kopia lustrzana https://gitlab.com/sane-project/backends
Moved from djpeg.README in backend directory
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The JPEG decoder in SANE is taken mostly without change from The
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Independent JPEG Group's JPEG software, release 6a. Their "djpeg.README"
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file is included below. The only changes are to file names, e.g.
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djpeg.c -> sanei_jpeg.c, and function names,
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e.g. sanei_jpeg_start_output_ppm()
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The Independent JPEG Group's JPEG software
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==========================================
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README for release 6a of 7-Feb-96
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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, Luis Ortiz, Jim
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Boucher, Lee Crocker, Julian Minguillon, George Phillips, Davide Rossi,
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Ge' Weijers, and other members of the Independent JPEG Group.
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IJG is not affiliated with the official ISO JPEG standards committee.
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DOCUMENTATION ROADMAP
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=====================
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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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User documentation:
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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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OVERVIEW
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========
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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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|
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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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|
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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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LEGAL ISSUES
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============
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In plain English:
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1. We don't promise that this software works. (But if you find any bugs,
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please let us know!)
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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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In legalese:
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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) 1991-1996, 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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conditions:
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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
|
||||
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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acknowledge us.
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|
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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
|
||||
software".
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|
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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
|
||||
assumed by the product vendor.
|
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|
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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
|
||||
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
|
||||
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.
|
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|
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The configuration script "configure" was produced with GNU Autoconf. It
|
||||
is copyright by the Free Software Foundation but is 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
|
||||
legally be used without obtaining one or more licenses. For this reason,
|
||||
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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code.
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|
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WARNING: Unisys has begun to enforce their patent on LZW compression against
|
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GIF encoders and decoders. You will need a license from Unisys to use the
|
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included rdgif.c or wrgif.c files in a commercial or shareware application.
|
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At this time, Unisys is not enforcing their patent against freeware, so
|
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distribution of this package remains legal. However, we intend to remove
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GIF support from the IJG package as soon as a suitable replacement format
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becomes reasonably popular.
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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
|
||||
CompuServe Incorporated."
|
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|
||||
|
||||
REFERENCES
|
||||
==========
|
||||
|
||||
We highly recommend reading one or more of these references before trying to
|
||||
understand the innards of the JPEG software.
|
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|
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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",
|
||||
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.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
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||||
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, published by M&T Books (Redwood
|
||||
City, CA), 1991, ISBN 1-55851-216-0. 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.
|
||||
|
||||
Extensions to the original JPEG standard are defined in JPEG Part 3, a new ISO
|
||||
document. Part 3 is undergoing ISO balloting and is expected to be approved
|
||||
by the end of 1995; it will have document numbers ISO/IEC IS 10918-3, ITU-T
|
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T.84. IJG currently does not support any Part 3 extensions.
|
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|
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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:
|
||||
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 at ftp.uu.net, file
|
||||
graphics/jpeg/jfif.ps.gz. It can also be obtained by e-mail from the C-Cube
|
||||
mail server, netlib@c3.pla.ca.us. Send the message "send jfif_ps from jpeg"
|
||||
to the server to obtain the JFIF document; send the message "help" if you have
|
||||
trouble.
|
||||
|
||||
The TIFF 6.0 file format specification can be obtained by FTP from sgi.com
|
||||
(192.48.153.1), file graphics/tiff/TIFF6.ps.Z; or you can order a printed
|
||||
copy from Aldus Corp. at (206) 628-6593. The JPEG incorporation scheme
|
||||
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).
|
||||
Instead, we recommend the JPEG design proposed by TIFF Technical Note #2
|
||||
(Compression tag 7). Copies of this Note can be obtained from sgi.com or
|
||||
from 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 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 graphics/jpeg/jpegsrc.v6a.tar.gz. If you are on the Internet, you
|
||||
can retrieve files from ftp.uu.net by standard anonymous FTP. If you don't
|
||||
have FTP 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.coast.net:/SimTel/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.
|
||||
You can always obtain the latest version from the news.answers archive at
|
||||
rtfm.mit.edu. By FTP, fetch /pub/usenet/news.answers/jpeg-faq/part1 and
|
||||
.../part2. If you don't have FTP, 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 image 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. You can obtain this package by FTP from ftp.x.org
|
||||
(contrib/pbmplus*.tar.Z) or ftp.ee.lbl.gov (pbmplus*.tar.Z). There is also
|
||||
a newer update of this package called NETPBM, available from
|
||||
wuarchive.wustl.edu under directory /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 havefun.stanford.edu in directory 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.
|
||||
|
||||
|
||||
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
|
||||
=====
|
||||
|
||||
In future versions, we are considering supporting some of the upcoming JPEG
|
||||
Part 3 extensions --- principally, variable quantization and the SPIFF file
|
||||
format.
|
||||
|
||||
Tuning the software for better behavior at low quality/high compression
|
||||
settings is also of interest. The current method for scaling the
|
||||
quantization tables is known not to be very good at low Q values.
|
||||
|
||||
As always, speeding things up is high on our priority list.
|
||||
|
||||
Please send bug reports, offers of help, etc. to jpeg-info@uunet.uu.net.
|
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