michael@0: libjpeg-turbo note: This file has been modified by The libjpeg-turbo Project michael@0: to include only information relevant to libjpeg-turbo, to wordsmith certain michael@0: sections, and to remove impolitic language that existed in the libjpeg v8 michael@0: README. It is included only for reference. Please see README-turbo.txt for michael@0: information specific to libjpeg-turbo. michael@0: michael@0: michael@0: The Independent JPEG Group's JPEG software michael@0: ========================================== michael@0: michael@0: This distribution contains a release of the Independent JPEG Group's free JPEG michael@0: software. You are welcome to redistribute this software and to use it for any michael@0: purpose, subject to the conditions under LEGAL ISSUES, below. michael@0: michael@0: This software is the work of Tom Lane, Guido Vollbeding, Philip Gladstone, michael@0: Bill Allombert, Jim Boucher, Lee Crocker, Bob Friesenhahn, Ben Jackson, michael@0: Julian Minguillon, Luis Ortiz, George Phillips, Davide Rossi, Ge' Weijers, michael@0: and other members of the Independent JPEG Group. michael@0: michael@0: IJG is not affiliated with the ISO/IEC JTC1/SC29/WG1 standards committee michael@0: (also known as JPEG, together with ITU-T SG16). michael@0: michael@0: michael@0: DOCUMENTATION ROADMAP michael@0: ===================== michael@0: michael@0: This file contains the following sections: michael@0: michael@0: OVERVIEW General description of JPEG and the IJG software. michael@0: LEGAL ISSUES Copyright, lack of warranty, terms of distribution. michael@0: REFERENCES Where to learn more about JPEG. michael@0: ARCHIVE LOCATIONS Where to find newer versions of this software. michael@0: FILE FORMAT WARS Software *not* to get. michael@0: TO DO Plans for future IJG releases. michael@0: michael@0: Other documentation files in the distribution are: michael@0: michael@0: User documentation: michael@0: install.txt How to configure and install the IJG software. michael@0: usage.txt Usage instructions for cjpeg, djpeg, jpegtran, michael@0: rdjpgcom, and wrjpgcom. michael@0: *.1 Unix-style man pages for programs (same info as usage.txt). michael@0: wizard.txt Advanced usage instructions for JPEG wizards only. michael@0: change.log Version-to-version change highlights. michael@0: Programmer and internal documentation: michael@0: libjpeg.txt How to use the JPEG library in your own programs. michael@0: example.c Sample code for calling the JPEG library. michael@0: structure.txt Overview of the JPEG library's internal structure. michael@0: coderules.txt Coding style rules --- please read if you contribute code. michael@0: michael@0: Please read at least the files install.txt and usage.txt. Some information michael@0: can also be found in the JPEG FAQ (Frequently Asked Questions) article. See michael@0: ARCHIVE LOCATIONS below to find out where to obtain the FAQ article. michael@0: michael@0: If you want to understand how the JPEG code works, we suggest reading one or michael@0: more of the REFERENCES, then looking at the documentation files (in roughly michael@0: the order listed) before diving into the code. michael@0: michael@0: michael@0: OVERVIEW michael@0: ======== michael@0: michael@0: This package contains C software to implement JPEG image encoding, decoding, michael@0: and transcoding. JPEG (pronounced "jay-peg") is a standardized compression michael@0: method for full-color and gray-scale images. JPEG's strong suit is compressing michael@0: photographic images or other types of images that have smooth color and michael@0: brightness transitions between neighboring pixels. Images with sharp lines or michael@0: other abrupt features may not compress well with JPEG, and a higher JPEG michael@0: quality may have to be used to avoid visible compression artifacts with such michael@0: images. michael@0: michael@0: JPEG is lossy, meaning that the output pixels are not necessarily identical to michael@0: the input pixels. However, on photographic content and other "smooth" images, michael@0: very good compression ratios can be obtained with no visible compression michael@0: artifacts, and extremely high compression ratios are possible if you are michael@0: willing to sacrifice image quality (by reducing the "quality" setting in the michael@0: compressor.) michael@0: michael@0: This software implements JPEG baseline, extended-sequential, and progressive michael@0: compression processes. Provision is made for supporting all variants of these michael@0: processes, although some uncommon parameter settings aren't implemented yet. michael@0: We have made no provision for supporting the hierarchical or lossless michael@0: processes defined in the standard. michael@0: michael@0: We provide a set of library routines for reading and writing JPEG image files, michael@0: plus two sample applications "cjpeg" and "djpeg", which use the library to michael@0: perform conversion between JPEG and some other popular image file formats. michael@0: The library is intended to be reused in other applications. michael@0: michael@0: In order to support file conversion and viewing software, we have included michael@0: considerable functionality beyond the bare JPEG coding/decoding capability; michael@0: for example, the color quantization modules are not strictly part of JPEG michael@0: decoding, but they are essential for output to colormapped file formats or michael@0: colormapped displays. These extra functions can be compiled out of the michael@0: library if not required for a particular application. michael@0: michael@0: We have also included "jpegtran", a utility for lossless transcoding between michael@0: different JPEG processes, and "rdjpgcom" and "wrjpgcom", two simple michael@0: applications for inserting and extracting textual comments in JFIF files. michael@0: michael@0: The emphasis in designing this software has been on achieving portability and michael@0: flexibility, while also making it fast enough to be useful. In particular, michael@0: the software is not intended to be read as a tutorial on JPEG. (See the michael@0: REFERENCES section for introductory material.) Rather, it is intended to michael@0: be reliable, portable, industrial-strength code. We do not claim to have michael@0: achieved that goal in every aspect of the software, but we strive for it. michael@0: michael@0: We welcome the use of this software as a component of commercial products. michael@0: No royalty is required, but we do ask for an acknowledgement in product michael@0: documentation, as described under LEGAL ISSUES. michael@0: michael@0: michael@0: LEGAL ISSUES michael@0: ============ michael@0: michael@0: In plain English: michael@0: michael@0: 1. We don't promise that this software works. (But if you find any bugs, michael@0: please let us know!) michael@0: 2. You can use this software for whatever you want. You don't have to pay us. michael@0: 3. You may not pretend that you wrote this software. If you use it in a michael@0: program, you must acknowledge somewhere in your documentation that michael@0: you've used the IJG code. michael@0: michael@0: In legalese: michael@0: michael@0: The authors make NO WARRANTY or representation, either express or implied, michael@0: with respect to this software, its quality, accuracy, merchantability, or michael@0: fitness for a particular purpose. This software is provided "AS IS", and you, michael@0: its user, assume the entire risk as to its quality and accuracy. michael@0: michael@0: This software is copyright (C) 1991-2012, Thomas G. Lane, Guido Vollbeding. michael@0: All Rights Reserved except as specified below. michael@0: michael@0: Permission is hereby granted to use, copy, modify, and distribute this michael@0: software (or portions thereof) for any purpose, without fee, subject to these michael@0: conditions: michael@0: (1) If any part of the source code for this software is distributed, then this michael@0: README file must be included, with this copyright and no-warranty notice michael@0: unaltered; and any additions, deletions, or changes to the original files michael@0: must be clearly indicated in accompanying documentation. michael@0: (2) If only executable code is distributed, then the accompanying michael@0: documentation must state that "this software is based in part on the work of michael@0: the Independent JPEG Group". michael@0: (3) Permission for use of this software is granted only if the user accepts michael@0: full responsibility for any undesirable consequences; the authors accept michael@0: NO LIABILITY for damages of any kind. michael@0: michael@0: These conditions apply to any software derived from or based on the IJG code, michael@0: not just to the unmodified library. If you use our work, you ought to michael@0: acknowledge us. michael@0: michael@0: Permission is NOT granted for the use of any IJG author's name or company name michael@0: in advertising or publicity relating to this software or products derived from michael@0: it. This software may be referred to only as "the Independent JPEG Group's michael@0: software". michael@0: michael@0: We specifically permit and encourage the use of this software as the basis of michael@0: commercial products, provided that all warranty or liability claims are michael@0: assumed by the product vendor. michael@0: michael@0: michael@0: The Unix configuration script "configure" was produced with GNU Autoconf. michael@0: It is copyright by the Free Software Foundation but is freely distributable. michael@0: The same holds for its supporting scripts (config.guess, config.sub, michael@0: ltmain.sh). Another support script, install-sh, is copyright by X Consortium michael@0: but is also freely distributable. michael@0: michael@0: The IJG distribution formerly included code to read and write GIF files. michael@0: To avoid entanglement with the Unisys LZW patent, GIF reading support has michael@0: been removed altogether, and the GIF writer has been simplified to produce michael@0: "uncompressed GIFs". This technique does not use the LZW algorithm; the michael@0: resulting GIF files are larger than usual, but are readable by all standard michael@0: GIF decoders. michael@0: michael@0: We are required to state that michael@0: "The Graphics Interchange Format(c) is the Copyright property of michael@0: CompuServe Incorporated. GIF(sm) is a Service Mark property of michael@0: CompuServe Incorporated." michael@0: michael@0: michael@0: REFERENCES michael@0: ========== michael@0: michael@0: We recommend reading one or more of these references before trying to michael@0: understand the innards of the JPEG software. michael@0: michael@0: The best short technical introduction to the JPEG compression algorithm is michael@0: Wallace, Gregory K. "The JPEG Still Picture Compression Standard", michael@0: Communications of the ACM, April 1991 (vol. 34 no. 4), pp. 30-44. michael@0: (Adjacent articles in that issue discuss MPEG motion picture compression, michael@0: applications of JPEG, and related topics.) If you don't have the CACM issue michael@0: handy, a PostScript file containing a revised version of Wallace's article is michael@0: available at http://www.ijg.org/files/wallace.ps.gz. The file (actually michael@0: a preprint for an article that appeared in IEEE Trans. Consumer Electronics) michael@0: omits the sample images that appeared in CACM, but it includes corrections michael@0: and some added material. Note: the Wallace article is copyright ACM and IEEE, michael@0: and it may not be used for commercial purposes. michael@0: michael@0: A somewhat less technical, more leisurely introduction to JPEG can be found in michael@0: "The Data Compression Book" by Mark Nelson and Jean-loup Gailly, published by michael@0: M&T Books (New York), 2nd ed. 1996, ISBN 1-55851-434-1. This book provides michael@0: good explanations and example C code for a multitude of compression methods michael@0: including JPEG. It is an excellent source if you are comfortable reading C michael@0: code but don't know much about data compression in general. The book's JPEG michael@0: sample code is far from industrial-strength, but when you are ready to look michael@0: at a full implementation, you've got one here... michael@0: michael@0: The best currently available description of JPEG is the textbook "JPEG Still michael@0: Image Data Compression Standard" by William B. Pennebaker and Joan L. michael@0: Mitchell, published by Van Nostrand Reinhold, 1993, ISBN 0-442-01272-1. michael@0: Price US$59.95, 638 pp. The book includes the complete text of the ISO JPEG michael@0: standards (DIS 10918-1 and draft DIS 10918-2). michael@0: michael@0: The original JPEG standard is divided into two parts, Part 1 being the actual michael@0: specification, while Part 2 covers compliance testing methods. Part 1 is michael@0: titled "Digital Compression and Coding of Continuous-tone Still Images, michael@0: Part 1: Requirements and guidelines" and has document numbers ISO/IEC IS michael@0: 10918-1, ITU-T T.81. Part 2 is titled "Digital Compression and Coding of michael@0: Continuous-tone Still Images, Part 2: Compliance testing" and has document michael@0: numbers ISO/IEC IS 10918-2, ITU-T T.83. michael@0: michael@0: The JPEG standard does not specify all details of an interchangeable file michael@0: format. For the omitted details we follow the "JFIF" conventions, revision michael@0: 1.02. JFIF 1.02 has been adopted as an Ecma International Technical Report michael@0: and thus received a formal publication status. It is available as a free michael@0: download in PDF format from michael@0: http://www.ecma-international.org/publications/techreports/E-TR-098.htm. michael@0: A PostScript version of the JFIF document is available at michael@0: http://www.ijg.org/files/jfif.ps.gz. There is also a plain text version at michael@0: http://www.ijg.org/files/jfif.txt.gz, but it is missing the figures. michael@0: michael@0: The TIFF 6.0 file format specification can be obtained by FTP from michael@0: ftp://ftp.sgi.com/graphics/tiff/TIFF6.ps.gz. The JPEG incorporation scheme michael@0: found in the TIFF 6.0 spec of 3-June-92 has a number of serious problems. michael@0: IJG does not recommend use of the TIFF 6.0 design (TIFF Compression tag 6). michael@0: Instead, we recommend the JPEG design proposed by TIFF Technical Note #2 michael@0: (Compression tag 7). Copies of this Note can be obtained from michael@0: http://www.ijg.org/files/. It is expected that the next revision michael@0: of the TIFF spec will replace the 6.0 JPEG design with the Note's design. michael@0: Although IJG's own code does not support TIFF/JPEG, the free libtiff library michael@0: uses our library to implement TIFF/JPEG per the Note. michael@0: michael@0: michael@0: ARCHIVE LOCATIONS michael@0: ================= michael@0: michael@0: The "official" archive site for this software is www.ijg.org. michael@0: The most recent released version can always be found there in michael@0: directory "files". This particular version will be archived as michael@0: http://www.ijg.org/files/jpegsrc.v8d.tar.gz, and in Windows-compatible michael@0: "zip" archive format as http://www.ijg.org/files/jpegsr8d.zip. michael@0: michael@0: The JPEG FAQ (Frequently Asked Questions) article is a source of some michael@0: general information about JPEG. michael@0: It is available on the World Wide Web at http://www.faqs.org/faqs/jpeg-faq/ michael@0: and other news.answers archive sites, including the official news.answers michael@0: archive at rtfm.mit.edu: ftp://rtfm.mit.edu/pub/usenet/news.answers/jpeg-faq/. michael@0: If you don't have Web or FTP access, send e-mail to mail-server@rtfm.mit.edu michael@0: with body michael@0: send usenet/news.answers/jpeg-faq/part1 michael@0: send usenet/news.answers/jpeg-faq/part2 michael@0: michael@0: michael@0: FILE FORMAT WARS michael@0: ================ michael@0: michael@0: The ISO/IEC JTC1/SC29/WG1 standards committee (also known as JPEG, together michael@0: with ITU-T SG16) currently promotes different formats containing the name michael@0: "JPEG" which are incompatible with original DCT-based JPEG. IJG therefore does michael@0: not support these formats (see REFERENCES). Indeed, one of the original michael@0: reasons for developing this free software was to help force convergence on michael@0: common, interoperable format standards for JPEG files. michael@0: Don't use an incompatible file format! michael@0: (In any case, our decoder will remain capable of reading existing JPEG michael@0: image files indefinitely.) michael@0: michael@0: michael@0: TO DO michael@0: ===== michael@0: michael@0: Please send bug reports, offers of help, etc. to jpeg-info@jpegclub.org.