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    1 The Independent JPEG Group's JPEG software
    2 ==========================================
    4 README for release 6b of 27-Mar-1998
    5 ====================================
    7 This distribution contains the sixth public release of the Independent JPEG
    8 Group's free JPEG software.  You are welcome to redistribute this software and
    9 to use it for any purpose, subject to the conditions under LEGAL ISSUES, below.
   11 Serious users of this software (particularly those incorporating it into
   12 larger programs) should contact IJG at jpeg-info@uunet.uu.net to be added to
   13 our electronic mailing list.  Mailing list members are notified of updates
   14 and have a chance to participate in technical discussions, etc.
   16 This software is the work of Tom Lane, Philip Gladstone, Jim Boucher,
   17 Lee Crocker, Julian Minguillon, Luis Ortiz, George Phillips, Davide Rossi,
   18 Guido Vollbeding, Ge' Weijers, and other members of the Independent JPEG
   19 Group.
   21 IJG is not affiliated with the official ISO JPEG standards committee.
   25 =====================
   27 This file contains the following sections:
   29 OVERVIEW            General description of JPEG and the IJG software.
   30 LEGAL ISSUES        Copyright, lack of warranty, terms of distribution.
   31 REFERENCES          Where to learn more about JPEG.
   32 ARCHIVE LOCATIONS   Where to find newer versions of this software.
   33 RELATED SOFTWARE    Other stuff you should get.
   34 FILE FORMAT WARS    Software *not* to get.
   35 TO DO               Plans for future IJG releases.
   37 Other documentation files in the distribution are:
   39 User documentation:
   40   install.doc       How to configure and install the IJG software.
   41   usage.doc         Usage instructions for cjpeg, djpeg, jpegtran,
   42                     rdjpgcom, and wrjpgcom.
   43   *.1               Unix-style man pages for programs (same info as usage.doc).
   44   wizard.doc        Advanced usage instructions for JPEG wizards only.
   45   change.log        Version-to-version change highlights.
   46 Programmer and internal documentation:
   47   libjpeg.doc       How to use the JPEG library in your own programs.
   48   example.c         Sample code for calling the JPEG library.
   49   structure.doc     Overview of the JPEG library's internal structure.
   50   filelist.doc      Road map of IJG files.
   51   coderules.doc     Coding style rules --- please read if you contribute code.
   53 Please read at least the files install.doc and usage.doc.  Useful information
   54 can also be found in the JPEG FAQ (Frequently Asked Questions) article.  See
   55 ARCHIVE LOCATIONS below to find out where to obtain the FAQ article.
   57 If you want to understand how the JPEG code works, we suggest reading one or
   58 more of the REFERENCES, then looking at the documentation files (in roughly
   59 the order listed) before diving into the code.
   63 ========
   65 This package contains C software to implement JPEG image compression and
   66 decompression.  JPEG (pronounced "jay-peg") is a standardized compression
   67 method for full-color and gray-scale images.  JPEG is intended for compressing
   68 "real-world" scenes; line drawings, cartoons and other non-realistic images
   69 are not its strong suit.  JPEG is lossy, meaning that the output image is not
   70 exactly identical to the input image.  Hence you must not use JPEG if you
   71 have to have identical output bits.  However, on typical photographic images,
   72 very good compression levels can be obtained with no visible change, and
   73 remarkably high compression levels are possible if you can tolerate a
   74 low-quality image.  For more details, see the references, or just experiment
   75 with various compression settings.
   77 This software implements JPEG baseline, extended-sequential, and progressive
   78 compression processes.  Provision is made for supporting all variants of these
   79 processes, although some uncommon parameter settings aren't implemented yet.
   80 For legal reasons, we are not distributing code for the arithmetic-coding
   81 variants of JPEG; see LEGAL ISSUES.  We have made no provision for supporting
   82 the hierarchical or lossless processes defined in the standard.
   84 We provide a set of library routines for reading and writing JPEG image files,
   85 plus two sample applications "cjpeg" and "djpeg", which use the library to
   86 perform conversion between JPEG and some other popular image file formats.
   87 The library is intended to be reused in other applications.
   89 In order to support file conversion and viewing software, we have included
   90 considerable functionality beyond the bare JPEG coding/decoding capability;
   91 for example, the color quantization modules are not strictly part of JPEG
   92 decoding, but they are essential for output to colormapped file formats or
   93 colormapped displays.  These extra functions can be compiled out of the
   94 library if not required for a particular application.  We have also included
   95 "jpegtran", a utility for lossless transcoding between different JPEG
   96 processes, and "rdjpgcom" and "wrjpgcom", two simple applications for
   97 inserting and extracting textual comments in JFIF files.
   99 The emphasis in designing this software has been on achieving portability and
  100 flexibility, while also making it fast enough to be useful.  In particular,
  101 the software is not intended to be read as a tutorial on JPEG.  (See the
  102 REFERENCES section for introductory material.)  Rather, it is intended to
  103 be reliable, portable, industrial-strength code.  We do not claim to have
  104 achieved that goal in every aspect of the software, but we strive for it.
  106 We welcome the use of this software as a component of commercial products.
  107 No royalty is required, but we do ask for an acknowledgement in product
  108 documentation, as described under LEGAL ISSUES.
  112 ============
  114 In plain English:
  116 1. We don't promise that this software works.  (But if you find any bugs,
  117    please let us know!)
  118 2. You can use this software for whatever you want.  You don't have to pay us.
  119 3. You may not pretend that you wrote this software.  If you use it in a
  120    program, you must acknowledge somewhere in your documentation that
  121    you've used the IJG code.
  123 In legalese:
  125 The authors make NO WARRANTY or representation, either express or implied,
  126 with respect to this software, its quality, accuracy, merchantability, or
  127 fitness for a particular purpose.  This software is provided "AS IS", and you,
  128 its user, assume the entire risk as to its quality and accuracy.
  130 This software is copyright (C) 1991-1998, Thomas G. Lane.
  131 All Rights Reserved except as specified below.
  133 Permission is hereby granted to use, copy, modify, and distribute this
  134 software (or portions thereof) for any purpose, without fee, subject to these
  135 conditions:
  136 (1) If any part of the source code for this software is distributed, then this
  137 README file must be included, with this copyright and no-warranty notice
  138 unaltered; and any additions, deletions, or changes to the original files
  139 must be clearly indicated in accompanying documentation.
  140 (2) If only executable code is distributed, then the accompanying
  141 documentation must state that "this software is based in part on the work of
  142 the Independent JPEG Group".
  143 (3) Permission for use of this software is granted only if the user accepts
  144 full responsibility for any undesirable consequences; the authors accept
  145 NO LIABILITY for damages of any kind.
  147 These conditions apply to any software derived from or based on the IJG code,
  148 not just to the unmodified library.  If you use our work, you ought to
  149 acknowledge us.
  151 Permission is NOT granted for the use of any IJG author's name or company name
  152 in advertising or publicity relating to this software or products derived from
  153 it.  This software may be referred to only as "the Independent JPEG Group's
  154 software".
  156 We specifically permit and encourage the use of this software as the basis of
  157 commercial products, provided that all warranty or liability claims are
  158 assumed by the product vendor.
  161 ansi2knr.c is included in this distribution by permission of L. Peter Deutsch,
  162 sole proprietor of its copyright holder, Aladdin Enterprises of Menlo Park, CA.
  163 ansi2knr.c is NOT covered by the above copyright and conditions, but instead
  164 by the usual distribution terms of the Free Software Foundation; principally,
  165 that you must include source code if you redistribute it.  (See the file
  166 ansi2knr.c for full details.)  However, since ansi2knr.c is not needed as part
  167 of any program generated from the IJG code, this does not limit you more than
  168 the foregoing paragraphs do.
  170 The Unix configuration script "configure" was produced with GNU Autoconf.
  171 It is copyright by the Free Software Foundation but is freely distributable.
  172 The same holds for its supporting scripts (config.guess, config.sub,
  173 ltconfig, ltmain.sh).  Another support script, install-sh, is copyright
  174 by M.I.T. but is also freely distributable.
  176 It appears that the arithmetic coding option of the JPEG spec is covered by
  177 patents owned by IBM, AT&T, and Mitsubishi.  Hence arithmetic coding cannot
  178 legally be used without obtaining one or more licenses.  For this reason,
  179 support for arithmetic coding has been removed from the free JPEG software.
  180 (Since arithmetic coding provides only a marginal gain over the unpatented
  181 Huffman mode, it is unlikely that very many implementations will support it.)
  182 So far as we are aware, there are no patent restrictions on the remaining
  183 code.
  185 The IJG distribution formerly included code to read and write GIF files.
  186 To avoid entanglement with the Unisys LZW patent, GIF reading support has
  187 been removed altogether, and the GIF writer has been simplified to produce
  188 "uncompressed GIFs".  This technique does not use the LZW algorithm; the
  189 resulting GIF files are larger than usual, but are readable by all standard
  190 GIF decoders.
  192 We are required to state that
  193     "The Graphics Interchange Format(c) is the Copyright property of
  194     CompuServe Incorporated.  GIF(sm) is a Service Mark property of
  195     CompuServe Incorporated."
  199 ==========
  201 We highly recommend reading one or more of these references before trying to
  202 understand the innards of the JPEG software.
  204 The best short technical introduction to the JPEG compression algorithm is
  205 	Wallace, Gregory K.  "The JPEG Still Picture Compression Standard",
  206 	Communications of the ACM, April 1991 (vol. 34 no. 4), pp. 30-44.
  207 (Adjacent articles in that issue discuss MPEG motion picture compression,
  208 applications of JPEG, and related topics.)  If you don't have the CACM issue
  209 handy, a PostScript file containing a revised version of Wallace's article is
  210 available at ftp://ftp.uu.net/graphics/jpeg/wallace.ps.gz.  The file (actually
  211 a preprint for an article that appeared in IEEE Trans. Consumer Electronics)
  212 omits the sample images that appeared in CACM, but it includes corrections
  213 and some added material.  Note: the Wallace article is copyright ACM and IEEE,
  214 and it may not be used for commercial purposes.
  216 A somewhat less technical, more leisurely introduction to JPEG can be found in
  217 "The Data Compression Book" by Mark Nelson and Jean-loup Gailly, published by
  218 M&T Books (New York), 2nd ed. 1996, ISBN 1-55851-434-1.  This book provides
  219 good explanations and example C code for a multitude of compression methods
  220 including JPEG.  It is an excellent source if you are comfortable reading C
  221 code but don't know much about data compression in general.  The book's JPEG
  222 sample code is far from industrial-strength, but when you are ready to look
  223 at a full implementation, you've got one here...
  225 The best full description of JPEG is the textbook "JPEG Still Image Data
  226 Compression Standard" by William B. Pennebaker and Joan L. Mitchell, published
  227 by Van Nostrand Reinhold, 1993, ISBN 0-442-01272-1.  Price US$59.95, 638 pp.
  228 The book includes the complete text of the ISO JPEG standards (DIS 10918-1
  229 and draft DIS 10918-2).  This is by far the most complete exposition of JPEG
  230 in existence, and we highly recommend it.
  232 The JPEG standard itself is not available electronically; you must order a
  233 paper copy through ISO or ITU.  (Unless you feel a need to own a certified
  234 official copy, we recommend buying the Pennebaker and Mitchell book instead;
  235 it's much cheaper and includes a great deal of useful explanatory material.)
  236 In the USA, copies of the standard may be ordered from ANSI Sales at (212)
  237 642-4900, or from Global Engineering Documents at (800) 854-7179.  (ANSI
  238 doesn't take credit card orders, but Global does.)  It's not cheap: as of
  239 1992, ANSI was charging $95 for Part 1 and $47 for Part 2, plus 7%
  240 shipping/handling.  The standard is divided into two parts, Part 1 being the
  241 actual specification, while Part 2 covers compliance testing methods.  Part 1
  242 is titled "Digital Compression and Coding of Continuous-tone Still Images,
  243 Part 1: Requirements and guidelines" and has document numbers ISO/IEC IS
  244 10918-1, ITU-T T.81.  Part 2 is titled "Digital Compression and Coding of
  245 Continuous-tone Still Images, Part 2: Compliance testing" and has document
  246 numbers ISO/IEC IS 10918-2, ITU-T T.83.
  248 Some extensions to the original JPEG standard are defined in JPEG Part 3,
  249 a newer ISO standard numbered ISO/IEC IS 10918-3 and ITU-T T.84.  IJG
  250 currently does not support any Part 3 extensions.
  252 The JPEG standard does not specify all details of an interchangeable file
  253 format.  For the omitted details we follow the "JFIF" conventions, revision
  254 1.02.  A copy of the JFIF spec is available from:
  255 	Literature Department
  256 	C-Cube Microsystems, Inc.
  257 	1778 McCarthy Blvd.
  258 	Milpitas, CA 95035
  259 	phone (408) 944-6300,  fax (408) 944-6314
  260 A PostScript version of this document is available by FTP at
  261 ftp://ftp.uu.net/graphics/jpeg/jfif.ps.gz.  There is also a plain text
  262 version at ftp://ftp.uu.net/graphics/jpeg/jfif.txt.gz, but it is missing
  263 the figures.
  265 The TIFF 6.0 file format specification can be obtained by FTP from
  266 ftp://ftp.sgi.com/graphics/tiff/TIFF6.ps.gz.  The JPEG incorporation scheme
  267 found in the TIFF 6.0 spec of 3-June-92 has a number of serious problems.
  268 IJG does not recommend use of the TIFF 6.0 design (TIFF Compression tag 6).
  269 Instead, we recommend the JPEG design proposed by TIFF Technical Note #2
  270 (Compression tag 7).  Copies of this Note can be obtained from ftp.sgi.com or
  271 from ftp://ftp.uu.net/graphics/jpeg/.  It is expected that the next revision
  272 of the TIFF spec will replace the 6.0 JPEG design with the Note's design.
  273 Although IJG's own code does not support TIFF/JPEG, the free libtiff library
  274 uses our library to implement TIFF/JPEG per the Note.  libtiff is available
  275 from ftp://ftp.sgi.com/graphics/tiff/.
  279 =================
  281 The "official" archive site for this software is ftp.uu.net (Internet
  282 address  The most recent released version can always be found
  283 there in directory graphics/jpeg.  This particular version will be archived
  284 as ftp://ftp.uu.net/graphics/jpeg/jpegsrc.v6b.tar.gz.  If you don't have
  285 direct Internet access, UUNET's archives are also available via UUCP; contact
  286 help@uunet.uu.net for information on retrieving files that way.
  288 Numerous Internet sites maintain copies of the UUNET files.  However, only
  289 ftp.uu.net is guaranteed to have the latest official version.
  291 You can also obtain this software in DOS-compatible "zip" archive format from
  292 the SimTel archives (ftp://ftp.simtel.net/pub/simtelnet/msdos/graphics/), or
  293 on CompuServe in the Graphics Support forum (GO CIS:GRAPHSUP), library 12
  294 "JPEG Tools".  Again, these versions may sometimes lag behind the ftp.uu.net
  295 release.
  297 The JPEG FAQ (Frequently Asked Questions) article is a useful source of
  298 general information about JPEG.  It is updated constantly and therefore is
  299 not included in this distribution.  The FAQ is posted every two weeks to
  300 Usenet newsgroups comp.graphics.misc, news.answers, and other groups.
  301 It is available on the World Wide Web at http://www.faqs.org/faqs/jpeg-faq/
  302 and other news.answers archive sites, including the official news.answers
  303 archive at rtfm.mit.edu: ftp://rtfm.mit.edu/pub/usenet/news.answers/jpeg-faq/.
  304 If you don't have Web or FTP access, send e-mail to mail-server@rtfm.mit.edu
  305 with body
  306 	send usenet/news.answers/jpeg-faq/part1
  307 	send usenet/news.answers/jpeg-faq/part2
  311 ================
  313 Numerous viewing and image manipulation programs now support JPEG.  (Quite a
  314 few of them use this library to do so.)  The JPEG FAQ described above lists
  315 some of the more popular free and shareware viewers, and tells where to
  316 obtain them on Internet.
  318 If you are on a Unix machine, we highly recommend Jef Poskanzer's free
  319 PBMPLUS software, which provides many useful operations on PPM-format image
  320 files.  In particular, it can convert PPM images to and from a wide range of
  321 other formats, thus making cjpeg/djpeg considerably more useful.  The latest
  322 version is distributed by the NetPBM group, and is available from numerous
  323 sites, notably ftp://wuarchive.wustl.edu/graphics/graphics/packages/NetPBM/.
  324 Unfortunately PBMPLUS/NETPBM is not nearly as portable as the IJG software is;
  325 you are likely to have difficulty making it work on any non-Unix machine.
  327 A different free JPEG implementation, written by the PVRG group at Stanford,
  328 is available from ftp://havefun.stanford.edu/pub/jpeg/.  This program
  329 is designed for research and experimentation rather than production use;
  330 it is slower, harder to use, and less portable than the IJG code, but it
  331 is easier to read and modify.  Also, the PVRG code supports lossless JPEG,
  332 which we do not.  (On the other hand, it doesn't do progressive JPEG.)
  336 ================
  338 Some JPEG programs produce files that are not compatible with our library.
  339 The root of the problem is that the ISO JPEG committee failed to specify a
  340 concrete file format.  Some vendors "filled in the blanks" on their own,
  341 creating proprietary formats that no one else could read.  (For example, none
  342 of the early commercial JPEG implementations for the Macintosh were able to
  343 exchange compressed files.)
  345 The file format we have adopted is called JFIF (see REFERENCES).  This format
  346 has been agreed to by a number of major commercial JPEG vendors, and it has
  347 become the de facto standard.  JFIF is a minimal or "low end" representation.
  348 We recommend the use of TIFF/JPEG (TIFF revision 6.0 as modified by TIFF
  349 Technical Note #2) for "high end" applications that need to record a lot of
  350 additional data about an image.  TIFF/JPEG is fairly new and not yet widely
  351 supported, unfortunately.
  353 The upcoming JPEG Part 3 standard defines a file format called SPIFF.
  354 SPIFF is interoperable with JFIF, in the sense that most JFIF decoders should
  355 be able to read the most common variant of SPIFF.  SPIFF has some technical
  356 advantages over JFIF, but its major claim to fame is simply that it is an
  357 official standard rather than an informal one.  At this point it is unclear
  358 whether SPIFF will supersede JFIF or whether JFIF will remain the de-facto
  359 standard.  IJG intends to support SPIFF once the standard is frozen, but we
  360 have not decided whether it should become our default output format or not.
  361 (In any case, our decoder will remain capable of reading JFIF indefinitely.)
  363 Various proprietary file formats incorporating JPEG compression also exist.
  364 We have little or no sympathy for the existence of these formats.  Indeed,
  365 one of the original reasons for developing this free software was to help
  366 force convergence on common, open format standards for JPEG files.  Don't
  367 use a proprietary file format!
  370 TO DO
  371 =====
  373 The major thrust for v7 will probably be improvement of visual quality.
  374 The current method for scaling the quantization tables is known not to be
  375 very good at low Q values.  We also intend to investigate block boundary
  376 smoothing, "poor man's variable quantization", and other means of improving
  377 quality-vs-file-size performance without sacrificing compatibility.
  379 In future versions, we are considering supporting some of the upcoming JPEG
  380 Part 3 extensions --- principally, variable quantization and the SPIFF file
  381 format.
  383 As always, speeding things up is of great interest.
  385 Please send bug reports, offers of help, etc. to jpeg-info@uunet.uu.net.