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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 jpeg-info@uunet.uu.net 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
|
| -Group.
|
| -
|
| -IJG is not affiliated with the official ISO JPEG standards committee.
|
| -
|
| -
|
| -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.
|
| -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.
|
| -
|
| -
|
| -OVERVIEW
|
| -========
|
| -
|
| -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.
|
| -
|
| -
|
| -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
|
| -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.
|
| -
|
| -
|
| -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, ltmain.sh). 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
|
| -code.
|
| -
|
| -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."
|
| -
|
| -
|
| -REFERENCES
|
| -==========
|
| -
|
| -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 ftp://ftp.uu.net/graphics/jpeg/wallace.ps.gz. 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
|
| -ftp://ftp.uu.net/graphics/jpeg/jfif.ps.gz. There is also a plain text
|
| -version at ftp://ftp.uu.net/graphics/jpeg/jfif.txt.gz, but it is missing
|
| -the figures.
|
| -
|
| -The TIFF 6.0 file format specification can be obtained by FTP from
|
| -ftp://ftp.sgi.com/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 ftp.sgi.com or
|
| -from ftp://ftp.uu.net/graphics/jpeg/. 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
|
| -from ftp://ftp.sgi.com/graphics/tiff/.
|
| -
|
| -
|
| -ARCHIVE LOCATIONS
|
| -=================
|
| -
|
| -The "official" archive site for this software is ftp.uu.net (Internet
|
| -address 192.48.96.9). The most recent released version can always be found
|
| -there in directory graphics/jpeg. This particular version will be archived
|
| -as ftp://ftp.uu.net/graphics/jpeg/jpegsrc.v6b.tar.gz. If you don't have
|
| -direct Internet access, UUNET's archives are also available via UUCP; contact
|
| -help@uunet.uu.net for information on retrieving files that way.
|
| -
|
| -Numerous Internet sites maintain copies of the UUNET files. However, only
|
| -ftp.uu.net is guaranteed to have the latest official version.
|
| -
|
| -You can also obtain this software in DOS-compatible "zip" archive format from
|
| -the SimTel archives (ftp://ftp.simtel.net/pub/simtelnet/msdos/graphics/), or
|
| -on CompuServe in the Graphics Support forum (GO CIS:GRAPHSUP), library 12
|
| -"JPEG Tools". Again, these versions may sometimes lag behind the ftp.uu.net
|
| -release.
|
| -
|
| -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 comp.graphics.misc, news.answers, and other groups.
|
| -It is available on the World Wide Web at http://www.faqs.org/faqs/jpeg-faq/
|
| -and other news.answers archive sites, including the official news.answers
|
| -archive at rtfm.mit.edu: ftp://rtfm.mit.edu/pub/usenet/news.answers/jpeg-faq/.
|
| -If you don't have Web or FTP access, send e-mail to mail-server@rtfm.mit.edu
|
| -with body
|
| - send usenet/news.answers/jpeg-faq/part1
|
| - send usenet/news.answers/jpeg-faq/part2
|
| -
|
| -
|
| -RELATED SOFTWARE
|
| -================
|
| -
|
| -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 ftp://wuarchive.wustl.edu/graphics/graphics/packages/NetPBM/.
|
| -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 ftp://havefun.stanford.edu/pub/jpeg/. 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.)
|
| -
|
| -
|
| -FILE FORMAT WARS
|
| -================
|
| -
|
| -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!
|
| -
|
| -
|
| -TO DO
|
| -=====
|
| -
|
| -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
|
| -format.
|
| -
|
| -As always, speeding things up is of great interest.
|
| -
|
| -Please send bug reports, offers of help, etc. to jpeg-info@uunet.uu.net.
|
|
|