985                      PNG_COLOR_MASK_PALETTE
 2986                      PNG_COLOR_MASK_COLOR
 2987                      PNG_COLOR_MASK_ALPHA
 2988 
 2989     interlace_type - PNG_INTERLACE_NONE or
 2990                      PNG_INTERLACE_ADAM7
 2991 
 2992     compression_type - (must be
 2993                      PNG_COMPRESSION_TYPE_DEFAULT)
 2994 
 2995     filter_method  - (must be PNG_FILTER_TYPE_DEFAULT
 2996                      or, if you are writing a PNG to
 2997                      be embedded in a MNG datastream,
 2998                      can also be
 2999                      PNG_INTRAPIXEL_DIFFERENCING)
 3000 
 3001 If you call png_set_IHDR(), the call must appear before any of the
 3002 other png_set_*() functions, because they might require access to some of
 3003 the IHDR settings.  The remaining png_set_*() functions can be called
 3004 in any order.
 3005 
 3006 If you wish, you can reset the compression_type, interlace_type, or
 3007 filter_method later by calling png_set_IHDR() again; if you do this, the
 3008 width, height, bit_depth, and color_type must be the same in each call.
 3009 
 3010     png_set_PLTE(png_ptr, info_ptr, palette,
 3011        num_palette);
 3012 
 3013     palette        - the palette for the file
 3014                      (array of png_color)
 3015     num_palette    - number of entries in the palette
 3016 
 3017 
 3018     png_set_gAMA(png_ptr, info_ptr, file_gamma);
 3019     png_set_gAMA_fixed(png_ptr, info_ptr, int_file_gamma);
 3020 
 3021     file_gamma     - the gamma at which the image was
 3022                      created (PNG_INFO_gAMA)
 3023 
 3024     int_file_gamma - 100,000 times the gamma at which
 3025                      the image was created
 3026 
 3027     png_set_cHRM(png_ptr, info_ptr,  white_x, white_y, red_x, red_y,
 3028                      green_x, green_y, blue_x, blue_y)
 3029     png_set_cHRM_XYZ(png_ptr, info_ptr, red_X, red_Y, red_Z, green_X,
 3030                      green_Y, green_Z, blue_X, blue_Y, blue_Z)
 3031     png_set_cHRM_fixed(png_ptr, info_ptr, int_white_x, int_white_y,
 3032                      int_red_x, int_red_y, int_green_x, int_green_y,
 3033                      int_blue_x, int_blue_y)
 3034     png_set_cHRM_XYZ_fixed(png_ptr, info_ptr, int_red_X, int_red_Y,
 3035                      int_red_Z, int_green_X, int_green_Y, int_green_Z,
 3036                      int_blue_X, int_blue_Y, int_blue_Z)
 3037 
 3038     {white,red,green,blue}_{x,y}
 3039                      A color space encoding specified using the chromaticities
 3040                      of the end points and the white point.
 3041 
 3042     {red,green,blue}_{X,Y,Z}
 3043                      A color space encoding specified using the encoding end
 3044                      points - the CIE tristimulus specification of the intended
 3045                      color of the red, green and blue channels in the PNG RGB
 3046                      data.  The white point is simply the sum of the three end
 3047                      points.
 3048 
 3049     png_set_sRGB(png_ptr, info_ptr, srgb_intent);
 3050 
 3051     srgb_intent    - the rendering intent
 3052                      (PNG_INFO_sRGB) The presence of
 3053                      the sRGB chunk means that the pixel
 3054                      data is in the sRGB color space.
 3055                      This chunk also implies specific
 3056                      values of gAMA and cHRM.  Rendering
 3057                      intent is the CSS-1 property that
 3058                      has been defined by the International
 3059                      Color Consortium
 3060                      (http://www.color.org).
 3061                      It can be one of
 3062                      PNG_sRGB_INTENT_SATURATION,
 3063                      PNG_sRGB_INTENT_PERCEPTUAL,
 3064                      PNG_sRGB_INTENT_ABSOLUTE, or
 3065                      PNG_sRGB_INTENT_RELATIVE.
 3066 
 3067 
 3068     png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr,
 3069        srgb_intent);
 3070 
 3071     srgb_intent    - the rendering intent
 3072                      (PNG_INFO_sRGB) The presence of the
 3073                      sRGB chunk means that the pixel
 3074                      data is in the sRGB color space.
 3075                      This function also causes gAMA and
 3076                      cHRM chunks with the specific values
 3077                      that are consistent with sRGB to be
 3078                      written.
 3079 
 3080     png_set_iCCP(png_ptr, info_ptr, name, compression_type,
 3081                        profile, proflen);
 3082 
 3083     name             - The profile name.
 3084 
 3085     compression_type - The compression type; always
 3086                        PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
 3087                        You may give NULL to this argument to
 3088                        ignore it.
 3089 
 3090     profile          - International Color Consortium color
 3091                        profile data. May contain NULs.
 3092 
 3093     proflen          - length of profile data in bytes.
 3094 
 3095     png_set_sBIT(png_ptr, info_ptr, sig_bit);
 3096 
 3097     sig_bit        - the number of significant bits for
 3098                      (PNG_INFO_sBIT) each of the gray, red,
 3099                      green, and blue channels, whichever are
 3100                      appropriate for the given color type
 3101                      (png_color_16)
 3102 
 3103     png_set_tRNS(png_ptr, info_ptr, trans_alpha,
 3104        num_trans, trans_color);
 3105 
 3106     trans_alpha    - array of alpha (transparency)
 3107                      entries for palette (PNG_INFO_tRNS)
 3108 
 3109     num_trans      - number of transparent entries
 3110                      (PNG_INFO_tRNS)
 3111 
 3112     trans_color    - graylevel or color sample values
 3113                      (in order red, green, blue) of the
 3114                      single transparent color for
 3115                      non-paletted images (PNG_INFO_tRNS)
 3116 
 3117     png_set_eXIf_1(png_ptr, info_ptr, num_exif, exif);
 3118 
 3119     exif           - Exif profile (array of
 3120                      png_byte) (PNG_INFO_eXIf)
 3121 
 3122     png_set_hIST(png_ptr, info_ptr, hist);
 3123 
 3124     hist           - histogram of palette (array of
 3125                      png_uint_16) (PNG_INFO_hIST)
 3126 
 3127     png_set_tIME(png_ptr, info_ptr, mod_time);
 3128 
 3129     mod_time       - time image was last modified
 3130                      (PNG_VALID_tIME)
 3131 
 3132     png_set_bKGD(png_ptr, info_ptr, background);
 3133 
 3134     background     - background color (of type
 3135                      png_color_16p) (PNG_VALID_bKGD)
 3136 
 3137     png_set_text(png_ptr, info_ptr, text_ptr, num_text);
 3138 
 3139     text_ptr       - array of png_text holding image
 3140                      comments
 3141 
 3142     text_ptr[i].compression - type of compression used
 3143                  on "text" PNG_TEXT_COMPRESSION_NONE
 3144                            PNG_TEXT_COMPRESSION_zTXt
 3145                            PNG_ITXT_COMPRESSION_NONE
 3146                            PNG_ITXT_COMPRESSION_zTXt
 3147     text_ptr[i].key   - keyword for comment.  Must contain
 3148                  1-79 characters.
 3149     text_ptr[i].text  - text comments for current
 3150                          keyword.  Can be NULL or empty.
 3151     text_ptr[i].text_length - length of text string,
 3152                  after decompression, 0 for iTXt
 3153     text_ptr[i].itxt_length - length of itxt string,
 3154                  after decompression, 0 for tEXt/zTXt
 3155     text_ptr[i].lang  - language of comment (NULL or
 3156                          empty for unknown).
 3157     text_ptr[i].translated_keyword  - keyword in UTF-8 (NULL
 3158                          or empty for unknown).
 3159 
 3160     Note that the itxt_length, lang, and lang_key
 3161     members of the text_ptr structure only exist when the
 3162     library is built with iTXt chunk support.  Prior to
 3163     libpng-1.4.0 the library was built by default without
 3164     iTXt support. Also note that when iTXt is supported,
 3165     they contain NULL pointers when the "compression"
 3166     field contains PNG_TEXT_COMPRESSION_NONE or
 3167     PNG_TEXT_COMPRESSION_zTXt.
 3168 
 3169     num_text       - number of comments
 3170 
 3171     png_set_sPLT(png_ptr, info_ptr, &palette_ptr,
 3172        num_spalettes);
 3173 
 3174     palette_ptr    - array of png_sPLT_struct structures
 3175                      to be added to the list of palettes
 3176                      in the info structure.
 3177     num_spalettes  - number of palette structures to be
 3178                      added.
 3179 
 3180     png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y,
 3181         unit_type);
 3182 
 3183     offset_x  - positive offset from the left
 3184                      edge of the screen
 3185 
 3186     offset_y  - positive offset from the top
 3187                      edge of the screen
 3188 
 3189     unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER
 3190 
 3191     png_set_pHYs(png_ptr, info_ptr, res_x, res_y,
 3192         unit_type);
 3193 
 3194     res_x       - pixels/unit physical resolution
 3195                   in x direction
 3196 
 3197     res_y       - pixels/unit physical resolution
 3198                   in y direction
 3199 
 3200     unit_type   - PNG_RESOLUTION_UNKNOWN,
 3201                   PNG_RESOLUTION_METER
 3202 
 3203     png_set_sCAL(png_ptr, info_ptr, unit, width, height)
 3204 
 3205     unit        - physical scale units (an integer)
 3206 
 3207     width       - width of a pixel in physical scale units
 3208 
 3209     height      - height of a pixel in physical scale units
 3210                   (width and height are doubles)
 3211 
 3212     png_set_sCAL_s(png_ptr, info_ptr, unit, width, height)
 3213 
 3214     unit        - physical scale units (an integer)
 3215 
 3216     width       - width of a pixel in physical scale units
 3217                   expressed as a string
 3218 
 3219     height      - height of a pixel in physical scale units
 3220                  (width and height are strings like "2.54")
 3221 
 3222     png_set_unknown_chunks(png_ptr, info_ptr, &unknowns,
 3223        num_unknowns)
 3224 
 3225     unknowns          - array of png_unknown_chunk
 3226                         structures holding unknown chunks
 3227     unknowns[i].name  - name of unknown chunk
 3228     unknowns[i].data  - data of unknown chunk
 3229     unknowns[i].size  - size of unknown chunk's data
 3230     unknowns[i].location - position to write chunk in file
 3231                            0: do not write chunk
 3232                            PNG_HAVE_IHDR: before PLTE
 3233                            PNG_HAVE_PLTE: before IDAT
 3234                            PNG_AFTER_IDAT: after IDAT
 3235 
 3236 The "location" member is set automatically according to
 3237 what part of the output file has already been written.
 3238 You can change its value after calling png_set_unknown_chunks()
 3239 as demonstrated in pngtest.c.  Within each of the "locations",
 3240 the chunks are sequenced according to their position in the
 3241 structure (that is, the value of "i", which is the order in which
 3242 the chunk was either read from the input file or defined with
 3243 png_set_unknown_chunks).
 3244 
 3245 A quick word about text and num_text.  text is an array of png_text
 3246 structures.  num_text is the number of valid structures in the array.
 3247 Each png_text structure holds a language code, a keyword, a text value,
 3248 and a compression type.
 3249 
 3250 The compression types have the same valid numbers as the compression
 3251 types of the image data.  Currently, the only valid number is zero.
 3252 However, you can store text either compressed or uncompressed, unlike
 3253 images, which always have to be compressed.  So if you don't want the
 3254 text compressed, set the compression type to PNG_TEXT_COMPRESSION_NONE.
 3255 Because tEXt and zTXt chunks don't have a language field, if you
 3256 specify PNG_TEXT_COMPRESSION_NONE or PNG_TEXT_COMPRESSION_zTXt
 3257 any language code or translated keyword will not be written out.
 3258 
 3259 Until text gets around a few hundred bytes, it is not worth compressing it.
 3260 After the text has been written out to the file, the compression type
 3261 is set to PNG_TEXT_COMPRESSION_NONE_WR or PNG_TEXT_COMPRESSION_zTXt_WR,
 3262 so that it isn't written out again at the end (in case you are calling
 3263 png_write_end() with the same struct).
 3264 
 3265 The keywords that are given in the PNG Specification are:
 3266 
 3267     Title            Short (one line) title or
 3268                      caption for image
 3269 
 3270     Author           Name of image's creator
 3271 
 3272     Description      Description of image (possibly long)
 3273 
 3274     Copyright        Copyright notice
 3275 
 3276     Creation Time    Time of original image creation
 3277                      (usually RFC 1123 format, see below)
 3278 
 3279     Software         Software used to create the image
 3280 
 3281     Disclaimer       Legal disclaimer
 3282 
 3283     Warning          Warning of nature of content
 3284 
 3285     Source           Device used to create the image
 3286 
 3287     Comment          Miscellaneous comment; conversion
 3288                      from other image format
 3289 
 3290 The keyword-text pairs work like this.  Keywords should be short
 3291 simple descriptions of what the comment is about.  Some typical
 3292 keywords are found in the PNG specification, as is some recommendations
 3293 on keywords.  You can repeat keywords in a file.  You can even write
 3294 some text before the image and some after.  For example, you may want
 3295 to put a description of the image before the image, but leave the
 3296 disclaimer until after, so viewers working over modem connections
 3297 don't have to wait for the disclaimer to go over the modem before
 3298 they start seeing the image.  Finally, keywords should be full
 3299 words, not abbreviations.  Keywords and text are in the ISO 8859-1
 3300 (Latin-1) character set (a superset of regular ASCII) and can not
 3301 contain NUL characters, and should not contain control or other
 3302 unprintable characters.  To make the comments widely readable, stick
 3303 with basic ASCII, and avoid machine specific character set extensions
 3304 like the IBM-PC character set.  The keyword must be present, but
 3305 you can leave off the text string on non-compressed pairs.
 3306 Compressed pairs must have a text string, as only the text string
 3307 is compressed anyway, so the compression would be meaningless.
 3308 
 3309 PNG supports modification time via the png_time structure.  Two
 3310 conversion routines are provided, png_convert_from_time_t() for
 3311 time_t and png_convert_from_struct_tm() for struct tm.  The
 3312 time_t routine uses gmtime().  You don't have to use either of
 3313 these, but if you wish to fill in the png_time structure directly,
 3314 you should provide the time in universal time (GMT) if possible
 3315 instead of your local time.  Note that the year number is the full
 3316 year (e.g. 1998, rather than 98 - PNG is year 2000 compliant!), and
 3317 that months start with 1.
 3318 
 3319 If you want to store the time of the original image creation, you should
 3320 use a plain tEXt chunk with the "Creation Time" keyword.  This is
 3321 necessary because the "creation time" of a PNG image is somewhat vague,
 3322 depending on whether you mean the PNG file, the time the image was
 3323 created in a non-PNG format, a still photo from which the image was
 3324 scanned, or possibly the subject matter itself.  In order to facilitate
 3325 machine-readable dates, it is recommended that the "Creation Time"
 3326 tEXt chunk use RFC 1123 format dates (e.g. "22 May 1997 18:07:10 GMT"),
 3327 although this isn't a requirement.  Unlike the tIME chunk, the
 3328 "Creation Time" tEXt chunk is not expected to be automatically changed
 3329 by the software.  To facilitate the use of RFC 1123 dates, a function
 3330 png_convert_to_rfc1123_buffer(buffer, png_timep) is provided to
 3331 convert from PNG time to an RFC 1123 format string.  The caller must provide
 3332 a writeable buffer of at least 29 bytes.
 3333 
 3334 Writing unknown chunks
 3335 
 3336 You can use the png_set_unknown_chunks function to queue up private chunks
 3337 for writing.  You give it a chunk name, location, raw data, and a size.  You
 3338 also must use png_set_keep_unknown_chunks() to ensure that libpng will
 3339 handle them.  That's all there is to it.  The chunks will be written by the
 3340 next following png_write_info_before_PLTE, png_write_info, or png_write_end
 3341 function, depending upon the specified location.  Any chunks previously
 3342 read into the info structure's unknown-chunk list will also be written out
 3343 in a sequence that satisfies the PNG specification's ordering rules.
 3344 
 3345 Here is an example of writing two private chunks, prVt and miNE:
 3346 
 3347     #ifdef PNG_WRITE_UNKNOWN_CHUNKS_SUPPORTED
 3348     /* Set unknown chunk data */
 3349     png_unknown_chunk unk_chunk[2];
 3350     strcpy((char *) unk_chunk[0].name, "prVt";
 3351     unk_chunk[0].data = (unsigned char *) "PRIVATE DATA";
 3352     unk_chunk[0].size = strlen(unk_chunk[0].data)+1;
 3353     unk_chunk[0].location = PNG_HAVE_IHDR;
 3354     strcpy((char *) unk_chunk[1].name, "miNE";
 3355     unk_chunk[1].data = (unsigned char *) "MY CHUNK DATA";
 3356     unk_chunk[1].size = strlen(unk_chunk[0].data)+1;
 3357     unk_chunk[1].location = PNG_AFTER_IDAT;
 3358     png_set_unknown_chunks(write_ptr, write_info_ptr,
 3359         unk_chunk, 2);
 3360     /* Needed because miNE is not safe-to-copy */
 3361     png_set_keep_unknown_chunks(png, PNG_HANDLE_CHUNK_ALWAYS,
 3362        (png_bytep) "miNE", 1);
 3363     # if PNG_LIBPNG_VER < 10600
 3364       /* Deal with unknown chunk location bug in 1.5.x and earlier */
 3365       png_set_unknown_chunk_location(png, info, 0, PNG_HAVE_IHDR);
 3366       png_set_unknown_chunk_location(png, info, 1, PNG_AFTER_IDAT);
 3367     # endif
 3368     # if PNG_LIBPNG_VER < 10500
 3369       /* PNG_AFTER_IDAT writes two copies of the chunk prior to libpng-1.5.0,
 3370        * one before IDAT and another after IDAT, so don't use it; only use
 3371        * PNG_HAVE_IHDR location.  This call resets the location previously
 3372        * set by assignment and png_set_unknown_chunk_location() for chunk 1.
 3373        */
 3374       png_set_unknown_chunk_location(png, info, 1, PNG_HAVE_IHDR);
 3375     # endif
 3376     #endif
 3377 
 3378 The high-level write interface
 3379 
 3380 At this point there are two ways to proceed; through the high-level
 3381 write interface, or through a sequence of low-level write operations.
 3382 You can use the high-level interface if your image data is present
 3383 in the info structure.  All defined output
 3384 transformations are permitted, enabled by the following masks.
 3385 
 3386     PNG_TRANSFORM_IDENTITY      No transformation
 3387     PNG_TRANSFORM_PACKING       Pack 1, 2 and 4-bit samples
 3388     PNG_TRANSFORM_PACKSWAP      Change order of packed
 3389                                 pixels to LSB first
 3390     PNG_TRANSFORM_INVERT_MONO   Invert monochrome images
 3391     PNG_TRANSFORM_SHIFT         Normalize pixels to the
 3392                                 sBIT depth
 3393     PNG_TRANSFORM_BGR           Flip RGB to BGR, RGBA
 3394                                 to BGRA
 3395     PNG_TRANSFORM_SWAP_ALPHA    Flip RGBA to ARGB or GA
 3396                                 to AG
 3397     PNG_TRANSFORM_INVERT_ALPHA  Change alpha from opacity
 3398                                 to transparency
 3399     PNG_TRANSFORM_SWAP_ENDIAN   Byte-swap 16-bit samples
 3400     PNG_TRANSFORM_STRIP_FILLER        Strip out filler
 3401                                       bytes (deprecated).
 3402     PNG_TRANSFORM_STRIP_FILLER_BEFORE Strip out leading
 3403                                       filler bytes
 3404     PNG_TRANSFORM_STRIP_FILLER_AFTER  Strip out trailing
 3405                                       filler bytes
 3406 
 3407 If you have valid image data in the info structure (you can use
 3408 png_set_rows() to put image data in the info structure), simply do this:
 3409 
 3410     png_write_png(png_ptr, info_ptr, png_transforms, NULL)
 3411 
 3412 where png_transforms is an integer containing the bitwise OR of some set of
 3413 transformation flags.  This call is equivalent to png_write_info(),
 3414 followed the set of transformations indicated by the transform mask,
 3415 then png_write_image(), and finally png_write_end().
 3416 
 3417 (The final parameter of this call is not yet used.  Someday it might point
 3418 to transformation parameters required by some future output transform.)
 3419 
 3420 You must use png_transforms and not call any png_set_transform() functions
 3421 when you use png_write_png().
 3422 
 3423 The low-level write interface
 3424 
 3425 If you are going the low-level route instead, you are now ready to
 3426 write all the file information up to the actual image data.  You do
 3427 this with a call to png_write_info().
 3428 
 3429     png_write_info(png_ptr, info_ptr);
 3430 
 3431 Note that there is one transformation you may need to do before
 3432 png_write_info().  In PNG files, the alpha channel in an image is the
 3433 level of opacity.  If your data is supplied as a level of transparency,
 3434 you can invert the alpha channel before you write it, so that 0 is
 3435 fully transparent and 255 (in 8-bit or paletted images) or 65535
 3436 (in 16-bit images) is fully opaque, with
 3437 
 3438     png_set_invert_alpha(png_ptr);
 3439 
 3440 This must appear before png_write_info() instead of later with the
 3441 other transformations because in the case of paletted images the tRNS
 3442 chunk data has to be inverted before the tRNS chunk is written.  If
 3443 your image is not a paletted image, the tRNS data (which in such cases
 3444 represents a single color to be rendered as transparent) won't need to
 3445 be changed, and you can safely do this transformation after your
 3446 png_write_info() call.
 3447 
 3448 If you need to write a private chunk that you want to appear before
 3449 the PLTE chunk when PLTE is present, you can write the PNG info in
 3450 two steps, and insert code to write your own chunk between them:
 3451 
 3452     png_write_info_before_PLTE(png_ptr, info_ptr);
 3453     png_set_unknown_chunks(png_ptr, info_ptr, ...);
 3454     png_write_info(png_ptr, info_ptr);
 3455 
 3456 After you've written the file information, you can set up the library
 3457 to handle any special transformations of the image data.  The various
 3458 ways to transform the data will be described in the order that they
 3459 should occur.  This is important, as some of these change the color
 3460 type and/or bit depth of the data, and some others only work on
 3461 certain color types and bit depths.  Even though each transformation
 3462 checks to see if it has data that it can do something with, you should
 3463 make sure to only enable a transformation if it will be valid for the
 3464 data.  For example, don't swap red and blue on grayscale data.
 3465 
 3466 PNG files store RGB pixels packed into 3 or 6 bytes.  This code tells
 3467 the library to strip input data that has 4 or 8 bytes per pixel down
 3468 to 3 or 6 bytes (or strip 2 or 4-byte grayscale+filler data to 1 or 2
 3469 bytes per pixel).
 3470 
 3471     png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);
 3472 
 3473 where the 0 is unused, and the location is either PNG_FILLER_BEFORE or
 3474 PNG_FILLER_AFTER, depending upon whether the filler byte in the pixel
 3475 is stored XRGB or RGBX.
 3476 
 3477 PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as
 3478 they can, resulting in, for example, 8 pixels per byte for 1 bit files.
 3479 If the data is supplied at 1 pixel per byte, use this code, which will
 3480 correctly pack the pixels into a single byte:
 3481 
 3482     png_set_packing(png_ptr);
 3483 
 3484 PNG files reduce possible bit depths to 1, 2, 4, 8, and 16.  If your
 3485 data is of another bit depth, you can write an sBIT chunk into the
 3486 file so that decoders can recover the original data if desired.
 3487 
 3488     /* Set the true bit depth of the image data */
 3489     if (color_type & PNG_COLOR_MASK_COLOR)
 3490     {
 3491        sig_bit.red = true_bit_depth;
 3492        sig_bit.green = true_bit_depth;
 3493        sig_bit.blue = true_bit_depth;
 3494     }
 3495 
 3496     else
 3497     {
 3498        sig_bit.gray = true_bit_depth;
 3499     }
 3500 
 3501     if (color_type & PNG_COLOR_MASK_ALPHA)
 3502     {
 3503        sig_bit.alpha = true_bit_depth;
 3504     }
 3505 
 3506     png_set_sBIT(png_ptr, info_ptr, &sig_bit);
 3507 
 3508 If the data is stored in the row buffer in a bit depth other than
 3509 one supported by PNG (e.g. 3 bit data in the range 0-7 for a 4-bit PNG),
 3510 this will scale the values to appear to be the correct bit depth as
 3511 is required by PNG.
 3512 
 3513     png_set_shift(png_ptr, &sig_bit);
 3514 
 3515 PNG files store 16-bit pixels in network byte order (big-endian,
 3516 ie. most significant bits first).  This code would be used if they are
 3517 supplied the other way (little-endian, i.e. least significant bits
 3518 first, the way PCs store them):
 3519 
 3520     if (bit_depth > 8)
 3521        png_set_swap(png_ptr);
 3522 
 3523 If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you
 3524 need to change the order the pixels are packed into bytes, you can use:
 3525 
 3526     if (bit_depth < 8)
 3527        png_set_packswap(png_ptr);
 3528 
 3529 PNG files store 3 color pixels in red, green, blue order.  This code
 3530 would be used if they are supplied as blue, green, red:
 3531 
 3532     png_set_bgr(png_ptr);
 3533 
 3534 PNG files describe monochrome as black being zero and white being
 3535 one. This code would be used if the pixels are supplied with this reversed
 3536 (black being one and white being zero):
 3537 
 3538     png_set_invert_mono(png_ptr);
 3539 
 3540 Finally, you can write your own transformation function if none of
 3541 the existing ones meets your needs.  This is done by setting a callback
 3542 with
 3543 
 3544     png_set_write_user_transform_fn(png_ptr,
 3545        write_transform_fn);
 3546 
 3547 You must supply the function
 3548 
 3549     void write_transform_fn(png_structp png_ptr, png_row_infop
 3550        row_info, png_bytep data)
 3551 
 3552 See pngtest.c for a working example.  Your function will be called
 3553 before any of the other transformations are processed.  If supported
 3554 libpng also supplies an information routine that may be called from
 3555 your callback:
 3556 
 3557    png_get_current_row_number(png_ptr);
 3558    png_get_current_pass_number(png_ptr);
 3559 
 3560 This returns the current row passed to the transform.  With interlaced
 3561 images the value returned is the row in the input sub-image image.  Use
 3562 PNG_ROW_FROM_PASS_ROW(row, pass) and PNG_COL_FROM_PASS_COL(col, pass) to
 3563 find the output pixel (x,y) given an interlaced sub-image pixel (row,col,pass).
 3564 
 3565 The discussion of interlace handling above contains more information on how to
 3566 use these values.
 3567 
 3568 You can also set up a pointer to a user structure for use by your
 3569 callback function.
 3570 
 3571     png_set_user_transform_info(png_ptr, user_ptr, 0, 0);
 3572 
 3573 The user_channels and user_depth parameters of this function are ignored
 3574 when writing; you can set them to zero as shown.
 3575 
 3576 You can retrieve the pointer via the function png_get_user_transform_ptr().
 3577 For example:
 3578 
 3579     voidp write_user_transform_ptr =
 3580        png_get_user_transform_ptr(png_ptr);
 3581 
 3582 It is possible to have libpng flush any pending output, either manually,
 3583 or automatically after a certain number of lines have been written.  To
 3584 flush the output stream a single time call:
 3585 
 3586     png_write_flush(png_ptr);
 3587 
 3588 and to have libpng flush the output stream periodically after a certain
 3589 number of scanlines have been written, call:
 3590 
 3591     png_set_flush(png_ptr, nrows);
 3592 
 3593 Note that the distance between rows is from the last time png_write_flush()
 3594 was called, or the first row of the image if it has never been called.
 3595 So if you write 50 lines, and then png_set_flush 25, it will flush the
 3596 output on the next scanline, and every 25 lines thereafter, unless
 3597 png_write_flush() is called before 25 more lines have been written.
 3598 If nrows is too small (less than about 10 lines for a 640 pixel wide
 3599 RGB image) the image compression may decrease noticeably (although this
 3600 may be acceptable for real-time applications).  Infrequent flushing will
 3601 only degrade the compression performance by a few percent over images
 3602 that do not use flushing.
 3603 
 3604 Writing the image data
 3605 
 3606 That's it for the transformations.  Now you can write the image data.
 3607 The simplest way to do this is in one function call.  If you have the
 3608 whole image in memory, you can just call png_write_image() and libpng
 3609 will write the image.  You will need to pass in an array of pointers to
 3610 each row.  This function automatically handles interlacing, so you don't
 3611 need to call png_set_interlace_handling() or call this function multiple
 3612 times, or any of that other stuff necessary with png_write_rows().
 3613 
 3614     png_write_image(png_ptr, row_pointers);
 3615 
 3616 where row_pointers is:
 3617 
 3618     png_byte *row_pointers[height];
 3619 
 3620 You can point to void or char or whatever you use for pixels.
 3621 
 3622 If you don't want to write the whole image at once, you can
 3623 use png_write_rows() instead.  If the file is not interlaced,
 3624 this is simple:
 3625 
 3626     png_write_rows(png_ptr, row_pointers,
 3627        number_of_rows);
 3628 
 3629 row_pointers is the same as in the png_write_image() call.
 3630 
 3631 If you are just writing one row at a time, you can do this with
 3632 a single row_pointer instead of an array of row_pointers:
 3633 
 3634     png_bytep row_pointer = row;
 3635 
 3636     png_write_row(png_ptr, row_pointer);
 3637 
 3638 When the file is interlaced, things can get a good deal more complicated.
 3639 The only currently (as of the PNG Specification version 1.2, dated July
 3640 1999) defined interlacing scheme for PNG files is the "Adam7" interlace
 3641 scheme, that breaks down an image into seven smaller images of varying
 3642 size.  libpng will build these images for you, or you can do them
 3643 yourself.  If you want to build them yourself, see the PNG specification
 3644 for details of which pixels to write when.
 3645 
 3646 If you don't want libpng to handle the interlacing details, just
 3647 use png_set_interlace_handling() and call png_write_rows() the
 3648 correct number of times to write all the sub-images
 3649 (png_set_interlace_handling() returns the number of sub-images.)
 3650 
 3651 If you want libpng to build the sub-images, call this before you start
 3652 writing any rows:
 3653 
 3654     number_of_passes = png_set_interlace_handling(png_ptr);
 3655 
 3656 This will return the number of passes needed.  Currently, this is seven,
 3657 but may change if another interlace type is added.
 3658 
 3659 Then write the complete image number_of_passes times.
 3660 
 3661     png_write_rows(png_ptr, row_pointers, number_of_rows);
 3662 
 3663 Think carefully before you write an interlaced image.  Typically code that
 3664 reads such images reads all the image data into memory, uncompressed, before
 3665 doing any processing.  Only code that can display an image on the fly can
 3666 take advantage of the interlacing and even then the image has to be exactly
 3667 the correct size for the output device, because scaling an image requires
 3668 adjacent pixels and these are not available until all the passes have been
 3669 read.
 3670 
 3671 If you do write an interlaced image you will hardly ever need to handle
 3672 the interlacing yourself.  Call png_set_interlace_handling() and use the
 3673 approach described above.
 3674 
 3675 The only time it is conceivable that you will really need to write an
 3676 interlaced image pass-by-pass is when you have read one pass by pass and
 3677 made some pixel-by-pixel transformation to it, as described in the read
 3678 code above.  In this case use the PNG_PASS_ROWS and PNG_PASS_COLS macros
 3679 to determine the size of each sub-image in turn and simply write the rows
 3680 you obtained from the read code.
 3681 
 3682 Finishing a sequential write
 3683 
 3684 After you are finished writing the image, you should finish writing
 3685 the file.  If you are interested in writing comments or time, you should
 3686 pass an appropriately filled png_info pointer.  If you are not interested,
 3687 you can pass NULL.
 3688 
 3689     png_write_end(png_ptr, info_ptr);
 3690 
 3691 When you are done, you can free all memory used by libpng like this:
 3692 
 3693     png_destroy_write_struct(&png_ptr, &info_ptr);
 3694 
 3695 It is also possible to individually free the info_ptr members that
 3696 point to libpng-allocated storage with the following function:
 3697 
 3698     png_free_data(png_ptr, info_ptr, mask, seq)
 3699 
 3700     mask  - identifies data to be freed, a mask
 3701             containing the bitwise OR of one or
 3702             more of
 3703               PNG_FREE_PLTE, PNG_FREE_TRNS,
 3704               PNG_FREE_HIST, PNG_FREE_ICCP,
 3705               PNG_FREE_PCAL, PNG_FREE_ROWS,
 3706               PNG_FREE_SCAL, PNG_FREE_SPLT,
 3707               PNG_FREE_TEXT, PNG_FREE_UNKN,
 3708             or simply PNG_FREE_ALL
 3709 
 3710     seq   - sequence number of item to be freed
 3711             (-1 for all items)
 3712 
 3713 This function may be safely called when the relevant storage has
 3714 already been freed, or has not yet been allocated, or was allocated
 3715 by the user  and not by libpng,  and will in those cases do nothing.
 3716 The "seq" parameter is ignored if only one item of the selected data
 3717 type, such as PLTE, is allowed.  If "seq" is not -1, and multiple items
 3718 are allowed for the data type identified in the mask, such as text or
 3719 sPLT, only the n'th item in the structure is freed, where n is "seq".
 3720 
 3721 If you allocated data such as a palette that you passed in to libpng
 3722 with png_set_*, you must not free it until just before the call to
 3723 png_destroy_write_struct().
 3724 
 3725 The default behavior is only to free data that was allocated internally
 3726 by libpng.  This can be changed, so that libpng will not free the data,
 3727 or so that it will free data that was allocated by the user with png_malloc()
 3728 or png_calloc() and passed in via a png_set_*() function, with
 3729 
 3730     png_data_freer(png_ptr, info_ptr, freer, mask)
 3731 
 3732     freer  - one of
 3733                PNG_DESTROY_WILL_FREE_DATA
 3734                PNG_SET_WILL_FREE_DATA
 3735                PNG_USER_WILL_FREE_DATA
 3736 
 3737     mask   - which data elements are affected
 3738              same choices as in png_free_data()
 3739 
 3740 For example, to transfer responsibility for some data from a read structure
 3741 to a write structure, you could use
 3742 
 3743     png_data_freer(read_ptr, read_info_ptr,
 3744        PNG_USER_WILL_FREE_DATA,
 3745        PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)
 3746 
 3747     png_data_freer(write_ptr, write_info_ptr,
 3748        PNG_DESTROY_WILL_FREE_DATA,
 3749        PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)
 3750 
 3751 thereby briefly reassigning responsibility for freeing to the user but
 3752 immediately afterwards reassigning it once more to the write_destroy
 3753 function.  Having done this, it would then be safe to destroy the read
 3754 structure and continue to use the PLTE, tRNS, and hIST data in the write
 3755 structure.
 3756 
 3757 This function only affects data that has already been allocated.
 3758 You can call this function before calling after the png_set_*() functions
 3759 to control whether the user or png_destroy_*() is supposed to free the data.
 3760 When the user assumes responsibility for libpng-allocated data, the
 3761 application must use
 3762 png_free() to free it, and when the user transfers responsibility to libpng
 3763 for data that the user has allocated, the user must have used png_malloc()
 3764 or png_calloc() to allocate it.
 3765 
 3766 If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword
 3767 separately, do not transfer responsibility for freeing text_ptr to libpng,
 3768 because when libpng fills a png_text structure it combines these members with
 3769 the key member, and png_free_data() will free only text_ptr.key.  Similarly,
 3770 if you transfer responsibility for free'ing text_ptr from libpng to your
 3771 application, your application must not separately free those members.
 3772 For a more compact example of writing a PNG image, see the file example.c.
 3773 
 3774 V. Simplified API
 3775 
 3776 The simplified API, which became available in libpng-1.6.0, hides the details
 3777 of both libpng and the PNG file format itself.
 3778 It allows PNG files to be read into a very limited number of
 3779 in-memory bitmap formats or to be written from the same formats.  If these
 3780 formats do not accommodate your needs then you can, and should, use the more
 3781 sophisticated APIs above - these support a wide variety of in-memory formats
 3782 and a wide variety of sophisticated transformations to those formats as well
 3783 as a wide variety of APIs to manipulate ancillary information.
 3784 
 3785 To read a PNG file using the simplified API:
 3786 
 3787   1) Declare a 'png_image' structure (see below) on the stack, set the
 3788      version field to PNG_IMAGE_VERSION and the 'opaque' pointer to NULL
 3789      (this is REQUIRED, your program may crash if you don't do it.)
 3790 
 3791   2) Call the appropriate png_image_begin_read... function.
 3792 
 3793   3) Set the png_image 'format' member to the required sample format.
 3794 
 3795   4) Allocate a buffer for the image and, if required, the color-map.
 3796 
 3797   5) Call png_image_finish_read to read the image and, if required, the
 3798      color-map into your buffers.
 3799 
 3800 There are no restrictions on the format of the PNG input itself; all valid
 3801 color types, bit depths, and interlace methods are acceptable, and the
 3802 input image is transformed as necessary to the requested in-memory format
 3803 during the png_image_finish_read() step.  The only caveat is that if you
 3804 request a color-mapped image from a PNG that is full-color or makes
 3805 complex use of an alpha channel the transformation is extremely lossy and the
 3806 result may look terrible.
 3807 
 3808 To write a PNG file using the simplified API:
 3809 
 3810   1) Declare a 'png_image' structure on the stack and memset()
 3811      it to all zero.
 3812 
 3813   2) Initialize the members of the structure that describe the
 3814      image, setting the 'format' member to the format of the
 3815      image samples.
 3816 
 3817   3) Call the appropriate png_image_write... function with a
 3818      pointer to the image and, if necessary, the color-map to write
 3819      the PNG data.
 3820 
 3821 png_image is a structure that describes the in-memory format of an image
 3822 when it is being read or defines the in-memory format of an image that you
 3823 need to write.  The "png_image" structure contains the following members:
 3824 
 3825    png_controlp opaque  Initialize to NULL, free with png_image_free
 3826    png_uint_32  version Set to PNG_IMAGE_VERSION
 3827    png_uint_32  width   Image width in pixels (columns)
 3828    png_uint_32  height  Image height in pixels (rows)
 3829    png_uint_32  format  Image format as defined below
 3830    png_uint_32  flags   A bit mask containing informational flags
 3831    png_uint_32  colormap_entries; Number of entries in the color-map
 3832    png_uint_32  warning_or_error;
 3833    char         message[64];
 3834 
 3835 In the event of an error or warning the "warning_or_error"
 3836 field will be set to a non-zero value and the 'message' field will contain
 3837 a '\0' terminated string with the libpng error or warning message.  If both
 3838 warnings and an error were encountered, only the error is recorded.  If there
 3839 are multiple warnings, only the first one is recorded.
 3840 
 3841 The upper 30 bits of the "warning_or_error" value are reserved; the low two
 3842 bits contain a two bit code such that a value more than 1 indicates a failure
 3843 in the API just called:
 3844 
 3845    0 - no warning or error
 3846    1 - warning
 3847    2 - error
 3848    3 - error preceded by warning
 3849 
 3850 The pixels (samples) of the image have one to four channels whose components
 3851 have original values in the range 0 to 1.0:
 3852 
 3853   1: A single gray or luminance channel (G).
 3854   2: A gray/luminance channel and an alpha channel (GA).
 3855   3: Three red, green, blue color channels (RGB).
 3856   4: Three color channels and an alpha channel (RGBA).
 3857 
 3858 The channels are encoded in one of two ways:
 3859 
 3860   a) As a small integer, value 0..255, contained in a single byte.  For the
 3861 alpha channel the original value is simply value/255.  For the color or
 3862 luminance channels the value is encoded according to the sRGB specification
 3863 and matches the 8-bit format expected by typical display devices.
 3864 
 3865 The color/gray channels are not scaled (pre-multiplied) by the alpha
 3866 channel and are suitable for passing to color management software.
 3867 
 3868   b) As a value in the range 0..65535, contained in a 2-byte integer, in
 3869 the native byte order of the platform on which the application is running.
 3870 All channels can be converted to the original value by dividing by 65535; all
 3871 channels are linear.  Color channels use the RGB encoding (RGB end-points) of
 3872 the sRGB specification.  This encoding is identified by the
 3873 PNG_FORMAT_FLAG_LINEAR flag below.
 3874 
 3875 When the simplified API needs to convert between sRGB and linear colorspaces,
 3876 the actual sRGB transfer curve defined in the sRGB specification (see the
 3877 article at https://en.wikipedia.org/wiki/SRGB) is used, not the gamma=1/2.2
 3878 approximation used elsewhere in libpng.
 3879 
 3880 When an alpha channel is present it is expected to denote pixel coverage
 3881 of the color or luminance channels and is returned as an associated alpha
 3882 channel: the color/gray channels are scaled (pre-multiplied) by the alpha
 3883 value.
 3884 
 3885 The samples are either contained directly in the image data, between 1 and 8
 3886 bytes per pixel according to the encoding, or are held in a color-map indexed
 3887 by bytes in the image data.  In the case of a color-map the color-map entries
 3888 are individual samples, encoded as above, and the image data has one byte per
 3889 pixel to select the relevant sample from the color-map.
 3890 
 3891 PNG_FORMAT_*
 3892 
 3893 The #defines to be used in png_image::format.  Each #define identifies a
 3894 particular layout of channel data and, if present, alpha values.  There are
 3895 separate defines for each of the two component encodings.
 3896 
 3897 A format is built up using single bit flag values.  All combinations are
 3898 valid.  Formats can be built up from the flag values or you can use one of
 3899 the predefined values below.  When testing formats always use the FORMAT_FLAG
 3900 macros to test for individual features - future versions of the library may
 3901 add new flags.
 3902 
 3903 When reading or writing color-mapped images the format should be set to the
 3904 format of the entries in the color-map then png_image_{read,write}_colormap
 3905 called to read or write the color-map and set the format correctly for the
 3906 image data.  Do not set the PNG_FORMAT_FLAG_COLORMAP bit directly!
 3907 
 3908 NOTE: libpng can be built with particular features disabled. If you see
 3909 compiler errors because the definition of one of the following flags has been
 3910 compiled out it is because libpng does not have the required support.  It is
 3911 possible, however, for the libpng configuration to enable the format on just
 3912 read or just write; in that case you may see an error at run time.
 3913 You can guard against this by checking for the definition of the
 3914 appropriate "_SUPPORTED" macro, one of:
 3915 
 3916    PNG_SIMPLIFIED_{READ,WRITE}_{BGR,AFIRST}_SUPPORTED
 3917 
 3918    PNG_FORMAT_FLAG_ALPHA    format with an alpha channel
 3919    PNG_FORMAT_FLAG_COLOR    color format: otherwise grayscale
 3920    PNG_FORMAT_FLAG_LINEAR   2-byte channels else 1-byte
 3921    PNG_FORMAT_FLAG_COLORMAP image data is color-mapped
 3922    PNG_FORMAT_FLAG_BGR      BGR colors, else order is RGB
 3923    PNG_FORMAT_FLAG_AFIRST   alpha channel comes first
 3924 
 3925 Supported formats are as follows.  Future versions of libpng may support more
 3926 formats; for compatibility with older versions simply check if the format
 3927 macro is defined using #ifdef.  These defines describe the in-memory layout
 3928 of the components of the pixels of the image.
 3929 
 3930 First the single byte (sRGB) formats:
 3931 
 3932    PNG_FORMAT_GRAY
 3933    PNG_FORMAT_GA
 3934    PNG_FORMAT_AG
 3935    PNG_FORMAT_RGB
 3936    PNG_FORMAT_BGR
 3937    PNG_FORMAT_RGBA
 3938    PNG_FORMAT_ARGB
 3939    PNG_FORMAT_BGRA
 3940    PNG_FORMAT_ABGR
 3941 
 3942 Then the linear 2-byte formats.  When naming these "Y" is used to
 3943 indicate a luminance (gray) channel.  The component order within the pixel
 3944 is always the same - there is no provision for swapping the order of the
 3945 components in the linear format.  The components are 16-bit integers in
 3946 the native byte order for your platform, and there is no provision for
 3947 swapping the bytes to a different endian condition.
 3948 
 3949    PNG_FORMAT_LINEAR_Y
 3950    PNG_FORMAT_LINEAR_Y_ALPHA
 3951    PNG_FORMAT_LINEAR_RGB
 3952    PNG_FORMAT_LINEAR_RGB_ALPHA
 3953 
 3954 With color-mapped formats the image data is one byte for each pixel. The byte
 3955 is an index into the color-map which is formatted as above.  To obtain a
 3956 color-mapped format it is sufficient just to add the PNG_FOMAT_FLAG_COLORMAP
 3957 to one of the above definitions, or you can use one of the definitions below.
 3958 
 3959    PNG_FORMAT_RGB_COLORMAP
 3960    PNG_FORMAT_BGR_COLORMAP
 3961    PNG_FORMAT_RGBA_COLORMAP
 3962    PNG_FORMAT_ARGB_COLORMAP
 3963    PNG_FORMAT_BGRA_COLORMAP
 3964    PNG_FORMAT_ABGR_COLORMAP
 3965 
 3966 PNG_IMAGE macros
 3967 
 3968 These are convenience macros to derive information from a png_image
 3969 structure.  The PNG_IMAGE_SAMPLE_ macros return values appropriate to the
 3970 actual image sample values - either the entries in the color-map or the
 3971 pixels in the image.  The PNG_IMAGE_PIXEL_ macros return corresponding values
 3972 for the pixels and will always return 1 for color-mapped formats.  The
 3973 remaining macros return information about the rows in the image and the
 3974 complete image.
 3975 
 3976 NOTE: All the macros that take a png_image::format parameter are compile time
 3977 constants if the format parameter is, itself, a constant.  Therefore these
 3978 macros can be used in array declarations and case labels where required.
 3979 Similarly the macros are also pre-processor constants (sizeof is not used) so
 3980 they can be used in #if tests.
 3981 
 3982   PNG_IMAGE_SAMPLE_CHANNELS(fmt)
 3983     Returns the total number of channels in a given format: 1..4
 3984 
 3985   PNG_IMAGE_SAMPLE_COMPONENT_SIZE(fmt)
 3986     Returns the size in bytes of a single component of a pixel or color-map
 3987     entry (as appropriate) in the image: 1 or 2.
 3988 
 3989   PNG_IMAGE_SAMPLE_SIZE(fmt)
 3990     This is the size of the sample data for one sample.  If the image is
 3991     color-mapped it is the size of one color-map entry (and image pixels are
 3992     one byte in size), otherwise it is the size of one image pixel.
 3993 
 3994   PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(fmt)
 3995     The maximum size of the color-map required by the format expressed in a
 3996     count of components.  This can be used to compile-time allocate a
 3997     color-map:
 3998 
 3999     png_uint_16 colormap[PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(linear_fmt)];
 4000 
 4001     png_byte colormap[PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(sRGB_fmt)];
 4002 
 4003     Alternatively use the PNG_IMAGE_COLORMAP_SIZE macro below to use the
 4004     information from one of the png_image_begin_read_ APIs and dynamically
 4005     allocate the required memory.
 4006 
 4007   PNG_IMAGE_COLORMAP_SIZE(fmt)
 4008    The size of the color-map required by the format; this is the size of the
 4009    color-map buffer passed to the png_image_{read,write}_colormap APIs. It is
 4010    a fixed number determined by the format so can easily be allocated on the
 4011    stack if necessary.
 4012 
 4013 Corresponding information about the pixels
 4014 
 4015   PNG_IMAGE_PIXEL_CHANNELS(fmt)
 4016    The number of separate channels (components) in a pixel; 1 for a
 4017    color-mapped image.
 4018 
 4019   PNG_IMAGE_PIXEL_COMPONENT_SIZE(fmt)\
 4020    The size, in bytes, of each component in a pixel; 1 for a color-mapped
 4021    image.
 4022 
 4023   PNG_IMAGE_PIXEL_SIZE(fmt)
 4024    The size, in bytes, of a complete pixel; 1 for a color-mapped image.
 4025 
 4026 Information about the whole row, or whole image
 4027 
 4028   PNG_IMAGE_ROW_STRIDE(image)
 4029    Returns the total number of components in a single row of the image; this
 4030    is the minimum 'row stride', the minimum count of components between each
 4031    row.  For a color-mapped image this is the minimum number of bytes in a
 4032    row.
 4033 
 4034    If you need the stride measured in bytes, row_stride_bytes is
 4035    PNG_IMAGE_ROW_STRIDE(image) * PNG_IMAGE_PIXEL_COMPONENT_SIZE(fmt)
 4036    plus any padding bytes that your application might need, for example
 4037    to start the next row on a 4-byte boundary.
 4038 
 4039   PNG_IMAGE_BUFFER_SIZE(image, row_stride)
 4040    Return the size, in bytes, of an image buffer given a png_image and a row
 4041    stride - the number of components to leave space for in each row.
 4042 
 4043   PNG_IMAGE_SIZE(image)
 4044    Return the size, in bytes, of the image in memory given just a png_image;
 4045    the row stride is the minimum stride required for the image.
 4046 
 4047   PNG_IMAGE_COLORMAP_SIZE(image)
 4048    Return the size, in bytes, of the color-map of this image.  If the image
 4049    format is not a color-map format this will return a size sufficient for
 4050    256 entries in the given format; check PNG_FORMAT_FLAG_COLORMAP if
 4051    you don't want to allocate a color-map in this case.
 4052 
 4053 PNG_IMAGE_FLAG_*
 4054 
 4055 Flags containing additional information about the image are held in
 4056 the 'flags' field of png_image.
 4057 
 4058   PNG_IMAGE_FLAG_COLORSPACE_NOT_sRGB == 0x01
 4059     This indicates that the RGB values of the in-memory bitmap do not
 4060     correspond to the red, green and blue end-points defined by sRGB.
 4061 
 4062   PNG_IMAGE_FLAG_FAST == 0x02
 4063    On write emphasise speed over compression; the resultant PNG file will be
 4064    larger but will be produced significantly faster, particular for large
 4065    images.  Do not use this option for images which will be distributed, only
 4066    used it when producing intermediate files that will be read back in
 4067    repeatedly.  For a typical 24-bit image the option will double the read
 4068    speed at the cost of increasing the image size by 25%, however for many
 4069    more compressible images the PNG file can be 10 times larger with only a
 4070    slight speed gain.
 4071 
 4072   PNG_IMAGE_FLAG_16BIT_sRGB == 0x04
 4073     On read if the image is a 16-bit per component image and there is no gAMA
 4074     or sRGB chunk assume that the components are sRGB encoded.  Notice that
 4075     images output by the simplified API always have gamma information; setting
 4076     this flag only affects the interpretation of 16-bit images from an
 4077     external source.  It is recommended that the application expose this flag
 4078     to the user; the user can normally easily recognize the difference between
 4079     linear and sRGB encoding.  This flag has no effect on write - the data
 4080     passed to the write APIs must have the correct encoding (as defined
 4081     above.)
 4082 
 4083     If the flag is not set (the default) input 16-bit per component data is
 4084     assumed to be linear.
 4085 
 4086     NOTE: the flag can only be set after the png_image_begin_read_ call,
 4087     because that call initializes the 'flags' field.
 4088 
 4089 READ APIs
 4090 
 4091    The png_image passed to the read APIs must have been initialized by setting
 4092    the png_controlp field 'opaque' to NULL (or, better, memset the whole thing.)
 4093 
 4094    int png_image_begin_read_from_file( png_imagep image,
 4095      const char *file_name)
 4096 
 4097      The named file is opened for read and the image header
 4098      is filled in from the PNG header in the file.
 4099 
 4100    int png_image_begin_read_from_stdio (png_imagep image,
 4101      FILE* file)
 4102 
 4103       The PNG header is read from the stdio FILE object.
 4104 
 4105    int png_image_begin_read_from_memory(png_imagep image,
 4106       png_const_voidp memory, size_t size)
 4107 
 4108       The PNG header is read from the given memory buffer.
 4109 
 4110    int png_image_finish_read(png_imagep image,
 4111       png_colorp background, void *buffer,
 4112       png_int_32 row_stride, void *colormap));
 4113 
 4114       Finish reading the image into the supplied buffer and
 4115       clean up the png_image structure.
 4116 
 4117       row_stride is the step, in png_byte or png_uint_16 units
 4118       as appropriate, between adjacent rows.  A positive stride
 4119       indicates that the top-most row is first in the buffer -
 4120       the normal top-down arrangement.  A negative stride
 4121       indicates that the bottom-most row is first in the buffer.
 4122 
 4123       background need only be supplied if an alpha channel must
 4124       be removed from a png_byte format and the removal is to be
 4125       done by compositing on a solid color; otherwise it may be
 4126       NULL and any composition will be done directly onto the
 4127       buffer.  The value is an sRGB color to use for the
 4128       background, for grayscale output the green channel is used.
 4129 
 4130       For linear output removing the alpha channel is always done
 4131       by compositing on black.
 4132 
 4133    void png_image_free(png_imagep image)
 4134 
 4135       Free any data allocated by libpng in image->opaque,
 4136       setting the pointer to NULL.  May be called at any time
 4137       after the structure is initialized.
 4138 
 4139 When the simplified API needs to convert between sRGB and linear colorspaces,
 4140 the actual sRGB transfer curve defined in the sRGB specification (see the
 4141 article at https://en.wikipedia.org/wiki/SRGB) is used, not the gamma=1/2.2
 4142 approximation used elsewhere in libpng.
 4143 
 4144 WRITE APIS
 4145 
 4146 For write you must initialize a png_image structure to describe the image to
 4147 be written:
 4148 
 4149    version: must be set to PNG_IMAGE_VERSION
 4150    opaque: must be initialized to NULL
 4151    width: image width in pixels
 4152    height: image height in rows
 4153    format: the format of the data you wish to write
 4154    flags: set to 0 unless one of the defined flags applies; set
 4155       PNG_IMAGE_FLAG_COLORSPACE_NOT_sRGB for color format images
 4156       where the RGB values do not correspond to the colors in sRGB.
 4157    colormap_entries: set to the number of entries in the color-map (0 to 256)
 4158 
 4159    int png_image_write_to_file, (png_imagep image,
 4160       const char *file, int convert_to_8bit, const void *buffer,
 4161       png_int_32 row_stride, const void *colormap));
 4162 
 4163       Write the image to the named file.
 4164 
 4165    int png_image_write_to_memory (png_imagep image, void *memory,
 4166       png_alloc_size_t * PNG_RESTRICT memory_bytes,
 4167       int convert_to_8_bit, const void *buffer, ptrdiff_t row_stride,
 4168       const void *colormap));
 4169 
 4170       Write the image to memory.
 4171 
 4172    int png_image_write_to_stdio(png_imagep image, FILE *file,
 4173       int convert_to_8_bit, const void *buffer,
 4174       png_int_32 row_stride, const void *colormap)
 4175 
 4176       Write the image to the given (FILE*).
 4177 
 4178 With all write APIs if image is in one of the linear formats with
 4179 (png_uint_16) data then setting convert_to_8_bit will cause the output to be
 4180 a (png_byte) PNG gamma encoded according to the sRGB specification, otherwise
 4181 a 16-bit linear encoded PNG file is written.
 4182 
 4183 With all APIs row_stride is handled as in the read APIs - it is the spacing
 4184 from one row to the next in component sized units (float) and if negative
 4185 indicates a bottom-up row layout in the buffer.  If you pass zero, libpng will
 4186 calculate the row_stride for you from the width and number of channels.
 4187 
 4188 Note that the write API does not support interlacing, sub-8-bit pixels,
 4189 indexed (paletted) images, or most ancillary chunks.
 4190 
 4191 VI. Modifying/Customizing libpng
 4192 
 4193 There are two issues here.  The first is changing how libpng does
 4194 standard things like memory allocation, input/output, and error handling.
 4195 The second deals with more complicated things like adding new chunks,
 4196 adding new transformations, and generally changing how libpng works.
 4197 Both of those are compile-time issues; that is, they are generally
 4198 determined at the time the code is written, and there is rarely a need
 4199 to provide the user with a means of changing them.
 4200 
 4201 Memory allocation, input/output, and error handling
 4202 
 4203 All of the memory allocation, input/output, and error handling in libpng
 4204 goes through callbacks that are user-settable.  The default routines are
 4205 in pngmem.c, pngrio.c, pngwio.c, and pngerror.c, respectively.  To change
 4206 these functions, call the appropriate png_set_*_fn() function.
 4207 
 4208 Memory allocation is done through the functions png_malloc(), png_calloc(),
 4209 and png_free().  The png_malloc() and png_free() functions currently just
 4210 call the standard C functions and png_calloc() calls png_malloc() and then
 4211 clears the newly allocated memory to zero; note that png_calloc(png_ptr, size)
 4212 is not the same as the calloc(number, size) function provided by stdlib.h.
 4213 There is limited support for certain systems with segmented memory
 4214 architectures and the types of pointers declared by png.h match this; you
 4215 will have to use appropriate pointers in your application.  If you prefer
 4216 to use a different method of allocating and freeing data, you can use
 4217 png_create_read_struct_2() or png_create_write_struct_2() to register your
 4218 own functions as described above.  These functions also provide a void
 4219 pointer that can be retrieved via
 4220 
 4221     mem_ptr=png_get_mem_ptr(png_ptr);
 4222 
 4223 Your replacement memory functions must have prototypes as follows:
 4224 
 4225     png_voidp malloc_fn(png_structp png_ptr,
 4226        png_alloc_size_t size);
 4227 
 4228     void free_fn(png_structp png_ptr, png_voidp ptr);
 4229 
 4230 Your malloc_fn() must return NULL in case of failure.  The png_malloc()
 4231 function will normally call png_error() if it receives a NULL from the
 4232 system memory allocator or from your replacement malloc_fn().
 4233 
 4234 Your free_fn() will never be called with a NULL ptr, since libpng's
 4235 png_free() checks for NULL before calling free_fn().
 4236 
 4237 Input/Output in libpng is done through png_read() and png_write(),
 4238 which currently just call fread() and fwrite().  The FILE * is stored in
 4239 png_struct and is initialized via png_init_io().  If you wish to change
 4240 the method of I/O, the library supplies callbacks that you can set
 4241 through the function png_set_read_fn() and png_set_write_fn() at run
 4242 time, instead of calling the png_init_io() function.  These functions
 4243 also provide a void pointer that can be retrieved via the function
 4244 png_get_io_ptr().  For example:
 4245 
 4246     png_set_read_fn(png_structp read_ptr,
 4247         voidp read_io_ptr, png_rw_ptr read_data_fn)
 4248 
 4249     png_set_write_fn(png_structp write_ptr,
 4250         voidp write_io_ptr, png_rw_ptr write_data_fn,
 4251         png_flush_ptr output_flush_fn);
 4252 
 4253     voidp read_io_ptr = png_get_io_ptr(read_ptr);
 4254     voidp write_io_ptr = png_get_io_ptr(write_ptr);
 4255 
 4256 The replacement I/O functions must have prototypes as follows:
 4257 
 4258     void user_read_data(png_structp png_ptr,
 4259         png_bytep data, size_t length);
 4260 
 4261     void user_write_data(png_structp png_ptr,
 4262         png_bytep data, size_t length);
 4263 
 4264     void user_flush_data(png_structp png_ptr);
 4265 
 4266 The user_read_data() function is responsible for detecting and
 4267 handling end-of-data errors.
 4268 
 4269 Supplying NULL for the read, write, or flush functions sets them back
 4270 to using the default C stream functions, which expect the io_ptr to
 4271 point to a standard *FILE structure.  It is probably a mistake
 4272 to use NULL for one of write_data_fn and output_flush_fn but not both
 4273 of them, unless you have built libpng with PNG_NO_WRITE_FLUSH defined.
 4274 It is an error to read from a write stream, and vice versa.
 4275 
 4276 Error handling in libpng is done through png_error() and png_warning().
 4277 Errors handled through png_error() are fatal, meaning that png_error()
 4278 should never return to its caller.  Currently, this is handled via
 4279 setjmp() and longjmp() (unless you have compiled libpng with
 4280 PNG_NO_SETJMP, in which case it is handled via PNG_ABORT()),
 4281 but you could change this to do things like exit() if you should wish,
 4282 as long as your function does not return.
 4283 
 4284 On non-fatal errors, png_warning() is called
 4285 to print a warning message, and then control returns to the calling code.
 4286 By default png_error() and png_warning() print a message on stderr via
 4287 fprintf() unless the library is compiled with PNG_NO_CONSOLE_IO defined
 4288 (because you don't want the messages) or PNG_NO_STDIO defined (because
 4289 fprintf() isn't available).  If you wish to change the behavior of the error
 4290 functions, you will need to set up your own message callbacks.  These
 4291 functions are normally supplied at the time that the png_struct is created.
 4292 It is also possible to redirect errors and warnings to your own replacement
 4293 functions after png_create_*_struct() has been called by calling:
 4294 
 4295     png_set_error_fn(png_structp png_ptr,
 4296         png_voidp error_ptr, png_error_ptr error_fn,
 4297         png_error_ptr warning_fn);
 4298 
 4299 If NULL is supplied for either error_fn or warning_fn, then the libpng
 4300 default function will be used, calling fprintf() and/or longjmp() if a
 4301 problem is encountered.  The replacement error functions should have
 4302 parameters as follows:
 4303 
 4304     void user_error_fn(png_structp png_ptr,
 4305         png_const_charp error_msg);
 4306 
 4307     void user_warning_fn(png_structp png_ptr,
 4308         png_const_charp warning_msg);
 4309 
 4310 Then, within your user_error_fn or user_warning_fn, you can retrieve
 4311 the error_ptr if you need it, by calling
 4312 
 4313     png_voidp error_ptr = png_get_error_ptr(png_ptr);
 4314 
 4315 The motivation behind using setjmp() and longjmp() is the C++ throw and
 4316 catch exception handling methods.  This makes the code much easier to write,
 4317 as there is no need to check every return code of every function call.
 4318 However, there are some uncertainties about the status of local variables
 4319 after a longjmp, so the user may want to be careful about doing anything
 4320 after setjmp returns non-zero besides returning itself.  Consult your
 4321 compiler documentation for more details.  For an alternative approach, you
 4322 may wish to use the "cexcept" facility (see https://cexcept.sourceforge.io/),
 4323 which is illustrated in pngvalid.c and in contrib/visupng.
 4324 
 4325 Beginning in libpng-1.4.0, the png_set_benign_errors() API became available.
 4326 You can use this to handle certain errors (normally handled as errors)
 4327 as warnings.
 4328 
 4329     png_set_benign_errors (png_ptr, int allowed);
 4330 
 4331     allowed: 0: treat png_benign_error() as an error.
 4332              1: treat png_benign_error() as a warning.
 4333 
 4334 As of libpng-1.6.0, the default condition is to treat benign errors as
 4335 warnings while reading and as errors while writing.
 4336 
 4337 Custom chunks
 4338 
 4339 If you need to read or write custom chunks, you may need to get deeper
 4340 into the libpng code.  The library now has mechanisms for storing
 4341 and writing chunks of unknown type; you can even declare callbacks
 4342 for custom chunks.  However, this may not be good enough if the
 4343 library code itself needs to know about interactions between your
 4344 chunk and existing `intrinsic' chunks.
 4345 
 4346 If you need to write a new intrinsic chunk, first read the PNG
 4347 specification. Acquire a first level of understanding of how it works.
 4348 Pay particular attention to the sections that describe chunk names,
 4349 and look at how other chunks were designed, so you can do things
 4350 similarly.  Second, check out the sections of libpng that read and
 4351 write chunks.  Try to find a chunk that is similar to yours and use
 4352 it as a template.  More details can be found in the comments inside
 4353 the code.  It is best to handle private or unknown chunks in a generic method,
 4354 via callback functions, instead of by modifying libpng functions. This
 4355 is illustrated in pngtest.c, which uses a callback function to handle a
 4356 private "vpAg" chunk and the new "sTER" chunk, which are both unknown to
 4357 libpng.
 4358 
 4359 If you wish to write your own transformation for the data, look through
 4360 the part of the code that does the transformations, and check out some of
 4361 the simpler ones to get an idea of how they work.  Try to find a similar
 4362 transformation to the one you want to add and copy off of it.  More details
 4363 can be found in the comments inside the code itself.
 4364 
 4365 Configuring for gui/windowing platforms:
 4366 
 4367 You will need to write new error and warning functions that use the GUI
 4368 interface, as described previously, and set them to be the error and
 4369 warning functions at the time that png_create_*_struct() is called,
 4370 in order to have them available during the structure initialization.
 4371 They can be changed later via png_set_error_fn().  On some compilers,
 4372 you may also have to change the memory allocators (png_malloc, etc.).
 4373 
 4374 Configuring zlib:
 4375 
 4376 There are special functions to configure the compression.  Perhaps the
 4377 most useful one changes the compression level, which currently uses
 4378 input compression values in the range 0 - 9.  The library normally
 4379 uses the default compression level (Z_DEFAULT_COMPRESSION = 6).  Tests
 4380 have shown that for a large majority of images, compression values in
 4381 the range 3-6 compress nearly as well as higher levels, and do so much
 4382 faster.  For online applications it may be desirable to have maximum speed
 4383 (Z_BEST_SPEED = 1).  With versions of zlib after v0.99, you can also
 4384 specify no compression (Z_NO_COMPRESSION = 0), but this would create
 4385 files larger than just storing the raw bitmap.  You can specify the
 4386 compression level by calling:
 4387 
 4388     #include zlib.h
 4389     png_set_compression_level(png_ptr, level);
 4390 
 4391 Another useful one is to reduce the memory level used by the library.
 4392 The memory level defaults to 8, but it can be lowered if you are
 4393 short on memory (running DOS, for example, where you only have 640K).
 4394 Note that the memory level does have an effect on compression; among
 4395 other things, lower levels will result in sections of incompressible
 4396 data being emitted in smaller stored blocks, with a correspondingly
 4397 larger relative overhead of up to 15% in the worst case.
 4398 
 4399     #include zlib.h
 4400     png_set_compression_mem_level(png_ptr, level);
 4401 
 4402 The other functions are for configuring zlib.  They are not recommended
 4403 for normal use and may result in writing an invalid PNG file.  See
 4404 zlib.h for more information on what these mean.
 4405 
 4406     #include zlib.h
 4407     png_set_compression_strategy(png_ptr,
 4408         strategy);
 4409 
 4410     png_set_compression_window_bits(png_ptr,
 4411         window_bits);
 4412 
 4413     png_set_compression_method(png_ptr, method);
 4414 
 4415 This controls the size of the IDAT chunks (default 8192):
 4416 
 4417     png_set_compression_buffer_size(png_ptr, size);
 4418 
 4419 As of libpng version 1.5.4, additional APIs became
 4420 available to set these separately for non-IDAT
 4421 compressed chunks such as zTXt, iTXt, and iCCP:
 4422 
 4423     #include zlib.h
 4424     #if PNG_LIBPNG_VER >= 10504
 4425     png_set_text_compression_level(png_ptr, level);
 4426 
 4427     png_set_text_compression_mem_level(png_ptr, level);
 4428 
 4429     png_set_text_compression_strategy(png_ptr,
 4430         strategy);
 4431 
 4432     png_set_text_compression_window_bits(png_ptr,
 4433         window_bits);
 4434 
 4435     png_set_text_compression_method(png_ptr, method);
 4436     #endif
 4437 
 4438 Controlling row filtering
 4439 
 4440 If you want to control whether libpng uses filtering or not, which
 4441 filters are used, and how it goes about picking row filters, you
 4442 can call one of these functions.  The selection and configuration
 4443 of row filters can have a significant impact on the size and
 4444 encoding speed and a somewhat lesser impact on the decoding speed
 4445 of an image.  Filtering is enabled by default for RGB and grayscale
 4446 images (with and without alpha), but not for paletted images nor
 4447 for any images with bit depths less than 8 bits/pixel.
 4448 
 4449 The 'method' parameter sets the main filtering method, which is
 4450 currently only '0' in the PNG 1.2 specification.  The 'filters'
 4451 parameter sets which filter(s), if any, should be used for each
 4452 scanline.  Possible values are PNG_ALL_FILTERS, PNG_NO_FILTERS,
 4453 or PNG_FAST_FILTERS to turn filtering on and off, or to turn on
 4454 just the fast-decoding subset of filters, respectively.
 4455 
 4456 Individual filter types are PNG_FILTER_NONE, PNG_FILTER_SUB,
 4457 PNG_FILTER_UP, PNG_FILTER_AVG, PNG_FILTER_PAETH, which can be bitwise
 4458 ORed together with '|' to specify one or more filters to use.
 4459 These filters are described in more detail in the PNG specification.
 4460 If you intend to change the filter type during the course of writing
 4461 the image, you should start with flags set for all of the filters
 4462 you intend to use so that libpng can initialize its internal
 4463 structures appropriately for all of the filter types.  (Note that this
 4464 means the first row must always be adaptively filtered, because libpng
 4465 currently does not allocate the filter buffers until png_write_row()
 4466 is called for the first time.)
 4467 
 4468     filters = PNG_NO_FILTERS;
 4469     filters = PNG_ALL_FILTERS;
 4470     filters = PNG_FAST_FILTERS;
 4471 
 4472     or
 4473 
 4474     filters = PNG_FILTER_NONE | PNG_FILTER_SUB |
 4475               PNG_FILTER_UP | PNG_FILTER_AVG |
 4476               PNG_FILTER_PAETH;
 4477 
 4478     png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE,
 4479        filters);
 4480 
 4481               The second parameter can also be
 4482               PNG_INTRAPIXEL_DIFFERENCING if you are
 4483               writing a PNG to be embedded in a MNG
 4484               datastream.  This parameter must be the
 4485               same as the value of filter_method used
 4486               in png_set_IHDR().
 4487 
 4488 Requesting debug printout
 4489 
 4490 The macro definition PNG_DEBUG can be used to request debugging
 4491 printout.  Set it to an integer value in the range 0 to 3.  Higher
 4492 numbers result in increasing amounts of debugging information.  The
 4493 information is printed to the "stderr" file, unless another file
 4494 name is specified in the PNG_DEBUG_FILE macro definition.
 4495 
 4496 When PNG_DEBUG > 0, the following functions (macros) become available:
 4497 
 4498    png_debug(level, message)
 4499    png_debug1(level, message, p1)
 4500    png_debug2(level, message, p1, p2)
 4501 
 4502 in which "level" is compared to PNG_DEBUG to decide whether to print
 4503 the message, "message" is the formatted string to be printed,
 4504 and p1 and p2 are parameters that are to be embedded in the string
 4505 according to printf-style formatting directives.  For example,
 4506 
 4507    png_debug1(2, "foo=%d", foo);
 4508 
 4509 is expanded to
 4510 
 4511    if (PNG_DEBUG > 2)
 4512       fprintf(PNG_DEBUG_FILE, "foo=%d\n", foo);
 4513 
 4514 When PNG_DEBUG is defined but is zero, the macros aren't defined, but you
 4515 can still use PNG_DEBUG to control your own debugging:
 4516 
 4517    #ifdef PNG_DEBUG
 4518        fprintf(stderr, ...
 4519    #endif
 4520 
 4521 When PNG_DEBUG = 1, the macros are defined, but only png_debug statements
 4522 having level = 0 will be printed.  There aren't any such statements in
 4523 this version of libpng, but if you insert some they will be printed.
 4524 
 4525 VII. MNG support
 4526 
 4527 The MNG specification (available at http://www.libpng.org/pub/mng) allows
 4528 certain extensions to PNG for PNG images that are embedded in MNG datastreams.
 4529 Libpng can support some of these extensions.  To enable them, use the
 4530 png_permit_mng_features() function:
 4531 
 4532    feature_set = png_permit_mng_features(png_ptr, mask)
 4533 
 4534    mask is a png_uint_32 containing the bitwise OR of the
 4535         features you want to enable.  These include
 4536         PNG_FLAG_MNG_EMPTY_PLTE
 4537         PNG_FLAG_MNG_FILTER_64
 4538         PNG_ALL_MNG_FEATURES
 4539 
 4540    feature_set is a png_uint_32 that is the bitwise AND of
 4541       your mask with the set of MNG features that is
 4542       supported by the version of libpng that you are using.
 4543 
 4544 It is an error to use this function when reading or writing a standalone
 4545 PNG file with the PNG 8-byte signature.  The PNG datastream must be wrapped
 4546 in a MNG datastream.  As a minimum, it must have the MNG 8-byte signature
 4547 and the MHDR and MEND chunks.  Libpng does not provide support for these
 4548 or any other MNG chunks; your application must provide its own support for
 4549 them.  You may wish to consider using libmng (available at
 4550 https://www.libmng.com/) instead.
 4551 
 4552 VIII. Changes to Libpng from version 0.88
 4553 
 4554 It should be noted that versions of libpng later than 0.96 are not
 4555 distributed by the original libpng author, Guy Schalnat, nor by
 4556 Andreas Dilger, who had taken over from Guy during 1996 and 1997, and
 4557 distributed versions 0.89 through 0.96, but rather by another member
 4558 of the original PNG Group, Glenn Randers-Pehrson.  Guy and Andreas are
 4559 still alive and well, but they have moved on to other things.
 4560 
 4561 The old libpng functions png_read_init(), png_write_init(),
 4562 png_info_init(), png_read_destroy(), and png_write_destroy() have been
 4563 moved to PNG_INTERNAL in version 0.95 to discourage their use.  These
 4564 functions will be removed from libpng version 1.4.0.
 4565 
 4566 The preferred method of creating and initializing the libpng structures is
 4567 via the png_create_read_struct(), png_create_write_struct(), and
 4568 png_create_info_struct() because they isolate the size of the structures
 4569 from the application, allow version error checking, and also allow the
 4570 use of custom error handling routines during the initialization, which
 4571 the old functions do not.  The functions png_read_destroy() and
 4572 png_write_destroy() do not actually free the memory that libpng
 4573 allocated for these structs, but just reset the data structures, so they
 4574 can be used instead of png_destroy_read_struct() and
 4575 png_destroy_write_struct() if you feel there is too much system overhead
 4576 allocating and freeing the png_struct for each image read.
 4577 
 4578 Setting the error callbacks via png_set_message_fn() before
 4579 png_read_init() as was suggested in libpng-0.88 is no longer supported
 4580 because this caused applications that do not use custom error functions
 4581 to fail if the png_ptr was not initialized to zero.  It is still possible
 4582 to set the error callbacks AFTER png_read_init(), or to change them with
 4583 png_set_error_fn(), which is essentially the same function, but with a new
 4584 name to force compilation errors with applications that try to use the old
 4585 method.
 4586 
 4587 Support for the sCAL, iCCP, iTXt, and sPLT chunks was added at libpng-1.0.6;
 4588 however, iTXt support was not enabled by default.
 4589 
 4590 Starting with version 1.0.7, you can find out which version of the library
 4591 you are using at run-time:
 4592 
 4593    png_uint_32 libpng_vn = png_access_version_number();
 4594 
 4595 The number libpng_vn is constructed from the major version, minor
 4596 version with leading zero, and release number with leading zero,
 4597 (e.g., libpng_vn for version 1.0.7 is 10007).
 4598 
 4599 Note that this function does not take a png_ptr, so you can call it
 4600 before you've created one.
 4601 
 4602 You can also check which version of png.h you used when compiling your
 4603 application:
 4604 
 4605    png_uint_32 application_vn = PNG_LIBPNG_VER;
 4606 
 4607 IX. Changes to Libpng from version 1.0.x to 1.2.x
 4608 
 4609 Support for user memory management was enabled by default.  To
 4610 accomplish this, the functions png_create_read_struct_2(),
 4611 png_create_write_struct_2(), png_set_mem_fn(), png_get_mem_ptr(),
 4612 png_malloc_default(), and png_free_default() were added.
 4613 
 4614 Support for the iTXt chunk has been enabled by default as of
 4615 version 1.2.41.
 4616 
 4617 Support for certain MNG features was enabled.
 4618 
 4619 Support for numbered error messages was added.  However, we never got
 4620 around to actually numbering the error messages.  The function
 4621 png_set_strip_error_numbers() was added (Note: the prototype for this
 4622 function was inadvertently removed from png.h in PNG_NO_ASSEMBLER_CODE
 4623 builds of libpng-1.2.15.  It was restored in libpng-1.2.36).
 4624 
 4625 The png_malloc_warn() function was added at libpng-1.2.3.  This issues
 4626 a png_warning and returns NULL instead of aborting when it fails to
 4627 acquire the requested memory allocation.
 4628 
 4629 Support for setting user limits on image width and height was enabled
 4630 by default.  The functions png_set_user_limits(), png_get_user_width_max(),
 4631 and png_get_user_height_max() were added at libpng-1.2.6.
 4632 
 4633 The png_set_add_alpha() function was added at libpng-1.2.7.
 4634 
 4635 The function png_set_expand_gray_1_2_4_to_8() was added at libpng-1.2.9.
 4636 Unlike png_set_gray_1_2_4_to_8(), the new function does not expand the
 4637 tRNS chunk to alpha. The png_set_gray_1_2_4_to_8() function is
 4638 deprecated.
 4639 
 4640 A number of macro definitions in support of runtime selection of
 4641 assembler code features (especially Intel MMX code support) were
 4642 added at libpng-1.2.0:
 4643 
 4644     PNG_ASM_FLAG_MMX_SUPPORT_COMPILED
 4645     PNG_ASM_FLAG_MMX_SUPPORT_IN_CPU
 4646     PNG_ASM_FLAG_MMX_READ_COMBINE_ROW
 4647     PNG_ASM_FLAG_MMX_READ_INTERLACE
 4648     PNG_ASM_FLAG_MMX_READ_FILTER_SUB
 4649     PNG_ASM_FLAG_MMX_READ_FILTER_UP
 4650     PNG_ASM_FLAG_MMX_READ_FILTER_AVG
 4651     PNG_ASM_FLAG_MMX_READ_FILTER_PAETH
 4652     PNG_ASM_FLAGS_INITIALIZED
 4653     PNG_MMX_READ_FLAGS
 4654     PNG_MMX_FLAGS
 4655     PNG_MMX_WRITE_FLAGS
 4656     PNG_MMX_FLAGS
 4657 
 4658 We added the following functions in support of runtime
 4659 selection of assembler code features:
 4660 
 4661     png_get_mmx_flagmask()
 4662     png_set_mmx_thresholds()
 4663     png_get_asm_flags()
 4664     png_get_mmx_bitdepth_threshold()
 4665     png_get_mmx_rowbytes_threshold()
 4666     png_set_asm_flags()
 4667 
 4668 We replaced all of these functions with simple stubs in libpng-1.2.20,
 4669 when the Intel assembler code was removed due to a licensing issue.
 4670 
 4671 These macros are deprecated:
 4672 
 4673     PNG_READ_TRANSFORMS_NOT_SUPPORTED
 4674     PNG_PROGRESSIVE_READ_NOT_SUPPORTED
 4675     PNG_NO_SEQUENTIAL_READ_SUPPORTED
 4676     PNG_WRITE_TRANSFORMS_NOT_SUPPORTED
 4677     PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED
 4678     PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED
 4679 
 4680 They have been replaced, respectively, by:
 4681 
 4682     PNG_NO_READ_TRANSFORMS
 4683     PNG_NO_PROGRESSIVE_READ
 4684     PNG_NO_SEQUENTIAL_READ
 4685     PNG_NO_WRITE_TRANSFORMS
 4686     PNG_NO_READ_ANCILLARY_CHUNKS
 4687     PNG_NO_WRITE_ANCILLARY_CHUNKS
 4688 
 4689 PNG_MAX_UINT was replaced with PNG_UINT_31_MAX.  It has been
 4690 deprecated since libpng-1.0.16 and libpng-1.2.6.
 4691 
 4692 The function
 4693     png_check_sig(sig, num)
 4694 was replaced with
 4695     !png_sig_cmp(sig, 0, num)
 4696 It has been deprecated since libpng-0.90.
 4697 
 4698 The function
 4699     png_set_gray_1_2_4_to_8()
 4700 which also expands tRNS to alpha was replaced with
 4701     png_set_expand_gray_1_2_4_to_8()
 4702 which does not. It has been deprecated since libpng-1.0.18 and 1.2.9.
 4703 
 4704 X. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x
 4705 
 4706 Private libpng prototypes and macro definitions were moved from
 4707 png.h and pngconf.h into a new pngpriv.h header file.
 4708 
 4709 Functions png_set_benign_errors(), png_benign_error(), and
 4710 png_chunk_benign_error() were added.
 4711 
 4712 Support for setting the maximum amount of memory that the application
 4713 will allocate for reading chunks was added, as a security measure.
 4714 The functions png_set_chunk_cache_max() and png_get_chunk_cache_max()
 4715 were added to the library.
 4716 
 4717 We implemented support for I/O states by adding png_ptr member io_state
 4718 and functions png_get_io_chunk_name() and png_get_io_state() in pngget.c
 4719 
 4720 We added PNG_TRANSFORM_GRAY_TO_RGB to the available high-level
 4721 input transforms.
 4722 
 4723 Checking for and reporting of errors in the IHDR chunk is more thorough.
 4724 
 4725 Support for global arrays was removed, to improve thread safety.
 4726 
 4727 Some obsolete/deprecated macros and functions have been removed.
 4728 
 4729 Typecasted NULL definitions such as
 4730    #define png_voidp_NULL            (png_voidp)NULL
 4731 were eliminated.  If you used these in your application, just use
 4732 NULL instead.
 4733 
 4734 The png_struct and info_struct members "trans" and "trans_values" were
 4735 changed to "trans_alpha" and "trans_color", respectively.
 4736 
 4737 The obsolete, unused pnggccrd.c and pngvcrd.c files and related makefiles
 4738 were removed.
 4739 
 4740 The PNG_1_0_X and PNG_1_2_X macros were eliminated.
 4741 
 4742 The PNG_LEGACY_SUPPORTED macro was eliminated.
 4743 
 4744 Many WIN32_WCE #ifdefs were removed.
 4745 
 4746 The functions png_read_init(info_ptr), png_write_init(info_ptr),
 4747 png_info_init(info_ptr), png_read_destroy(), and png_write_destroy()
 4748 have been removed.  They have been deprecated since libpng-0.95.
 4749 
 4750 The png_permit_empty_plte() was removed. It has been deprecated
 4751 since libpng-1.0.9.  Use png_permit_mng_features() instead.
 4752 
 4753 We removed the obsolete stub functions png_get_mmx_flagmask(),
 4754 png_set_mmx_thresholds(), png_get_asm_flags(),
 4755 png_get_mmx_bitdepth_threshold(), png_get_mmx_rowbytes_threshold(),
 4756 png_set_asm_flags(), and png_mmx_supported()
 4757 
 4758 We removed the obsolete png_check_sig(), png_memcpy_check(), and
 4759 png_memset_check() functions.  Instead use !png_sig_cmp(), memcpy(),
 4760 and memset(), respectively.
 4761 
 4762 The function png_set_gray_1_2_4_to_8() was removed. It has been
 4763 deprecated since libpng-1.0.18 and 1.2.9, when it was replaced with
 4764 png_set_expand_gray_1_2_4_to_8() because the former function also
 4765 expanded any tRNS chunk to an alpha channel.
 4766 
 4767 Macros for png_get_uint_16, png_get_uint_32, and png_get_int_32
 4768 were added and are used by default instead of the corresponding
 4769 functions. Unfortunately,
 4770 from libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro (but not the
 4771 function) incorrectly returned a value of type png_uint_32.
 4772 
 4773 We changed the prototype for png_malloc() from
 4774     png_malloc(png_structp png_ptr, png_uint_32 size)
 4775 to
 4776     png_malloc(png_structp png_ptr, png_alloc_size_t size)
 4777 
 4778 This also applies to the prototype for the user replacement malloc_fn().
 4779 
 4780 The png_calloc() function was added and is used in place of
 4781 of "png_malloc(); memset();" except in the case in png_read_png()
 4782 where the array consists of pointers; in this case a "for" loop is used
 4783 after the png_malloc() to set the pointers to NULL, to give robust.
 4784 behavior in case the application runs out of memory part-way through
 4785 the process.
 4786 
 4787 We changed the prototypes of png_get_compression_buffer_size() and
 4788 png_set_compression_buffer_size() to work with size_t instead of
 4789 png_uint_32.
 4790 
 4791 Support for numbered error messages was removed by default, since we
 4792 never got around to actually numbering the error messages. The function
 4793 png_set_strip_error_numbers() was removed from the library by default.
 4794 
 4795 The png_zalloc() and png_zfree() functions are no longer exported.
 4796 The png_zalloc() function no longer zeroes out the memory that it
 4797 allocates.  Applications that called png_zalloc(png_ptr, number, size)
 4798 can call png_calloc(png_ptr, number*size) instead, and can call
 4799 png_free() instead of png_zfree().
 4800 
 4801 Support for dithering was disabled by default in libpng-1.4.0, because
 4802 it has not been well tested and doesn't actually "dither".
 4803 The code was not
 4804 removed, however, and could be enabled by building libpng with
 4805 PNG_READ_DITHER_SUPPORTED defined.  In libpng-1.4.2, this support
 4806 was re-enabled, but the function was renamed png_set_quantize() to
 4807 reflect more accurately what it actually does.  At the same time,
 4808 the PNG_DITHER_[RED,GREEN_BLUE]_BITS macros were also renamed to
 4809 PNG_QUANTIZE_[RED,GREEN,BLUE]_BITS, and PNG_READ_DITHER_SUPPORTED
 4810 was renamed to PNG_READ_QUANTIZE_SUPPORTED.
 4811 
 4812 We removed the trailing '.' from the warning and error messages.
 4813 
 4814 XI. Changes to Libpng from version 1.4.x to 1.5.x
 4815 
 4816 From libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro (but not the
 4817 function) incorrectly returned a value of type png_uint_32.
 4818 The incorrect macro was removed from libpng-1.4.5.
 4819 
 4820 Checking for invalid palette index on write was added at libpng
 4821 1.5.10.  If a pixel contains an invalid (out-of-range) index libpng issues
 4822 a benign error.  This is enabled by default because this condition is an
 4823 error according to the PNG specification, Clause 11.3.2, but the error can
 4824 be ignored in each png_ptr with
 4825 
 4826    png_set_check_for_invalid_index(png_ptr, allowed);
 4827 
 4828       allowed  - one of
 4829                  0: disable benign error (accept the
 4830                     invalid data without warning).
 4831                  1: enable benign error (treat the
 4832                     invalid data as an error or a
 4833                     warning).
 4834 
 4835 If the error is ignored, or if png_benign_error() treats it as a warning,
 4836 any invalid pixels are decoded as opaque black by the decoder and written
 4837 as-is by the encoder.
 4838 
 4839 Retrieving the maximum palette index found was added at libpng-1.5.15.
 4840 This statement must appear after png_read_png() or png_read_image() while
 4841 reading, and after png_write_png() or png_write_image() while writing.
 4842 
 4843    int max_palette = png_get_palette_max(png_ptr, info_ptr);
 4844 
 4845 This will return the maximum palette index found in the image, or "-1" if
 4846 the palette was not checked, or "0" if no palette was found.  Note that this
 4847 does not account for any palette index used by ancillary chunks such as the
 4848 bKGD chunk; you must check those separately to determine the maximum
 4849 palette index actually used.
 4850 
 4851 There are no substantial API changes between the non-deprecated parts of
 4852 the 1.4.5 API and the 1.5.0 API; however, the ability to directly access
 4853 members of the main libpng control structures, png_struct and png_info,
 4854 deprecated in earlier versions of libpng, has been completely removed from
 4855 libpng 1.5, and new private "pngstruct.h", "pnginfo.h", and "pngdebug.h"
 4856 header files were created.
 4857 
 4858 We no longer include zlib.h in png.h.  The include statement has been moved
 4859 to pngstruct.h, where it is not accessible by applications. Applications that
 4860 need access to information in zlib.h will need to add the '#include "zlib.h"'
 4861 directive.  It does not matter whether this is placed prior to or after
 4862 the '"#include png.h"' directive.
 4863 
 4864 The png_sprintf(), png_strcpy(), and png_strncpy() macros are no longer used
 4865 and were removed.
 4866 
 4867 We moved the png_strlen(), png_memcpy(), png_memset(), and png_memcmp()
 4868 macros into a private header file (pngpriv.h) that is not accessible to
 4869 applications.
 4870 
 4871 In png_get_iCCP, the type of "profile" was changed from png_charpp
 4872 to png_bytepp, and in png_set_iCCP, from png_charp to png_const_bytep.
 4873 
 4874 There are changes of form in png.h, including new and changed macros to
 4875 declare parts of the API.  Some API functions with arguments that are
 4876 pointers to data not modified within the function have been corrected to
 4877 declare these arguments with const.
 4878 
 4879 Much of the internal use of C macros to control the library build has also
 4880 changed and some of this is visible in the exported header files, in
 4881 particular the use of macros to control data and API elements visible
 4882 during application compilation may require significant revision to
 4883 application code.  (It is extremely rare for an application to do this.)
 4884 
 4885 Any program that compiled against libpng 1.4 and did not use deprecated
 4886 features or access internal library structures should compile and work
 4887 against libpng 1.5, except for the change in the prototype for
 4888 png_get_iCCP() and png_set_iCCP() API functions mentioned above.
 4889 
 4890 libpng 1.5.0 adds PNG_ PASS macros to help in the reading and writing of
 4891 interlaced images.  The macros return the number of rows and columns in
 4892 each pass and information that can be used to de-interlace and (if
 4893 absolutely necessary) interlace an image.
 4894 
 4895 libpng 1.5.0 adds an API png_longjmp(png_ptr, value).  This API calls
 4896 the application-provided png_longjmp_ptr on the internal, but application
 4897 initialized, longjmp buffer.  It is provided as a convenience to avoid
 4898 the need to use the png_jmpbuf macro, which had the unnecessary side
 4899 effect of resetting the internal png_longjmp_ptr value.
 4900 
 4901 libpng 1.5.0 includes a complete fixed point API.  By default this is
 4902 present along with the corresponding floating point API.  In general the
 4903 fixed point API is faster and smaller than the floating point one because
 4904 the PNG file format used fixed point, not floating point.  This applies
 4905 even if the library uses floating point in internal calculations.  A new
 4906 macro, PNG_FLOATING_ARITHMETIC_SUPPORTED, reveals whether the library
 4907 uses floating point arithmetic (the default) or fixed point arithmetic
 4908 internally for performance critical calculations such as gamma correction.
 4909 In some cases, the gamma calculations may produce slightly different
 4910 results.  This has changed the results in png_rgb_to_gray and in alpha
 4911 composition (png_set_background for example). This applies even if the
 4912 original image was already linear (gamma == 1.0) and, therefore, it is
 4913 not necessary to linearize the image.  This is because libpng has *not*
 4914 been changed to optimize that case correctly, yet.
 4915 
 4916 Fixed point support for the sCAL chunk comes with an important caveat;
 4917 the sCAL specification uses a decimal encoding of floating point values
 4918 and the accuracy of PNG fixed point values is insufficient for
 4919 representation of these values. Consequently a "string" API
 4920 (png_get_sCAL_s and png_set_sCAL_s) is the only reliable way of reading
 4921 arbitrary sCAL chunks in the absence of either the floating point API or
 4922 internal floating point calculations.  Starting with libpng-1.5.0, both
 4923 of these functions are present when PNG_sCAL_SUPPORTED is defined.  Prior
 4924 to libpng-1.5.0, their presence also depended upon PNG_FIXED_POINT_SUPPORTED
 4925 being defined and PNG_FLOATING_POINT_SUPPORTED not being defined.
 4926 
 4927 Applications no longer need to include the optional distribution header
 4928 file pngusr.h or define the corresponding macros during application
 4929 build in order to see the correct variant of the libpng API.  From 1.5.0
 4930 application code can check for the corresponding _SUPPORTED macro:
 4931 
 4932 #ifdef PNG_INCH_CONVERSIONS_SUPPORTED
 4933    /* code that uses the inch conversion APIs. */
 4934 #endif
 4935 
 4936 This macro will only be defined if the inch conversion functions have been
 4937 compiled into libpng.  The full set of macros, and whether or not support
 4938 has been compiled in, are available in the header file pnglibconf.h.
 4939 This header file is specific to the libpng build.  Notice that prior to
 4940 1.5.0 the _SUPPORTED macros would always have the default definition unless
 4941 reset by pngusr.h or by explicit settings on the compiler command line.
 4942 These settings may produce compiler warnings or errors in 1.5.0 because
 4943 of macro redefinition.
 4944 
 4945 Applications can now choose whether to use these macros or to call the
 4946 corresponding function by defining PNG_USE_READ_MACROS or
 4947 PNG_NO_USE_READ_MACROS before including png.h.  Notice that this is
 4948 only supported from 1.5.0; defining PNG_NO_USE_READ_MACROS prior to 1.5.0
 4949 will lead to a link failure.
 4950 
 4951 Prior to libpng-1.5.4, the zlib compressor used the same set of parameters
 4952 when compressing the IDAT data and textual data such as zTXt and iCCP.
 4953 In libpng-1.5.4 we reinitialized the zlib stream for each type of data.
 4954 We added five png_set_text_*() functions for setting the parameters to
 4955 use with textual data.
 4956 
 4957 Prior to libpng-1.5.4, the PNG_READ_16_TO_8_ACCURATE_SCALE_SUPPORTED
 4958 option was off by default, and slightly inaccurate scaling occurred.
 4959 This option can no longer be turned off, and the choice of accurate
 4960 or inaccurate 16-to-8 scaling is by using the new png_set_scale_16_to_8()
 4961 API for accurate scaling or the old png_set_strip_16_to_8() API for simple
 4962 chopping.  In libpng-1.5.4, the PNG_READ_16_TO_8_ACCURATE_SCALE_SUPPORTED
 4963 macro became PNG_READ_SCALE_16_TO_8_SUPPORTED, and the PNG_READ_16_TO_8
 4964 macro became PNG_READ_STRIP_16_TO_8_SUPPORTED, to enable the two
 4965 png_set_*_16_to_8() functions separately.
 4966 
 4967 Prior to libpng-1.5.4, the png_set_user_limits() function could only be
 4968 used to reduce the width and height limits from the value of
 4969 PNG_USER_WIDTH_MAX and PNG_USER_HEIGHT_MAX, although this document said
 4970 that it could be used to override them.  Now this function will reduce or
 4971 increase the limits.
 4972 
 4973 Starting in libpng-1.5.22, default user limits were established. These
 4974 can be overridden by application calls to png_set_user_limits(),
 4975 png_set_user_chunk_cache_max(), and/or png_set_user_malloc_max().
 4976 The limits are now
 4977                              max possible  default
 4978    png_user_width_max        0x7fffffff    1,000,000
 4979    png_user_height_max       0x7fffffff    1,000,000
 4980    png_user_chunk_cache_max  0 (unlimited) 1000
 4981    png_user_chunk_malloc_max 0 (unlimited) 8,000,000
 4982 
 4983 The png_set_option() function (and the "options" member of the png struct) was
 4984 added to libpng-1.5.15, with option PNG_ARM_NEON.
 4985 
 4986 The library now supports a complete fixed point implementation and can
 4987 thus be used on systems that have no floating point support or very
 4988 limited or slow support.  Previously gamma correction, an essential part
 4989 of complete PNG support, required reasonably fast floating point.
 4990 
 4991 As part of this the choice of internal implementation has been made
 4992 independent of the choice of fixed versus floating point APIs and all the
 4993 missing fixed point APIs have been implemented.
 4994 
 4995 The exact mechanism used to control attributes of API functions has
 4996 changed, as described in the INSTALL file.
 4997 
 4998 A new test program, pngvalid, is provided in addition to pngtest.
 4999 pngvalid validates the arithmetic accuracy of the gamma correction
 5000 calculations and includes a number of validations of the file format.
 5001 A subset of the full range of tests is run when "make check" is done
 5002 (in the 'configure' build.)  pngvalid also allows total allocated memory
 5003 usage to be evaluated and performs additional memory overwrite validation.
 5004 
 5005 Many changes to individual feature macros have been made. The following
 5006 are the changes most likely to be noticed by library builders who
 5007 configure libpng:
 5008 
 5009 1) All feature macros now have consistent naming:
 5010 
 5011 #define PNG_NO_feature turns the feature off
 5012 #define PNG_feature_SUPPORTED turns the feature on
 5013 
 5014 pnglibconf.h contains one line for each feature macro which is either:
 5015 
 5016 #define PNG_feature_SUPPORTED
 5017 
 5018 if the feature is supported or:
 5019 
 5020 /*#undef PNG_feature_SUPPORTED*/
 5021 
 5022 if it is not.  Library code consistently checks for the 'SUPPORTED' macro.
 5023 It does not, and libpng applications should not, check for the 'NO' macro
 5024 which will not normally be defined even if the feature is not supported.
 5025 The 'NO' macros are only used internally for setting or not setting the
 5026 corresponding 'SUPPORTED' macros.
 5027 
 5028 Compatibility with the old names is provided as follows:
 5029 
 5030 PNG_INCH_CONVERSIONS turns on PNG_INCH_CONVERSIONS_SUPPORTED
 5031 
 5032 And the following definitions disable the corresponding feature:
 5033 
 5034 PNG_SETJMP_NOT_SUPPORTED disables SETJMP
 5035 PNG_READ_TRANSFORMS_NOT_SUPPORTED disables READ_TRANSFORMS
 5036 PNG_NO_READ_COMPOSITED_NODIV disables READ_COMPOSITE_NODIV
 5037 PNG_WRITE_TRANSFORMS_NOT_SUPPORTED disables WRITE_TRANSFORMS
 5038 PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED disables READ_ANCILLARY_CHUNKS
 5039 PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED disables WRITE_ANCILLARY_CHUNKS
 5040 
 5041 Library builders should remove use of the above, inconsistent, names.
 5042 
 5043 2) Warning and error message formatting was previously conditional on
 5044 the STDIO feature. The library has been changed to use the
 5045 CONSOLE_IO feature instead. This means that if CONSOLE_IO is disabled
 5046 the library no longer uses the printf(3) functions, even though the
 5047 default read/write implementations use (FILE) style stdio.h functions.
 5048 
 5049 3) Three feature macros now control the fixed/floating point decisions:
 5050 
 5051 PNG_FLOATING_POINT_SUPPORTED enables the floating point APIs
 5052 
 5053 PNG_FIXED_POINT_SUPPORTED enables the fixed point APIs; however, in
 5054 practice these are normally required internally anyway (because the PNG
 5055 file format is fixed point), therefore in most cases PNG_NO_FIXED_POINT
 5056 merely stops the function from being exported.
 5057 
 5058 PNG_FLOATING_ARITHMETIC_SUPPORTED chooses between the internal floating
 5059 point implementation or the fixed point one.  Typically the fixed point
 5060 implementation is larger and slower than the floating point implementation
 5061 on a system that supports floating point; however, it may be faster on a
 5062 system which lacks floating point hardware and therefore uses a software
 5063 emulation.
 5064 
 5065 4) Added PNG_{READ,WRITE}_INT_FUNCTIONS_SUPPORTED.  This allows the
 5066 functions to read and write ints to be disabled independently of
 5067 PNG_USE_READ_MACROS, which allows libpng to be built with the functions
 5068 even though the default is to use the macros - this allows applications
 5069 to choose at app buildtime whether or not to use macros (previously
 5070 impossible because the functions weren't in the default build.)
 5071 
 5072 XII. Changes to Libpng from version 1.5.x to 1.6.x
 5073 
 5074 A "simplified API" has been added (see documentation in png.h and a simple
 5075 example in contrib/examples/pngtopng.c).  The new publicly visible API
 5076 includes the following:
 5077 
 5078    macros:
 5079      PNG_FORMAT_*
 5080      PNG_IMAGE_*
 5081    structures:
 5082      png_control
 5083      png_image
 5084    read functions
 5085      png_image_begin_read_from_file()
 5086      png_image_begin_read_from_stdio()
 5087      png_image_begin_read_from_memory()
 5088      png_image_finish_read()
 5089      png_image_free()
 5090    write functions
 5091      png_image_write_to_file()
 5092      png_image_write_to_memory()
 5093      png_image_write_to_stdio()
 5094 
 5095 Starting with libpng-1.6.0, you can configure libpng to prefix all exported
 5096 symbols, using the PNG_PREFIX macro.
 5097 
 5098 We no longer include string.h in png.h.  The include statement has been moved
 5099 to pngpriv.h, where it is not accessible by applications.  Applications that
 5100 need access to information in string.h must add an '#include <string.h>'
 5101 directive.  It does not matter whether this is placed prior to or after
 5102 the '#include "png.h"' directive.
 5103 
 5104 The following API are now DEPRECATED:
 5105    png_info_init_3()
 5106    png_convert_to_rfc1123() which has been replaced
 5107      with png_convert_to_rfc1123_buffer()
 5108    png_malloc_default()
 5109    png_free_default()
 5110    png_reset_zstream()
 5111 
 5112 The following have been removed:
 5113    png_get_io_chunk_name(), which has been replaced
 5114      with png_get_io_chunk_type().  The new
 5115      function returns a 32-bit integer instead of
 5116      a string.
 5117    The png_sizeof(), png_strlen(), png_memcpy(), png_memcmp(), and
 5118      png_memset() macros are no longer used in the libpng sources and
 5119      have been removed.  These had already been made invisible to applications
 5120      (i.e., defined in the private pngpriv.h header file) since libpng-1.5.0.
 5121 
 5122 The signatures of many exported functions were changed, such that
 5123    png_structp became png_structrp or png_const_structrp
 5124    png_infop became png_inforp or png_const_inforp
 5125 where "rp" indicates a "restricted pointer".
 5126 
 5127 Dropped support for 16-bit platforms. The support for FAR/far types has
 5128 been eliminated and the definition of png_alloc_size_t is now controlled
 5129 by a flag so that 'small size_t' systems can select it if necessary.
 5130 
 5131 Error detection in some chunks has improved; in particular the iCCP chunk
 5132 reader now does pretty complete validation of the basic format.  Some bad
 5133 profiles that were previously accepted are now accepted with a warning or
 5134 rejected, depending upon the png_set_benign_errors() setting, in particular
 5135 the very old broken Microsoft/HP 3144-byte sRGB profile.  Starting with
 5136 libpng-1.6.11, recognizing and checking sRGB profiles can be avoided by
 5137 means of
 5138 
 5139     #if defined(PNG_SKIP_sRGB_CHECK_PROFILE) && \
 5140         defined(PNG_SET_OPTION_SUPPORTED)
 5141        png_set_option(png_ptr, PNG_SKIP_sRGB_CHECK_PROFILE,
 5142            PNG_OPTION_ON);
 5143     #endif
 5144 
 5145 It's not a good idea to do this if you are using the "simplified API",
 5146 which needs to be able to recognize sRGB profiles conveyed via the iCCP
 5147 chunk.
 5148 
 5149 The PNG spec requirement that only grayscale profiles may appear in images
 5150 with color type 0 or 4 and that even if the image only contains gray pixels,
 5151 only RGB profiles may appear in images with color type 2, 3, or 6, is now
 5152 enforced.  The sRGB chunk is allowed to appear in images with any color type
 5153 and is interpreted by libpng to convey a one-tracer-curve gray profile or a
 5154 three-tracer-curve RGB profile as appropriate.
 5155 
 5156 Libpng 1.5.x erroneously used /MD for Debug DLL builds; if you used the debug
 5157 builds in your app and you changed your app to use /MD you will need to
 5158 change it back to /MDd for libpng 1.6.x.
 5159 
 5160 Prior to libpng-1.6.0 a warning would be issued if the iTXt chunk contained
 5161 an empty language field or an empty translated keyword.  Both of these
 5162 are allowed by the PNG specification, so these warnings are no longer issued.
 5163 
 5164 The library now issues an error if the application attempts to set a
 5165 transform after it calls png_read_update_info() or if it attempts to call
 5166 both png_read_update_info() and png_start_read_image() or to call either
 5167 of them more than once.
 5168 
 5169 The default condition for benign_errors is now to treat benign errors as
 5170 warnings while reading and as errors while writing.
 5171 
 5172 The library now issues a warning if both background processing and RGB to
 5173 gray are used when gamma correction happens. As with previous versions of
 5174 the library the results are numerically very incorrect in this case.
 5175 
 5176 There are some minor arithmetic changes in some transforms such as
 5177 png_set_background(), that might be detected by certain regression tests.
 5178 
 5179 Unknown chunk handling has been improved internally, without any API change.
 5180 This adds more correct option control of the unknown handling, corrects
 5181 a pre-existing bug where the per-chunk 'keep' setting is ignored, and makes
 5182 it possible to skip IDAT chunks in the sequential reader.
 5183 
 5184 The machine-generated configure files are no longer included in branches
 5185 libpng16 and later of the GIT repository.  They continue to be included
 5186 in the tarball releases, however.
 5187 
 5188 Libpng-1.6.0 through 1.6.2 used the CMF bytes at the beginning of the IDAT
 5189 stream to set the size of the sliding window for reading instead of using the
 5190 default 32-kbyte sliding window size.  It was discovered that there are
 5191 hundreds of PNG files in the wild that have incorrect CMF bytes that caused
 5192 zlib to issue the "invalid distance too far back" error and reject the file.
 5193 Libpng-1.6.3 and later calculate their own safe CMF from the image dimensions,
 5194 provide a way to revert to the libpng-1.5.x behavior (ignoring the CMF bytes
 5195 and using a 32-kbyte sliding window), by using
 5196 
 5197     png_set_option(png_ptr, PNG_MAXIMUM_INFLATE_WINDOW,
 5198         PNG_OPTION_ON);
 5199 
 5200 and provide a tool (contrib/tools/pngfix) for rewriting a PNG file while
 5201 optimizing the CMF bytes in its IDAT chunk correctly.
 5202 
 5203 Libpng-1.6.0 and libpng-1.6.1 wrote uncompressed iTXt chunks with the wrong
 5204 length, which resulted in PNG files that cannot be read beyond the bad iTXt
 5205 chunk.  This error was fixed in libpng-1.6.3, and a tool (called
 5206 contrib/tools/png-fix-itxt) has been added to the libpng distribution.
 5207 
 5208 Starting with libpng-1.6.17, the PNG_SAFE_LIMITS macro was eliminated
 5209 and safe limits are used by default (users who need larger limits
 5210 can still override them at compile time or run time, as described above).
 5211 
 5212 The new limits are
 5213                                 default   spec limit
 5214    png_user_width_max         1,000,000  2,147,483,647
 5215    png_user_height_max        1,000,000  2,147,483,647
 5216    png_user_chunk_cache_max         128  unlimited
 5217    png_user_chunk_malloc_max  8,000,000  unlimited
 5218 
 5219 Starting with libpng-1.6.18, a PNG_RELEASE_BUILD macro was added, which allows
 5220 library builders to control compilation for an installed system (a release build).
 5221 It can be set for testing debug or beta builds to ensure that they will compile
 5222 when the build type is switched to RC or STABLE. In essence this overrides the
 5223 PNG_LIBPNG_BUILD_BASE_TYPE definition which is not directly user controllable.
 5224 
 5225 Starting with libpng-1.6.19, attempting to set an over-length PLTE chunk
 5226 is an error. Previously this requirement of the PNG specification was not
 5227 enforced, and the palette was always limited to 256 entries. An over-length
 5228 PLTE chunk found in an input PNG is silently truncated.
 5229 
 5230 Starting with libpng-1.6.31, the eXIf chunk is supported. Libpng does not
 5231 attempt to decode the Exif profile; it simply returns a byte array
 5232 containing the profile to the calling application which must do its own
 5233 decoding.
 5234 
 5235 XIII. Detecting libpng
 5236 
 5237 The png_get_io_ptr() function has been present since libpng-0.88, has never
 5238 changed, and is unaffected by conditional compilation macros.  It is the
 5239 best choice for use in configure scripts for detecting the presence of any
 5240 libpng version since 0.88.  In an autoconf "configure.in" you could use
 5241 
 5242     AC_CHECK_LIB(png, png_get_io_ptr, ...
 5243 
 5244 XV. Source code repository
 5245 
 5246 Since about February 2009, version 1.2.34, libpng has been under "git" source
 5247 control.  The git repository was built from old libpng-x.y.z.tar.gz files
 5248 going back to version 0.70.  You can access the git repository (read only)
 5249 at
 5250 
 5251     https://github.com/glennrp/libpng or
 5252     https://git.code.sf.net/p/libpng/code.git
 5253 
 5254 or you can browse it with a web browser at
 5255 
 5256     https://github.com/glennrp/libpng or
 5257     https://sourceforge.net/p/libpng/code/ci/libpng16/tree/
 5258 
 5259 Patches can be sent to png-mng-implement at lists.sourceforge.net or
 5260 uploaded to the libpng bug tracker at
 5261 
 5262     https://libpng.sourceforge.io/
 5263 
 5264 or as a "pull request" to
 5265 
 5266     https://github.com/glennrp/libpng/pulls
 5267 
 5268 We also accept patches built from the tar or zip distributions, and
 5269 simple verbal descriptions of bug fixes, reported either to the
 5270 SourceForge bug tracker, to the png-mng-implement at lists.sf.net
 5271 mailing list, as github issues.
 5272 
 5273 XV. Coding style
 5274 
 5275 Our coding style is similar to the "Allman" style
 5276 (See https://en.wikipedia.org/wiki/Indent_style#Allman_style), with curly
 5277 braces on separate lines:
 5278 
 5279     if (condition)
 5280     {
 5281        action;
 5282     }
 5283 
 5284     else if (another condition)
 5285     {
 5286        another action;
 5287     }
 5288 
 5289 The braces can be omitted from simple one-line actions:
 5290 
 5291     if (condition)
 5292        return 0;
 5293 
 5294 We use 3-space indentation, except for continued statements which
 5295 are usually indented the same as the first line of the statement
 5296 plus four more spaces.
 5297 
 5298 For macro definitions we use 2-space indentation, always leaving the "#"
 5299 in the first column.
 5300 
 5301     #ifndef PNG_NO_FEATURE
 5302     #  ifndef PNG_FEATURE_SUPPORTED
 5303     #    define PNG_FEATURE_SUPPORTED
 5304     #  endif
 5305     #endif
 5306 
 5307 Comments appear with the leading "/*" at the same indentation as
 5308 the statement that follows the comment:
 5309 
 5310     /* Single-line comment */
 5311     statement;
 5312 
 5313     /* This is a multiple-line
 5314      * comment.
 5315      */
 5316     statement;
 5317 
 5318 Very short comments can be placed after the end of the statement
 5319 to which they pertain:
 5320 
 5321     statement;    /* comment */
 5322 
 5323 We don't use C++ style ("//") comments. We have, however,
 5324 used them in the past in some now-abandoned MMX assembler
 5325 code.
 5326 
 5327 Functions and their curly braces are not indented, and
 5328 exported functions are marked with PNGAPI:
 5329 
 5330  /* This is a public function that is visible to
 5331   * application programmers. It does thus-and-so.
 5332   */
 5333  void PNGAPI
 5334  png_exported_function(png_ptr, png_info, foo)
 5335  {
 5336     body;
 5337  }
 5338 
 5339 The return type and decorations are placed on a separate line
 5340 ahead of the function name, as illustrated above.
 5341 
 5342 The prototypes for all exported functions appear in png.h,
 5343 above the comment that says
 5344 
 5345     /* Maintainer: Put new public prototypes here ... */
 5346 
 5347 We mark all non-exported functions with "/* PRIVATE */"":
 5348 
 5349  void /* PRIVATE */
 5350  png_non_exported_function(png_ptr, png_info, foo)
 5351  {
 5352     body;
 5353  }
 5354 
 5355 The prototypes for non-exported functions (except for those in
 5356 pngtest) appear in pngpriv.h above the comment that says
 5357 
 5358   /* Maintainer: Put new private prototypes here ^ */
 5359 
 5360 To avoid polluting the global namespace, the names of all exported
 5361 functions and variables begin with "png_", and all publicly visible C
 5362 preprocessor macros begin with "PNG".  We request that applications that
 5363 use libpng *not* begin any of their own symbols with either of these strings.
 5364 
 5365 We put a space after the "sizeof" operator and we omit the
 5366 optional parentheses around its argument when the argument
 5367 is an expression, not a type name, and we always enclose the
 5368 sizeof operator, with its argument, in parentheses:
 5369 
 5370   (sizeof (png_uint_32))
 5371   (sizeof array)
 5372 
 5373 Prior to libpng-1.6.0 we used a "png_sizeof()" macro, formatted as
 5374 though it were a function.
 5375 
 5376 Control keywords if, for, while, and switch are always followed by a space
 5377 to distinguish them from function calls, which have no trailing space.
 5378 
 5379 We put a space after each comma and after each semicolon
 5380 in "for" statements, and we put spaces before and after each
 5381 C binary operator and after "for" or "while", and before
 5382 "?".  We don't put a space between a typecast and the expression
 5383 being cast, nor do we put one between a function name and the
 5384 left parenthesis that follows it:
 5385 
 5386     for (i = 2; i > 0; --i)
 5387        y[i] = a(x) + (int)b;
 5388 
 5389 We prefer #ifdef and #ifndef to #if defined() and #if !defined()
 5390 when there is only one macro being tested.  We always use parentheses
 5391 with "defined".
 5392 
 5393 We express integer constants that are used as bit masks in hex format,
 5394 with an even number of lower-case hex digits, and to make them unsigned
 5395 (e.g., 0x00U, 0xffU, 0x0100U) and long if they are greater than 0x7fff
 5396 (e.g., 0xffffUL).
 5397 
 5398 We prefer to use underscores rather than camelCase in names, except
 5399 for a few type names that we inherit from zlib.h.
 5400 
 5401 We prefer "if (something != 0)" and "if (something == 0)" over
 5402 "if (something)" and if "(!something)", respectively, and for pointers
 5403 we prefer "if (some_pointer != NULL)" or "if (some_pointer == NULL)".
 5404 
 5405 We do not use the TAB character for indentation in the C sources.
 5406 
 5407 Lines do not exceed 80 characters.
 5408 
 5409 Other rules can be inferred by inspecting the libpng source.