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+The Independent JPEG Group's JPEG software
+README for release 6b of 27-Mar-1998
+This distribution contains the sixth public 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.
+Serious users of this software (particularly those incorporating it into
+larger programs) should contact IJG at to be added to
+our electronic mailing list. Mailing list members are notified of updates
+and have a chance to participate in technical discussions, etc.
+This software is the work of Tom Lane, Philip Gladstone, Jim Boucher,
+Lee Crocker, Julian Minguillon, Luis Ortiz, George Phillips, Davide Rossi,
+Guido Vollbeding, Ge' Weijers, and other members of the Independent JPEG
+IJG is not affiliated with the official ISO JPEG standards committee.
+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.
+RELATED SOFTWARE Other stuff you should get.
+FILE FORMAT WARS Software *not* to get.
+TO DO Plans for future IJG releases.
+Other documentation files in the distribution are:
+User documentation:
+ install.doc How to configure and install the IJG software.
+ usage.doc Usage instructions for cjpeg, djpeg, jpegtran,
+ rdjpgcom, and wrjpgcom.
+ *.1 Unix-style man pages for programs (same info as usage.doc).
+ wizard.doc Advanced usage instructions for JPEG wizards only.
+ change.log Version-to-version change highlights.
+Programmer and internal documentation:
+ libjpeg.doc How to use the JPEG library in your own programs.
+ example.c Sample code for calling the JPEG library.
+ structure.doc Overview of the JPEG library's internal structure.
+ filelist.doc Road map of IJG files.
+ coderules.doc Coding style rules --- please read if you contribute code.
+Please read at least the files install.doc and usage.doc. Useful 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.
+This package contains C software to implement JPEG image compression and
+decompression. JPEG (pronounced "jay-peg") is a standardized compression
+method for full-color and gray-scale images. JPEG is intended for compressing
+"real-world" scenes; line drawings, cartoons and other non-realistic images
+are not its strong suit. JPEG is lossy, meaning that the output image is not
+exactly identical to the input image. Hence you must not use JPEG if you
+have to have identical output bits. However, on typical photographic images,
+very good compression levels can be obtained with no visible change, and
+remarkably high compression levels are possible if you can tolerate a
+low-quality image. For more details, see the references, or just experiment
+with various compression settings.
+This software implements JPEG baseline, extended-sequential, and progressive
+compression processes. Provision is made for supporting all variants of these
+processes, although some uncommon parameter settings aren't implemented yet.
+For legal reasons, we are not distributing code for the arithmetic-coding
+variants of JPEG; see LEGAL ISSUES. We have made no provision for supporting
+the hierarchical or lossless 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 or
+colormapped displays. 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.
+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) 1991-1998, Thomas G. Lane.
+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
+(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
+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.
+ansi2knr.c is included in this distribution by permission of L. Peter Deutsch,
+sole proprietor of its copyright holder, Aladdin Enterprises of Menlo Park, CA.
+ansi2knr.c is NOT covered by the above copyright and conditions, but instead
+by the usual distribution terms of the Free Software Foundation; principally,
+that you must include source code if you redistribute it. (See the file
+ansi2knr.c for full details.) However, since ansi2knr.c is not needed as part
+of any program generated from the IJG code, this does not limit you more than
+the foregoing paragraphs do.
+The Unix configuration script "configure" was produced with GNU Autoconf.
+It is copyright by the Free Software Foundation but is freely distributable.
+The same holds for its supporting scripts (config.guess, config.sub,
+ltconfig, Another support script, install-sh, is copyright
+by M.I.T. but is also freely distributable.
+It appears that the arithmetic coding option of the JPEG spec is covered by
+patents owned by IBM, AT&T, and Mitsubishi. Hence arithmetic coding cannot
+legally be used without obtaining one or more licenses. For this reason,
+support for arithmetic coding has been removed from the free JPEG software.
+(Since arithmetic coding provides only a marginal gain over the unpatented
+Huffman mode, it is unlikely that very many implementations will support it.)
+So far as we are aware, there are no patent restrictions on the remaining
+The IJG distribution formerly included code to read and write GIF files.
+To avoid entanglement with the Unisys LZW patent, GIF reading support has
+been removed altogether, and the GIF writer has been simplified to produce
+"uncompressed GIFs". This technique does not use the LZW algorithm; the
+resulting GIF files are larger than usual, but are readable by all standard
+GIF decoders.
+We are required to state that
+ "The Graphics Interchange Format(c) is the Copyright property of
+ CompuServe Incorporated. GIF(sm) is a Service Mark property of
+ CompuServe Incorporated."
+We highly 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 PostScript file containing a revised version of Wallace's article is
+available at 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 full 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). This is by far the most complete exposition of JPEG
+in existence, and we highly recommend it.
+The JPEG standard itself is not available electronically; you must order a
+paper copy through ISO or ITU. (Unless you feel a need to own a certified
+official copy, we recommend buying the Pennebaker and Mitchell book instead;
+it's much cheaper and includes a great deal of useful explanatory material.)
+In the USA, copies of the standard may be ordered from ANSI Sales at (212)
+642-4900, or from Global Engineering Documents at (800) 854-7179. (ANSI
+doesn't take credit card orders, but Global does.) It's not cheap: as of
+1992, ANSI was charging $95 for Part 1 and $47 for Part 2, plus 7%
+shipping/handling. The 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.
+Some extensions to the original JPEG standard are defined in JPEG Part 3,
+a newer ISO standard numbered ISO/IEC IS 10918-3 and ITU-T T.84. IJG
+currently does not support any Part 3 extensions.
+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. A copy of the JFIF spec is available from:
+ Literature Department
+ C-Cube Microsystems, Inc.
+ 1778 McCarthy Blvd.
+ Milpitas, CA 95035
+ phone (408) 944-6300, fax (408) 944-6314
+A PostScript version of this document is available by FTP at
+ There is also a plain text
+version at, but it is missing
+the figures.
+The TIFF 6.0 file format specification can be obtained by FTP from
+ 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 or
+from 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. libtiff is available
+The "official" archive site for this software is (Internet
+address The most recent released version can always be found
+there in directory graphics/jpeg. This particular version will be archived
+as If you don't have
+direct Internet access, UUNET's archives are also available via UUCP; contact for information on retrieving files that way.
+Numerous Internet sites maintain copies of the UUNET files. However, only is guaranteed to have the latest official version.
+You can also obtain this software in DOS-compatible "zip" archive format from
+the SimTel archives (, or
+on CompuServe in the Graphics Support forum (GO CIS:GRAPHSUP), library 12
+"JPEG Tools". Again, these versions may sometimes lag behind the
+The JPEG FAQ (Frequently Asked Questions) article is a useful source of
+general information about JPEG. It is updated constantly and therefore is
+not included in this distribution. The FAQ is posted every two weeks to
+Usenet newsgroups, news.answers, and other groups.
+It is available on the World Wide Web at
+and other news.answers archive sites, including the official news.answers
+archive at
+If you don't have Web or FTP access, send e-mail to
+with body
+ send usenet/news.answers/jpeg-faq/part1
+ send usenet/news.answers/jpeg-faq/part2
+Numerous viewing and image manipulation programs now support JPEG. (Quite a
+few of them use this library to do so.) The JPEG FAQ described above lists
+some of the more popular free and shareware viewers, and tells where to
+obtain them on Internet.
+If you are on a Unix machine, we highly recommend Jef Poskanzer's free
+PBMPLUS software, which provides many useful operations on PPM-format image
+files. In particular, it can convert PPM images to and from a wide range of
+other formats, thus making cjpeg/djpeg considerably more useful. The latest
+version is distributed by the NetPBM group, and is available from numerous
+sites, notably
+Unfortunately PBMPLUS/NETPBM is not nearly as portable as the IJG software is;
+you are likely to have difficulty making it work on any non-Unix machine.
+A different free JPEG implementation, written by the PVRG group at Stanford,
+is available from This program
+is designed for research and experimentation rather than production use;
+it is slower, harder to use, and less portable than the IJG code, but it
+is easier to read and modify. Also, the PVRG code supports lossless JPEG,
+which we do not. (On the other hand, it doesn't do progressive JPEG.)
+Some JPEG programs produce files that are not compatible with our library.
+The root of the problem is that the ISO JPEG committee failed to specify a
+concrete file format. Some vendors "filled in the blanks" on their own,
+creating proprietary formats that no one else could read. (For example, none
+of the early commercial JPEG implementations for the Macintosh were able to
+exchange compressed files.)
+The file format we have adopted is called JFIF (see REFERENCES). This format
+has been agreed to by a number of major commercial JPEG vendors, and it has
+become the de facto standard. JFIF is a minimal or "low end" representation.
+We recommend the use of TIFF/JPEG (TIFF revision 6.0 as modified by TIFF
+Technical Note #2) for "high end" applications that need to record a lot of
+additional data about an image. TIFF/JPEG is fairly new and not yet widely
+supported, unfortunately.
+The upcoming JPEG Part 3 standard defines a file format called SPIFF.
+SPIFF is interoperable with JFIF, in the sense that most JFIF decoders should
+be able to read the most common variant of SPIFF. SPIFF has some technical
+advantages over JFIF, but its major claim to fame is simply that it is an
+official standard rather than an informal one. At this point it is unclear
+whether SPIFF will supersede JFIF or whether JFIF will remain the de-facto
+standard. IJG intends to support SPIFF once the standard is frozen, but we
+have not decided whether it should become our default output format or not.
+(In any case, our decoder will remain capable of reading JFIF indefinitely.)
+Various proprietary file formats incorporating JPEG compression also exist.
+We have little or no sympathy for the existence of these formats. Indeed,
+one of the original reasons for developing this free software was to help
+force convergence on common, open format standards for JPEG files. Don't
+use a proprietary file format!
+The major thrust for v7 will probably be improvement of visual quality.
+The current method for scaling the quantization tables is known not to be
+very good at low Q values. We also intend to investigate block boundary
+smoothing, "poor man's variable quantization", and other means of improving
+quality-vs-file-size performance without sacrificing compatibility.
+In future versions, we are considering supporting some of the upcoming JPEG
+Part 3 extensions --- principally, variable quantization and the SPIFF file
+As always, speeding things up is of great interest.
+Please send bug reports, offers of help, etc. to

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