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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. |