зеркало из https://github.com/mozilla/mozjpeg.git
386 строки
20 KiB
Plaintext
386 строки
20 KiB
Plaintext
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.
|