19c791cdac
With rare exceptions ...
- Always separate line continuation characters by one space from
preceding code.
- Always use two-space indentation. Never use tabs.
- Always use K&R-style conditional blocks.
- Always surround operators with spaces, except in raw assembly code.
- Always put a space after, but not before, a comma.
- Never put a space between type casts and variables/function calls.
- Never put a space between the function name and the argument list in
function declarations and prototypes.
- Always surround braces ('{' and '}') with spaces.
- Always surround statements (if, for, else, catch, while, do, switch)
with spaces.
- Always attach pointer symbols ('*' and '**') to the variable or
function name.
- Always precede pointer symbols ('*' and '**') by a space in type
casts.
- Use the MIN() macro from jpegint.h within the libjpeg and TurboJPEG
API libraries (using min() from tjutil.h is still necessary for
TJBench.)
- Where it makes sense (particularly in the TurboJPEG code), put a blank
line after variable declaration blocks.
- Always separate statements in one-liners by two spaces.
The purpose of this was to ease maintenance on my part and also to make
it easier for contributors to figure out how to format patch
submissions. This was admittedly confusing (even to me sometimes) when
we had 3 or 4 different style conventions in the same source tree. The
new convention is more consistent with the formatting of other OSS code
bases.
This commit corrects deviations from the chosen formatting style in the
libjpeg API code and reformats the TurboJPEG API code such that it
conforms to the same standard.
NOTES:
- Although it is no longer necessary for the function name in function
declarations to begin in Column 1 (this was historically necessary
because of the ansi2knr utility, which allowed libjpeg to be built
with non-ANSI compilers), we retain that formatting for the libjpeg
code because it improves readability when using libjpeg's function
attribute macros (GLOBAL(), etc.)
- This reformatting project was accomplished with the help of AStyle and
Uncrustify, although neither was completely up to the task, and thus
a great deal of manual tweaking was required. Note to developers of
code formatting utilities: the libjpeg-turbo code base is an
excellent test bed, because AFAICT, it breaks every single one of the
utilities that are currently available.
- The legacy (MMX, SSE, 3DNow!) assembly code for i386 has been
formatted to match the SSE2 code (refer to
ff5685d5344273df321eb63a005eaae19d2496e3.) I hadn't intended to
bother with this, but the Loongson MMI implementation demonstrated
that there is still academic value to the MMX implementation, as an
algorithmic model for other 64-bit vector implementations. Thus, it
is desirable to improve its readability in the same manner as that of
the SSE2 implementation.
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libjpeg-turbo note: This file has been modified by The libjpeg-turbo Project
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to include only information relevant to libjpeg-turbo, to wordsmith certain
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sections, and to remove impolitic language that existed in the libjpeg v8
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README. It is included only for reference. Please see README.md for
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information specific to libjpeg-turbo.
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The Independent JPEG Group's JPEG software
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==========================================
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This distribution contains a release of the Independent JPEG Group's free JPEG
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software. You are welcome to redistribute this software and to use it for any
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purpose, subject to the conditions under LEGAL ISSUES, below.
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This software is the work of Tom Lane, Guido Vollbeding, Philip Gladstone,
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Bill Allombert, Jim Boucher, Lee Crocker, Bob Friesenhahn, Ben Jackson,
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Julian Minguillon, Luis Ortiz, George Phillips, Davide Rossi, Ge' Weijers,
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and other members of the Independent JPEG Group.
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IJG is not affiliated with the ISO/IEC JTC1/SC29/WG1 standards committee
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(also known as JPEG, together with ITU-T SG16).
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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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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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usage.txt 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.txt).
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wizard.txt 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.txt How to use the JPEG library in your own programs.
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example.txt Sample code for calling the JPEG library.
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structure.txt Overview of the JPEG library's internal structure.
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coderules.txt Coding style rules --- please read if you contribute code.
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Please read at least usage.txt. Some information can also be found in the JPEG
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FAQ (Frequently Asked Questions) article. See ARCHIVE LOCATIONS below to find
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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 encoding, decoding,
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and transcoding. JPEG (pronounced "jay-peg") is a standardized compression
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method for full-color and grayscale images. JPEG's strong suit is compressing
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photographic images or other types of images that have smooth color and
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brightness transitions between neighboring pixels. Images with sharp lines or
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other abrupt features may not compress well with JPEG, and a higher JPEG
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quality may have to be used to avoid visible compression artifacts with such
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images.
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JPEG is lossy, meaning that the output pixels are not necessarily identical to
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the input pixels. However, on photographic content and other "smooth" images,
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very good compression ratios can be obtained with no visible compression
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artifacts, and extremely high compression ratios are possible if you are
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willing to sacrifice image quality (by reducing the "quality" setting in the
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compressor.)
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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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We have made no provision for supporting the hierarchical or lossless
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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.
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We have also included "jpegtran", a utility for lossless transcoding between
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different JPEG processes, and "rdjpgcom" and "wrjpgcom", two simple
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applications for inserting and extracting textual comments in JFIF files.
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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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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) 1991-2016, Thomas G. Lane, Guido Vollbeding.
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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
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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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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
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software".
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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
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assumed by the product vendor.
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The IJG distribution formerly included code to read and write GIF files.
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To avoid entanglement with the Unisys LZW patent (now expired), GIF reading
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support has been removed altogether, and the GIF writer has been simplified
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to produce "uncompressed GIFs". This technique does not use the LZW
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algorithm; the resulting GIF files are larger than usual, but are readable
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by all standard GIF decoders.
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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
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CompuServe Incorporated."
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REFERENCES
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==========
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We recommend reading one or more of these references before trying to
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understand the innards of the JPEG software.
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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",
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Communications of the ACM, April 1991 (vol. 34 no. 4), pp. 30-44.
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(Adjacent articles in that issue discuss MPEG motion picture compression,
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applications of JPEG, and related topics.) If you don't have the CACM issue
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handy, a PDF file containing a revised version of Wallace's article is
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available at http://www.ijg.org/files/Wallace.JPEG.pdf. The file (actually
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a preprint for an article that appeared in IEEE Trans. Consumer Electronics)
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omits the sample images that appeared in CACM, but it includes corrections
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and some added material. Note: the Wallace article is copyright ACM and IEEE,
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and it may not be used for commercial purposes.
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A somewhat less technical, more leisurely introduction to JPEG can be found in
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"The Data Compression Book" by Mark Nelson and Jean-loup Gailly, published by
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M&T Books (New York), 2nd ed. 1996, ISBN 1-55851-434-1. This book provides
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good explanations and example C code for a multitude of compression methods
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including JPEG. It is an excellent source if you are comfortable reading C
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code but don't know much about data compression in general. The book's JPEG
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sample code is far from industrial-strength, but when you are ready to look
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at a full implementation, you've got one here...
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The best currently available description of JPEG is the textbook "JPEG Still
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Image Data Compression Standard" by William B. Pennebaker and Joan L.
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Mitchell, published by Van Nostrand Reinhold, 1993, ISBN 0-442-01272-1.
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Price US$59.95, 638 pp. The book includes the complete text of the ISO JPEG
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standards (DIS 10918-1 and draft DIS 10918-2).
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The original JPEG standard is divided into two parts, Part 1 being the actual
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specification, while Part 2 covers compliance testing methods. Part 1 is
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titled "Digital Compression and Coding of Continuous-tone Still Images,
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Part 1: Requirements and guidelines" and has document numbers ISO/IEC IS
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10918-1, ITU-T T.81. Part 2 is titled "Digital Compression and Coding of
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Continuous-tone Still Images, Part 2: Compliance testing" and has document
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numbers ISO/IEC IS 10918-2, ITU-T T.83.
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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. JFIF 1.02 has been adopted as an Ecma International Technical Report
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and thus received a formal publication status. It is available as a free
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download in PDF format from
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http://www.ecma-international.org/publications/techreports/E-TR-098.htm.
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A PostScript version of the JFIF document is available at
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http://www.ijg.org/files/jfif.ps.gz. There is also a plain text version at
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http://www.ijg.org/files/jfif.txt.gz, but it is missing the figures.
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The TIFF 6.0 file format specification can be obtained by FTP from
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ftp://ftp.sgi.com/graphics/tiff/TIFF6.ps.gz. The JPEG incorporation scheme
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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).
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Instead, we recommend the JPEG design proposed by TIFF Technical Note #2
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(Compression tag 7). Copies of this Note can be obtained from
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http://www.ijg.org/files/. It is expected that the next revision
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of the TIFF spec will replace the 6.0 JPEG design with the Note's design.
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Although IJG's own code does not support TIFF/JPEG, the free libtiff library
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uses our library to implement TIFF/JPEG per the Note.
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ARCHIVE LOCATIONS
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=================
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The "official" archive site for this software is www.ijg.org.
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The most recent released version can always be found there in
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directory "files".
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The JPEG FAQ (Frequently Asked Questions) article is a source of some
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general information about JPEG.
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It is available on the World Wide Web at http://www.faqs.org/faqs/jpeg-faq/
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and other news.answers archive sites, including the official news.answers
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archive at rtfm.mit.edu: ftp://rtfm.mit.edu/pub/usenet/news.answers/jpeg-faq/.
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If you don't have Web or FTP access, send e-mail to mail-server@rtfm.mit.edu
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with body
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send usenet/news.answers/jpeg-faq/part1
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send usenet/news.answers/jpeg-faq/part2
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FILE FORMAT WARS
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================
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The ISO/IEC JTC1/SC29/WG1 standards committee (also known as JPEG, together
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with ITU-T SG16) currently promotes different formats containing the name
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"JPEG" which are incompatible with original DCT-based JPEG. IJG therefore does
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not support these formats (see REFERENCES). Indeed, one of the original
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reasons for developing this free software was to help force convergence on
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common, interoperable format standards for JPEG files.
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Don't use an incompatible file format!
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(In any case, our decoder will remain capable of reading existing JPEG
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image files indefinitely.)
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TO DO
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=====
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Please send bug reports, offers of help, etc. to jpeg-info@jpegclub.org.
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