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    1 libjpeg-turbo note:  This file has been modified by The libjpeg-turbo Project
    2 to include only information relevant to libjpeg-turbo, to wordsmith certain
    3 sections, and to remove impolitic language that existed in the libjpeg v8
    4 README.  It is included only for reference.  Please see README.md for
    5 information specific to libjpeg-turbo.
    8 The Independent JPEG Group's JPEG software
    9 ==========================================
   11 This distribution contains a release of the Independent JPEG Group's free JPEG
   12 software.  You are welcome to redistribute this software and to use it for any
   13 purpose, subject to the conditions under LEGAL ISSUES, below.
   15 This software is the work of Tom Lane, Guido Vollbeding, Philip Gladstone,
   16 Bill Allombert, Jim Boucher, Lee Crocker, Bob Friesenhahn, Ben Jackson,
   17 Julian Minguillon, Luis Ortiz, George Phillips, Davide Rossi, Ge' Weijers,
   18 and other members of the Independent JPEG Group.
   20 IJG is not affiliated with the ISO/IEC JTC1/SC29/WG1 standards committee
   21 (also known as JPEG, together with ITU-T SG16).
   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 FILE FORMAT WARS    Software *not* to get.
   34 TO DO               Plans for future IJG releases.
   36 Other documentation files in the distribution are:
   38 User documentation:
   39   usage.txt         Usage instructions for cjpeg, djpeg, jpegtran,
   40                     rdjpgcom, and wrjpgcom.
   41   *.1               Unix-style man pages for programs (same info as usage.txt).
   42   wizard.txt        Advanced usage instructions for JPEG wizards only.
   43   change.log        Version-to-version change highlights.
   44 Programmer and internal documentation:
   45   libjpeg.txt       How to use the JPEG library in your own programs.
   46   example.txt       Sample code for calling the JPEG library.
   47   structure.txt     Overview of the JPEG library's internal structure.
   48   coderules.txt     Coding style rules --- please read if you contribute code.
   50 Please read at least usage.txt.  Some information can also be found in the JPEG
   51 FAQ (Frequently Asked Questions) article.  See ARCHIVE LOCATIONS below to find
   52 out where to obtain the FAQ article.
   54 If you want to understand how the JPEG code works, we suggest reading one or
   55 more of the REFERENCES, then looking at the documentation files (in roughly
   56 the order listed) before diving into the code.
   60 ========
   62 This package contains C software to implement JPEG image encoding, decoding,
   63 and transcoding.  JPEG (pronounced "jay-peg") is a standardized compression
   64 method for full-color and grayscale images.  JPEG's strong suit is compressing
   65 photographic images or other types of images that have smooth color and
   66 brightness transitions between neighboring pixels.  Images with sharp lines or
   67 other abrupt features may not compress well with JPEG, and a higher JPEG
   68 quality may have to be used to avoid visible compression artifacts with such
   69 images.
   71 JPEG is lossy, meaning that the output pixels are not necessarily identical to
   72 the input pixels.  However, on photographic content and other "smooth" images,
   73 very good compression ratios can be obtained with no visible compression
   74 artifacts, and extremely high compression ratios are possible if you are
   75 willing to sacrifice image quality (by reducing the "quality" setting in the
   76 compressor.)
   78 This software implements JPEG baseline, extended-sequential, and progressive
   79 compression processes.  Provision is made for supporting all variants of these
   80 processes, although some uncommon parameter settings aren't implemented yet.
   81 We have made no provision for supporting the hierarchical or lossless
   82 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.
   96 We have also included "jpegtran", a utility for lossless transcoding between
   97 different JPEG processes, and "rdjpgcom" and "wrjpgcom", two simple
   98 applications for inserting and extracting textual comments in JFIF files.
  100 The emphasis in designing this software has been on achieving portability and
  101 flexibility, while also making it fast enough to be useful.  In particular,
  102 the software is not intended to be read as a tutorial on JPEG.  (See the
  103 REFERENCES section for introductory material.)  Rather, it is intended to
  104 be reliable, portable, industrial-strength code.  We do not claim to have
  105 achieved that goal in every aspect of the software, but we strive for it.
  107 We welcome the use of this software as a component of commercial products.
  108 No royalty is required, but we do ask for an acknowledgement in product
  109 documentation, as described under LEGAL ISSUES.
  113 ============
  115 In plain English:
  117 1. We don't promise that this software works.  (But if you find any bugs,
  118    please let us know!)
  119 2. You can use this software for whatever you want.  You don't have to pay us.
  120 3. You may not pretend that you wrote this software.  If you use it in a
  121    program, you must acknowledge somewhere in your documentation that
  122    you've used the IJG code.
  124 In legalese:
  126 The authors make NO WARRANTY or representation, either express or implied,
  127 with respect to this software, its quality, accuracy, merchantability, or
  128 fitness for a particular purpose.  This software is provided "AS IS", and you,
  129 its user, assume the entire risk as to its quality and accuracy.
  131 This software is copyright (C) 1991-2016, Thomas G. Lane, Guido Vollbeding.
  132 All Rights Reserved except as specified below.
  134 Permission is hereby granted to use, copy, modify, and distribute this
  135 software (or portions thereof) for any purpose, without fee, subject to these
  136 conditions:
  137 (1) If any part of the source code for this software is distributed, then this
  138 README file must be included, with this copyright and no-warranty notice
  139 unaltered; and any additions, deletions, or changes to the original files
  140 must be clearly indicated in accompanying documentation.
  141 (2) If only executable code is distributed, then the accompanying
  142 documentation must state that "this software is based in part on the work of
  143 the Independent JPEG Group".
  144 (3) Permission for use of this software is granted only if the user accepts
  145 full responsibility for any undesirable consequences; the authors accept
  146 NO LIABILITY for damages of any kind.
  148 These conditions apply to any software derived from or based on the IJG code,
  149 not just to the unmodified library.  If you use our work, you ought to
  150 acknowledge us.
  152 Permission is NOT granted for the use of any IJG author's name or company name
  153 in advertising or publicity relating to this software or products derived from
  154 it.  This software may be referred to only as "the Independent JPEG Group's
  155 software".
  157 We specifically permit and encourage the use of this software as the basis of
  158 commercial products, provided that all warranty or liability claims are
  159 assumed by the product vendor.
  162 The IJG distribution formerly included code to read and write GIF files.
  163 To avoid entanglement with the Unisys LZW patent (now expired), GIF reading
  164 support has been removed altogether, and the GIF writer has been simplified
  165 to produce "uncompressed GIFs".  This technique does not use the LZW
  166 algorithm; the resulting GIF files are larger than usual, but are readable
  167 by all standard GIF decoders.
  169 We are required to state that
  170     "The Graphics Interchange Format(c) is the Copyright property of
  171     CompuServe Incorporated.  GIF(sm) is a Service Mark property of
  172     CompuServe Incorporated."
  176 ==========
  178 We recommend reading one or more of these references before trying to
  179 understand the innards of the JPEG software.
  181 The best short technical introduction to the JPEG compression algorithm is
  182         Wallace, Gregory K.  "The JPEG Still Picture Compression Standard",
  183         Communications of the ACM, April 1991 (vol. 34 no. 4), pp. 30-44.
  184 (Adjacent articles in that issue discuss MPEG motion picture compression,
  185 applications of JPEG, and related topics.)  If you don't have the CACM issue
  186 handy, a PDF file containing a revised version of Wallace's article is
  187 available at http://www.ijg.org/files/Wallace.JPEG.pdf.  The file (actually
  188 a preprint for an article that appeared in IEEE Trans. Consumer Electronics)
  189 omits the sample images that appeared in CACM, but it includes corrections
  190 and some added material.  Note: the Wallace article is copyright ACM and IEEE,
  191 and it may not be used for commercial purposes.
  193 A somewhat less technical, more leisurely introduction to JPEG can be found in
  194 "The Data Compression Book" by Mark Nelson and Jean-loup Gailly, published by
  195 M&T Books (New York), 2nd ed. 1996, ISBN 1-55851-434-1.  This book provides
  196 good explanations and example C code for a multitude of compression methods
  197 including JPEG.  It is an excellent source if you are comfortable reading C
  198 code but don't know much about data compression in general.  The book's JPEG
  199 sample code is far from industrial-strength, but when you are ready to look
  200 at a full implementation, you've got one here...
  202 The best currently available description of JPEG is the textbook "JPEG Still
  203 Image Data Compression Standard" by William B. Pennebaker and Joan L.
  204 Mitchell, published by Van Nostrand Reinhold, 1993, ISBN 0-442-01272-1.
  205 Price US$59.95, 638 pp.  The book includes the complete text of the ISO JPEG
  206 standards (DIS 10918-1 and draft DIS 10918-2).
  208 The original JPEG standard is divided into two parts, Part 1 being the actual
  209 specification, while Part 2 covers compliance testing methods.  Part 1 is
  210 titled "Digital Compression and Coding of Continuous-tone Still Images,
  211 Part 1: Requirements and guidelines" and has document numbers ISO/IEC IS
  212 10918-1, ITU-T T.81.  Part 2 is titled "Digital Compression and Coding of
  213 Continuous-tone Still Images, Part 2: Compliance testing" and has document
  214 numbers ISO/IEC IS 10918-2, ITU-T T.83.
  216 The JPEG standard does not specify all details of an interchangeable file
  217 format.  For the omitted details, we follow the "JFIF" conventions, revision
  218 1.02.  JFIF version 1 has been adopted as ISO/IEC 10918-5 (05/2013) and
  219 Recommendation ITU-T T.871 (05/2011): Information technology - Digital
  220 compression and coding of continuous-tone still images: JPEG File Interchange
  221 Format (JFIF).  It is available as a free download in PDF file format from
  222 https://www.iso.org/standard/54989.html and http://www.itu.int/rec/T-REC-T.871.
  223 A PDF file of the older JFIF 1.02 specification is available at
  224 http://www.w3.org/Graphics/JPEG/jfif3.pdf.
  226 The TIFF 6.0 file format specification can be obtained by FTP from
  227 ftp://ftp.sgi.com/graphics/tiff/TIFF6.ps.gz.  The JPEG incorporation scheme
  228 found in the TIFF 6.0 spec of 3-June-92 has a number of serious problems.
  229 IJG does not recommend use of the TIFF 6.0 design (TIFF Compression tag 6).
  230 Instead, we recommend the JPEG design proposed by TIFF Technical Note #2
  231 (Compression tag 7).  Copies of this Note can be obtained from
  232 http://www.ijg.org/files/.  It is expected that the next revision
  233 of the TIFF spec will replace the 6.0 JPEG design with the Note's design.
  234 Although IJG's own code does not support TIFF/JPEG, the free libtiff library
  235 uses our library to implement TIFF/JPEG per the Note.
  239 =================
  241 The "official" archive site for this software is www.ijg.org.
  242 The most recent released version can always be found there in
  243 directory "files".
  245 The JPEG FAQ (Frequently Asked Questions) article is a source of some
  246 general information about JPEG.
  247 It is available on the World Wide Web at http://www.faqs.org/faqs/jpeg-faq/
  248 and other news.answers archive sites, including the official news.answers
  249 archive at rtfm.mit.edu: ftp://rtfm.mit.edu/pub/usenet/news.answers/jpeg-faq/.
  250 If you don't have Web or FTP access, send e-mail to mail-server@rtfm.mit.edu
  251 with body
  252         send usenet/news.answers/jpeg-faq/part1
  253         send usenet/news.answers/jpeg-faq/part2
  257 =========================
  259 This software implements ITU T.81 | ISO/IEC 10918 with some extensions from
  260 ITU T.871 | ISO/IEC 10918-5 (JPEG File Interchange Format-- see REFERENCES).
  261 Informally, the term "JPEG image" or "JPEG file" most often refers to JFIF or
  262 a subset thereof, but there are other formats containing the name "JPEG" that
  263 are incompatible with the DCT-based JPEG standard or with JFIF (for instance,
  264 JPEG 2000 and JPEG XR).  This software therefore does not support these
  265 formats.  Indeed, one of the original reasons for developing this free software
  266 was to help force convergence on a common, interoperable format standard for
  267 JPEG files.
  269 JFIF is a minimal or "low end" representation.  TIFF/JPEG (TIFF revision 6.0 as
  270 modified by TIFF Technical Note #2) can be used for "high end" applications
  271 that need to record a lot of additional data about an image.
  274 TO DO
  275 =====
  277 Please send bug reports, offers of help, etc. to jpeg-info@jpegclub.org.