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The Independent JPEG Group's JPEG software
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README for release 6b of 27-Mar-1998
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====================================
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This distribution contains the sixth public release of the Independent JPEG
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Group's free JPEG software. You are welcome to redistribute this software and
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to use it for any purpose, subject to the conditions under LEGAL ISSUES, below.
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Serious users of this software (particularly those incorporating it into
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larger programs) should contact IJG at jpeg-info@uunet.uu.net to be added to
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our electronic mailing list. Mailing list members are notified of updates
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and have a chance to participate in technical discussions, etc.
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This software is the work of Tom Lane, Philip Gladstone, Jim Boucher,
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Lee Crocker, Julian Minguillon, Luis Ortiz, George Phillips, Davide Rossi,
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Guido Vollbeding, Ge' Weijers, and other members of the Independent JPEG
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IJG is not affiliated with the official ISO JPEG standards committee.
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This file contains the following sections:
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OVERVIEW General description of JPEG and the IJG software.
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LEGAL ISSUES Copyright, lack of warranty, terms of distribution.
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REFERENCES Where to learn more about JPEG.
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ARCHIVE LOCATIONS Where to find newer versions of this software.
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RELATED SOFTWARE Other stuff you should get.
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FILE FORMAT WARS Software *not* to get.
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TO DO Plans for future IJG releases.
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Other documentation files in the distribution are:
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install.doc How to configure and install the IJG software.
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usage.doc Usage instructions for cjpeg, djpeg, jpegtran,
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rdjpgcom, and wrjpgcom.
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*.1 Unix-style man pages for programs (same info as usage.doc).
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wizard.doc Advanced usage instructions for JPEG wizards only.
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change.log Version-to-version change highlights.
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Programmer and internal documentation:
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libjpeg.doc How to use the JPEG library in your own programs.
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example.c Sample code for calling the JPEG library.
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structure.doc Overview of the JPEG library's internal structure.
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filelist.doc Road map of IJG files.
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coderules.doc Coding style rules --- please read if you contribute code.
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Please read at least the files install.doc and usage.doc. Useful information
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can also be found in the JPEG FAQ (Frequently Asked Questions) article. See
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ARCHIVE LOCATIONS below to find out where to obtain the FAQ article.
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If you want to understand how the JPEG code works, we suggest reading one or
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more of the REFERENCES, then looking at the documentation files (in roughly
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the order listed) before diving into the code.
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This package contains C software to implement JPEG image compression and
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decompression. JPEG (pronounced "jay-peg") is a standardized compression
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method for full-color and gray-scale images. JPEG is intended for compressing
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"real-world" scenes; line drawings, cartoons and other non-realistic images
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are not its strong suit. JPEG is lossy, meaning that the output image is not
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exactly identical to the input image. Hence you must not use JPEG if you
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have to have identical output bits. However, on typical photographic images,
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very good compression levels can be obtained with no visible change, and
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remarkably high compression levels are possible if you can tolerate a
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low-quality image. For more details, see the references, or just experiment
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with various compression settings.
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This software implements JPEG baseline, extended-sequential, and progressive
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compression processes. Provision is made for supporting all variants of these
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processes, although some uncommon parameter settings aren't implemented yet.
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For legal reasons, we are not distributing code for the arithmetic-coding
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variants of JPEG; see LEGAL ISSUES. We have made no provision for supporting
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the hierarchical or lossless processes defined in the standard.
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We provide a set of library routines for reading and writing JPEG image files,
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plus two sample applications "cjpeg" and "djpeg", which use the library to
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perform conversion between JPEG and some other popular image file formats.
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The library is intended to be reused in other applications.
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In order to support file conversion and viewing software, we have included
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considerable functionality beyond the bare JPEG coding/decoding capability;
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for example, the color quantization modules are not strictly part of JPEG
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decoding, but they are essential for output to colormapped file formats or
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colormapped displays. These extra functions can be compiled out of the
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library if not required for a particular application. We have also included
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"jpegtran", a utility for lossless transcoding between different JPEG
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processes, and "rdjpgcom" and "wrjpgcom", two simple applications for
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inserting and extracting textual comments in JFIF files.
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The emphasis in designing this software has been on achieving portability and
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flexibility, while also making it fast enough to be useful. In particular,
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the software is not intended to be read as a tutorial on JPEG. (See the
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REFERENCES section for introductory material.) Rather, it is intended to
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be reliable, portable, industrial-strength code. We do not claim to have
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achieved that goal in every aspect of the software, but we strive for it.
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We welcome the use of this software as a component of commercial products.
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No royalty is required, but we do ask for an acknowledgement in product
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documentation, as described under LEGAL ISSUES.
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1. We don't promise that this software works. (But if you find any bugs,
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2. You can use this software for whatever you want. You don't have to pay us.
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3. You may not pretend that you wrote this software. If you use it in a
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program, you must acknowledge somewhere in your documentation that
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you've used the IJG code.
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The authors make NO WARRANTY or representation, either express or implied,
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with respect to this software, its quality, accuracy, merchantability, or
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fitness for a particular purpose. This software is provided "AS IS", and you,
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its user, assume the entire risk as to its quality and accuracy.
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This software is copyright (C) 1991-1998, Thomas G. Lane.
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All Rights Reserved except as specified below.
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Permission is hereby granted to use, copy, modify, and distribute this
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software (or portions thereof) for any purpose, without fee, subject to these
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(1) If any part of the source code for this software is distributed, then this
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README file must be included, with this copyright and no-warranty notice
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unaltered; and any additions, deletions, or changes to the original files
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must be clearly indicated in accompanying documentation.
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(2) If only executable code is distributed, then the accompanying
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documentation must state that "this software is based in part on the work of
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the Independent JPEG Group".
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(3) Permission for use of this software is granted only if the user accepts
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full responsibility for any undesirable consequences; the authors accept
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NO LIABILITY for damages of any kind.
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These conditions apply to any software derived from or based on the IJG code,
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not just to the unmodified library. If you use our work, you ought to
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Permission is NOT granted for the use of any IJG author's name or company name
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in advertising or publicity relating to this software or products derived from
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it. This software may be referred to only as "the Independent JPEG Group's
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We specifically permit and encourage the use of this software as the basis of
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commercial products, provided that all warranty or liability claims are
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assumed by the product vendor.
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ansi2knr.c is included in this distribution by permission of L. Peter Deutsch,
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sole proprietor of its copyright holder, Aladdin Enterprises of Menlo Park, CA.
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ansi2knr.c is NOT covered by the above copyright and conditions, but instead
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by the usual distribution terms of the Free Software Foundation; principally,
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that you must include source code if you redistribute it. (See the file
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ansi2knr.c for full details.) However, since ansi2knr.c is not needed as part
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of any program generated from the IJG code, this does not limit you more than
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the foregoing paragraphs do.
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The Unix configuration script "configure" was produced with GNU Autoconf.
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It is copyright by the Free Software Foundation but is freely distributable.
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The same holds for its supporting scripts (config.guess, config.sub,
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ltconfig, ltmain.sh). Another support script, install-sh, is copyright
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by M.I.T. but is also freely distributable.
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It appears that the arithmetic coding option of the JPEG spec is covered by
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patents owned by IBM, AT&T, and Mitsubishi. Hence arithmetic coding cannot
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legally be used without obtaining one or more licenses. For this reason,
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support for arithmetic coding has been removed from the free JPEG software.
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(Since arithmetic coding provides only a marginal gain over the unpatented
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Huffman mode, it is unlikely that very many implementations will support it.)
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So far as we are aware, there are no patent restrictions on the remaining
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The IJG distribution formerly included code to read and write GIF files.
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To avoid entanglement with the Unisys LZW patent, GIF reading support has
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been removed altogether, and the GIF writer has been simplified to produce
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"uncompressed GIFs". This technique does not use the LZW algorithm; the
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resulting GIF files are larger than usual, but are readable by all standard
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We are required to state that
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"The Graphics Interchange Format(c) is the Copyright property of
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CompuServe Incorporated. GIF(sm) is a Service Mark property of
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CompuServe Incorporated."
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We highly recommend reading one or more of these references before trying to
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understand the innards of the JPEG software.
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The best short technical introduction to the JPEG compression algorithm is
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Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
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Communications of the ACM, April 1991 (vol. 34 no. 4), pp. 30-44.
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(Adjacent articles in that issue discuss MPEG motion picture compression,
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applications of JPEG, and related topics.) If you don't have the CACM issue
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handy, a PostScript file containing a revised version of Wallace's article is
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available at ftp://ftp.uu.net/graphics/jpeg/wallace.ps.gz. The file (actually
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a preprint for an article that appeared in IEEE Trans. Consumer Electronics)
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omits the sample images that appeared in CACM, but it includes corrections
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and some added material. Note: the Wallace article is copyright ACM and IEEE,
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and it may not be used for commercial purposes.
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A somewhat less technical, more leisurely introduction to JPEG can be found in
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"The Data Compression Book" by Mark Nelson and Jean-loup Gailly, published by
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M&T Books (New York), 2nd ed. 1996, ISBN 1-55851-434-1. This book provides
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good explanations and example C code for a multitude of compression methods
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including JPEG. It is an excellent source if you are comfortable reading C
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code but don't know much about data compression in general. The book's JPEG
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sample code is far from industrial-strength, but when you are ready to look
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at a full implementation, you've got one here...
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The best full description of JPEG is the textbook "JPEG Still Image Data
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Compression Standard" by William B. Pennebaker and Joan L. Mitchell, published
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by Van Nostrand Reinhold, 1993, ISBN 0-442-01272-1. Price US$59.95, 638 pp.
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The book includes the complete text of the ISO JPEG standards (DIS 10918-1
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and draft DIS 10918-2). This is by far the most complete exposition of JPEG
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in existence, and we highly recommend it.
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The JPEG standard itself is not available electronically; you must order a
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paper copy through ISO or ITU. (Unless you feel a need to own a certified
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official copy, we recommend buying the Pennebaker and Mitchell book instead;
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it's much cheaper and includes a great deal of useful explanatory material.)
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In the USA, copies of the standard may be ordered from ANSI Sales at (212)
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642-4900, or from Global Engineering Documents at (800) 854-7179. (ANSI
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doesn't take credit card orders, but Global does.) It's not cheap: as of
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1992, ANSI was charging $95 for Part 1 and $47 for Part 2, plus 7%
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shipping/handling. The standard is divided into two parts, Part 1 being the
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actual specification, while Part 2 covers compliance testing methods. Part 1
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is titled "Digital Compression and Coding of Continuous-tone Still Images,
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Part 1: Requirements and guidelines" and has document numbers ISO/IEC IS
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10918-1, ITU-T T.81. Part 2 is titled "Digital Compression and Coding of
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Continuous-tone Still Images, Part 2: Compliance testing" and has document
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numbers ISO/IEC IS 10918-2, ITU-T T.83.
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Some extensions to the original JPEG standard are defined in JPEG Part 3,
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a newer ISO standard numbered ISO/IEC IS 10918-3 and ITU-T T.84. IJG
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currently does not support any Part 3 extensions.
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The JPEG standard does not specify all details of an interchangeable file
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format. For the omitted details we follow the "JFIF" conventions, revision
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1.02. A copy of the JFIF spec is available from:
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Literature Department
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C-Cube Microsystems, Inc.
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phone (408) 944-6300, fax (408) 944-6314
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A PostScript version of this document is available by FTP at
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ftp://ftp.uu.net/graphics/jpeg/jfif.ps.gz. There is also a plain text
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version at ftp://ftp.uu.net/graphics/jpeg/jfif.txt.gz, but it is missing
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The TIFF 6.0 file format specification can be obtained by FTP from
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ftp://ftp.sgi.com/graphics/tiff/TIFF6.ps.gz. The JPEG incorporation scheme
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found in the TIFF 6.0 spec of 3-June-92 has a number of serious problems.
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IJG does not recommend use of the TIFF 6.0 design (TIFF Compression tag 6).
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Instead, we recommend the JPEG design proposed by TIFF Technical Note #2
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(Compression tag 7). Copies of this Note can be obtained from ftp.sgi.com or
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from ftp://ftp.uu.net/graphics/jpeg/. It is expected that the next revision
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of the TIFF spec will replace the 6.0 JPEG design with the Note's design.
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Although IJG's own code does not support TIFF/JPEG, the free libtiff library
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uses our library to implement TIFF/JPEG per the Note. libtiff is available
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from ftp://ftp.sgi.com/graphics/tiff/.
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The "official" archive site for this software is ftp.uu.net (Internet
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address 192.48.96.9). The most recent released version can always be found
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there in directory graphics/jpeg. This particular version will be archived
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as ftp://ftp.uu.net/graphics/jpeg/jpegsrc.v6b.tar.gz. If you don't have
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direct Internet access, UUNET's archives are also available via UUCP; contact
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help@uunet.uu.net for information on retrieving files that way.
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Numerous Internet sites maintain copies of the UUNET files. However, only
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ftp.uu.net is guaranteed to have the latest official version.
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You can also obtain this software in DOS-compatible "zip" archive format from
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the SimTel archives (ftp://ftp.simtel.net/pub/simtelnet/msdos/graphics/), or
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on CompuServe in the Graphics Support forum (GO CIS:GRAPHSUP), library 12
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"JPEG Tools". Again, these versions may sometimes lag behind the ftp.uu.net
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The JPEG FAQ (Frequently Asked Questions) article is a useful source of
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general information about JPEG. It is updated constantly and therefore is
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not included in this distribution. The FAQ is posted every two weeks to
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Usenet newsgroups comp.graphics.misc, news.answers, and other groups.
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It is available on the World Wide Web at http://www.faqs.org/faqs/jpeg-faq/
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and other news.answers archive sites, including the official news.answers
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archive at rtfm.mit.edu: ftp://rtfm.mit.edu/pub/usenet/news.answers/jpeg-faq/.
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If you don't have Web or FTP access, send e-mail to mail-server@rtfm.mit.edu
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send usenet/news.answers/jpeg-faq/part1
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send usenet/news.answers/jpeg-faq/part2
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Numerous viewing and image manipulation programs now support JPEG. (Quite a
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few of them use this library to do so.) The JPEG FAQ described above lists
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some of the more popular free and shareware viewers, and tells where to
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obtain them on Internet.
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If you are on a Unix machine, we highly recommend Jef Poskanzer's free
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PBMPLUS software, which provides many useful operations on PPM-format image
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files. In particular, it can convert PPM images to and from a wide range of
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other formats, thus making cjpeg/djpeg considerably more useful. The latest
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version is distributed by the NetPBM group, and is available from numerous
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sites, notably ftp://wuarchive.wustl.edu/graphics/graphics/packages/NetPBM/.
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Unfortunately PBMPLUS/NETPBM is not nearly as portable as the IJG software is;
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you are likely to have difficulty making it work on any non-Unix machine.
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A different free JPEG implementation, written by the PVRG group at Stanford,
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is available from ftp://havefun.stanford.edu/pub/jpeg/. This program
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is designed for research and experimentation rather than production use;
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it is slower, harder to use, and less portable than the IJG code, but it
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is easier to read and modify. Also, the PVRG code supports lossless JPEG,
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which we do not. (On the other hand, it doesn't do progressive JPEG.)
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Some JPEG programs produce files that are not compatible with our library.
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The root of the problem is that the ISO JPEG committee failed to specify a
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concrete file format. Some vendors "filled in the blanks" on their own,
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creating proprietary formats that no one else could read. (For example, none
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of the early commercial JPEG implementations for the Macintosh were able to
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exchange compressed files.)
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The file format we have adopted is called JFIF (see REFERENCES). This format
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has been agreed to by a number of major commercial JPEG vendors, and it has
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become the de facto standard. JFIF is a minimal or "low end" representation.
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We recommend the use of TIFF/JPEG (TIFF revision 6.0 as modified by TIFF
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Technical Note #2) for "high end" applications that need to record a lot of
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additional data about an image. TIFF/JPEG is fairly new and not yet widely
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supported, unfortunately.
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The upcoming JPEG Part 3 standard defines a file format called SPIFF.
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SPIFF is interoperable with JFIF, in the sense that most JFIF decoders should
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be able to read the most common variant of SPIFF. SPIFF has some technical
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advantages over JFIF, but its major claim to fame is simply that it is an
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official standard rather than an informal one. At this point it is unclear
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whether SPIFF will supersede JFIF or whether JFIF will remain the de-facto
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standard. IJG intends to support SPIFF once the standard is frozen, but we
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have not decided whether it should become our default output format or not.
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(In any case, our decoder will remain capable of reading JFIF indefinitely.)
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Various proprietary file formats incorporating JPEG compression also exist.
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We have little or no sympathy for the existence of these formats. Indeed,
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one of the original reasons for developing this free software was to help
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force convergence on common, open format standards for JPEG files. Don't
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use a proprietary file format!
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The major thrust for v7 will probably be improvement of visual quality.
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The current method for scaling the quantization tables is known not to be
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very good at low Q values. We also intend to investigate block boundary
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smoothing, "poor man's variable quantization", and other means of improving
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quality-vs-file-size performance without sacrificing compatibility.
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In future versions, we are considering supporting some of the upcoming JPEG
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Part 3 extensions --- principally, variable quantization and the SPIFF file
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As always, speeding things up is of great interest.
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Please send bug reports, offers of help, etc. to jpeg-info@uunet.uu.net.
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