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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) 1991-2020, 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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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 version 1 has been adopted as ISO/IEC 10918-5 (05/2013) and
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Recommendation ITU-T T.871 (05/2011): Information technology - Digital
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compression and coding of continuous-tone still images: JPEG File Interchange
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Format (JFIF). It is available as a free download in PDF file format from
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https://www.iso.org/standard/54989.html and http://www.itu.int/rec/T-REC-T.871.
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A PDF file of the older JFIF 1.02 specification is available at
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http://www.w3.org/Graphics/JPEG/jfif3.pdf.
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The TIFF 6.0 file format specification can be obtained from
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http://mirrors.ctan.org/graphics/tiff/TIFF6.ps.gz. The JPEG incorporation
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scheme found in the TIFF 6.0 spec of 3-June-92 has a number of serious
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problems. IJG does not recommend use of the TIFF 6.0 design (TIFF Compression
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tag 6). Instead, we recommend the JPEG design proposed by TIFF Technical Note
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#2 (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. It is available at
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http://www.faqs.org/faqs/jpeg-faq.
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FILE FORMAT COMPATIBILITY
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=========================
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This software implements ITU T.81 | ISO/IEC 10918 with some extensions from
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ITU T.871 | ISO/IEC 10918-5 (JPEG File Interchange Format-- see REFERENCES).
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Informally, the term "JPEG image" or "JPEG file" most often refers to JFIF or
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a subset thereof, but there are other formats containing the name "JPEG" that
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are incompatible with the DCT-based JPEG standard or with JFIF (for instance,
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JPEG 2000 and JPEG XR). This software therefore does not support these
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formats. Indeed, one of the original reasons for developing this free software
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was to help force convergence on a common, interoperable format standard for
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JPEG files.
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JFIF is a minimal or "low end" representation. TIFF/JPEG (TIFF revision 6.0 as
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modified by TIFF Technical Note #2) can be used for "high end" applications
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that need to record a lot of additional data about an image.
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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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