Revision control
++ libjpeg-turbo note: This file has been modified by The libjpeg-turbo Project
++ to include only information relevant to libjpeg-turbo, to wordsmith certain
++ sections, and to remove impolitic language that existed in the libjpeg v8
++ README. It is included only for reference. Please see README.md for
++ information specific to libjpeg-turbo.
++
++
++ The Independent JPEG Group's JPEG software
++ ==========================================
++
++ This distribution contains a release of the Independent JPEG Group's free JPEG
++ software. You are welcome to redistribute this software and to use it for any
++ purpose, subject to the conditions under LEGAL ISSUES, below.
++
++ This software is the work of Tom Lane, Guido Vollbeding, Philip Gladstone,
++ Bill Allombert, Jim Boucher, Lee Crocker, Bob Friesenhahn, Ben Jackson,
++ Julian Minguillon, Luis Ortiz, George Phillips, Davide Rossi, Ge' Weijers,
++ and other members of the Independent JPEG Group.
++
++ IJG is not affiliated with the ISO/IEC JTC1/SC29/WG1 standards committee
++ (also known as JPEG, together with ITU-T SG16).
++
++
++ DOCUMENTATION ROADMAP
++ =====================
++
++ This file contains the following sections:
++
++ OVERVIEW General description of JPEG and the IJG software.
++ LEGAL ISSUES Copyright, lack of warranty, terms of distribution.
++ REFERENCES Where to learn more about JPEG.
++ ARCHIVE LOCATIONS Where to find newer versions of this software.
++ FILE FORMAT WARS Software *not* to get.
++ TO DO Plans for future IJG releases.
++
++ Other documentation files in the distribution are:
++
++ User documentation:
++ doc/usage.txt Usage instructions for cjpeg, djpeg, jpegtran,
++ rdjpgcom, and wrjpgcom.
++ doc/*.1 Unix-style man pages for programs (same info as
++ usage.txt).
++ doc/wizard.txt Advanced usage instructions for JPEG wizards only.
++ doc/change.log Version-to-version change highlights.
++ Programmer and internal documentation:
++ doc/libjpeg.txt How to use the JPEG library in your own programs.
++ src/example.c Sample code for calling the JPEG library.
++ doc/structure.txt Overview of the JPEG library's internal structure.
++ doc/coderules.txt Coding style rules --- please read if you contribute
++ code.
++
++ Please read at least usage.txt. Some information can also be found in the JPEG
++ FAQ (Frequently Asked Questions) article. See ARCHIVE LOCATIONS below to find
++ out where to obtain the FAQ article.
++
++ If you want to understand how the JPEG code works, we suggest reading one or
++ more of the REFERENCES, then looking at the documentation files (in roughly
++ the order listed) before diving into the code.
++
++
++ OVERVIEW
++ ========
++
++ This package contains C software to implement JPEG image encoding, decoding,
++ and transcoding. JPEG (pronounced "jay-peg") is a standardized compression
++ method for full-color and grayscale images. JPEG's strong suit is compressing
++ photographic images or other types of images that have smooth color and
++ brightness transitions between neighboring pixels. Images with sharp lines or
++ other abrupt features may not compress well with JPEG, and a higher JPEG
++ quality may have to be used to avoid visible compression artifacts with such
++ images.
++
++ JPEG is normally lossy, meaning that the output pixels are not necessarily
++ identical to the input pixels. However, on photographic content and other
++ "smooth" images, very good compression ratios can be obtained with no visible
++ compression artifacts, and extremely high compression ratios are possible if
++ you are willing to sacrifice image quality (by reducing the "quality" setting
++ in the compressor.)
++
++ This software implements JPEG baseline, extended-sequential, progressive, and
++ lossless compression processes. Provision is made for supporting all variants
++ of these processes, although some uncommon parameter settings aren't
++ implemented yet. We have made no provision for supporting the hierarchical
++ processes defined in the standard.
++
++ We provide a set of library routines for reading and writing JPEG image files,
++ plus two sample applications "cjpeg" and "djpeg", which use the library to
++ perform conversion between JPEG and some other popular image file formats.
++ The library is intended to be reused in other applications.
++
++ In order to support file conversion and viewing software, we have included
++ considerable functionality beyond the bare JPEG coding/decoding capability;
++ for example, the color quantization modules are not strictly part of JPEG
++ decoding, but they are essential for output to colormapped file formats. These
++ extra functions can be compiled out of the library if not required for a
++ particular application.
++
++ We have also included "jpegtran", a utility for lossless transcoding between
++ different JPEG processes, and "rdjpgcom" and "wrjpgcom", two simple
++ applications for inserting and extracting textual comments in JFIF files.
++
++ The emphasis in designing this software has been on achieving portability and
++ flexibility, while also making it fast enough to be useful. In particular,
++ the software is not intended to be read as a tutorial on JPEG. (See the
++ REFERENCES section for introductory material.) Rather, it is intended to
++ be reliable, portable, industrial-strength code. We do not claim to have
++ achieved that goal in every aspect of the software, but we strive for it.
++
++ We welcome the use of this software as a component of commercial products.
++ No royalty is required, but we do ask for an acknowledgement in product
++ documentation, as described under LEGAL ISSUES.
++
++
++ LEGAL ISSUES
++ ============
++
++ In plain English:
++
++ 1. We don't promise that this software works. (But if you find any bugs,
++ please let us know!)
++ 2. You can use this software for whatever you want. You don't have to pay us.
++ 3. You may not pretend that you wrote this software. If you use it in a
++ program, you must acknowledge somewhere in your documentation that
++ you've used the IJG code.
++
++ In legalese:
++
++ The authors make NO WARRANTY or representation, either express or implied,
++ with respect to this software, its quality, accuracy, merchantability, or
++ fitness for a particular purpose. This software is provided "AS IS", and you,
++ its user, assume the entire risk as to its quality and accuracy.
++
++ This software is copyright (C) 1991-2020, Thomas G. Lane, Guido Vollbeding.
++ All Rights Reserved except as specified below.
++
++ Permission is hereby granted to use, copy, modify, and distribute this
++ software (or portions thereof) for any purpose, without fee, subject to these
++ conditions:
++ (1) If any part of the source code for this software is distributed, then this
++ README file must be included, with this copyright and no-warranty notice
++ unaltered; and any additions, deletions, or changes to the original files
++ must be clearly indicated in accompanying documentation.
++ (2) If only executable code is distributed, then the accompanying
++ documentation must state that "this software is based in part on the work of
++ the Independent JPEG Group".
++ (3) Permission for use of this software is granted only if the user accepts
++ full responsibility for any undesirable consequences; the authors accept
++ NO LIABILITY for damages of any kind.
++
++ These conditions apply to any software derived from or based on the IJG code,
++ not just to the unmodified library. If you use our work, you ought to
++ acknowledge us.
++
++ Permission is NOT granted for the use of any IJG author's name or company name
++ in advertising or publicity relating to this software or products derived from
++ it. This software may be referred to only as "the Independent JPEG Group's
++ software".
++
++ We specifically permit and encourage the use of this software as the basis of
++ commercial products, provided that all warranty or liability claims are
++ assumed by the product vendor.
++
++
++ REFERENCES
++ ==========
++
++ We recommend reading one or more of these references before trying to
++ understand the innards of the JPEG software.
++
++ The best short technical introduction to the JPEG compression algorithm is
++ Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
++ Communications of the ACM, April 1991 (vol. 34 no. 4), pp. 30-44.
++ (Adjacent articles in that issue discuss MPEG motion picture compression,
++ applications of JPEG, and related topics.) If you don't have the CACM issue
++ handy, a PDF file containing a revised version of Wallace's article is
++ available at http://www.ijg.org/files/Wallace.JPEG.pdf. The file (actually
++ a preprint for an article that appeared in IEEE Trans. Consumer Electronics)
++ omits the sample images that appeared in CACM, but it includes corrections
++ and some added material. Note: the Wallace article is copyright ACM and IEEE,
++ and it may not be used for commercial purposes.
++
++ A somewhat less technical, more leisurely introduction to JPEG can be found in
++ "The Data Compression Book" by Mark Nelson and Jean-loup Gailly, published by
++ M&T Books (New York), 2nd ed. 1996, ISBN 1-55851-434-1. This book provides
++ good explanations and example C code for a multitude of compression methods
++ including JPEG. It is an excellent source if you are comfortable reading C
++ code but don't know much about data compression in general. The book's JPEG
++ sample code is far from industrial-strength, but when you are ready to look
++ at a full implementation, you've got one here...
++
++ The best currently available description of JPEG is the textbook "JPEG Still
++ Image Data Compression Standard" by William B. Pennebaker and Joan L.
++ Mitchell, published by Van Nostrand Reinhold, 1993, ISBN 0-442-01272-1.
++ Price US$59.95, 638 pp. The book includes the complete text of the ISO JPEG
++ standards (DIS 10918-1 and draft DIS 10918-2).
++
++ The original JPEG standard is divided into two parts, Part 1 being the actual
++ specification, while Part 2 covers compliance testing methods. Part 1 is
++ titled "Digital Compression and Coding of Continuous-tone Still Images,
++ Part 1: Requirements and guidelines" and has document numbers ISO/IEC IS
++ 10918-1, ITU-T T.81. Part 2 is titled "Digital Compression and Coding of
++ Continuous-tone Still Images, Part 2: Compliance testing" and has document
++ numbers ISO/IEC IS 10918-2, ITU-T T.83.
++
++ The JPEG standard does not specify all details of an interchangeable file
++ format. For the omitted details, we follow the "JFIF" conventions, revision
++ 1.02. JFIF version 1 has been adopted as ISO/IEC 10918-5 (05/2013) and
++ Recommendation ITU-T T.871 (05/2011): Information technology - Digital
++ compression and coding of continuous-tone still images: JPEG File Interchange
++ Format (JFIF). It is available as a free download in PDF file format from
++ A PDF file of the older JFIF 1.02 specification is available at
++
++ The TIFF 6.0 file format specification can be obtained from
++ http://mirrors.ctan.org/graphics/tiff/TIFF6.ps.gz. The JPEG incorporation
++ scheme found in the TIFF 6.0 spec of 3-June-92 has a number of serious
++ problems. IJG does not recommend use of the TIFF 6.0 design (TIFF Compression
++ tag 6). Instead, we recommend the JPEG design proposed by TIFF Technical Note
++ #2 (Compression tag 7). Copies of this Note can be obtained from
++ http://www.ijg.org/files/. It is expected that the next revision
++ of the TIFF spec will replace the 6.0 JPEG design with the Note's design.
++ Although IJG's own code does not support TIFF/JPEG, the free libtiff library
++ uses our library to implement TIFF/JPEG per the Note.
++
++
++ ARCHIVE LOCATIONS
++ =================
++
++ The "official" archive site for this software is www.ijg.org.
++ The most recent released version can always be found there in
++ directory "files".
++
++ The JPEG FAQ (Frequently Asked Questions) article is a source of some
++ general information about JPEG. It is available at
++
++
++ FILE FORMAT COMPATIBILITY
++ =========================
++
++ This software implements ITU T.81 | ISO/IEC 10918 with some extensions from
++ ITU T.871 | ISO/IEC 10918-5 (JPEG File Interchange Format-- see REFERENCES).
++ Informally, the term "JPEG image" or "JPEG file" most often refers to JFIF or
++ a subset thereof, but there are other formats containing the name "JPEG" that
++ are incompatible with the original JPEG standard or with JFIF (for instance,
++ JPEG 2000 and JPEG XR). This software therefore does not support these
++ formats. Indeed, one of the original reasons for developing this free software
++ was to help force convergence on a common, interoperable format standard for
++ JPEG files.
++
++ JFIF is a minimal or "low end" representation. TIFF/JPEG (TIFF revision 6.0 as
++ modified by TIFF Technical Note #2) can be used for "high end" applications
++ that need to record a lot of additional data about an image.
++
++
++ TO DO
++ =====
++
++ Please send bug reports, offers of help, etc. to jpeg-info@jpegclub.org.