| libjpeg-turbo note: This file has been modified by The libjpeg-turbo Project |
| to include only information relevant to libjpeg-turbo, to wordsmith certain |
| sections, and to remove impolitic language that existed in the libjpeg v8 |
| README. It is included only for reference. Please see README.md for |
| information specific to libjpeg-turbo. |
| |
| |
| The Independent JPEG Group's JPEG software |
| ========================================== |
| |
| This distribution contains a 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. |
| |
| This software is the work of Tom Lane, Guido Vollbeding, Philip Gladstone, |
| Bill Allombert, Jim Boucher, Lee Crocker, Bob Friesenhahn, Ben Jackson, |
| Julian Minguillon, Luis Ortiz, George Phillips, Davide Rossi, Ge' Weijers, |
| and other members of the Independent JPEG Group. |
| |
| IJG is not affiliated with the ISO/IEC JTC1/SC29/WG1 standards committee |
| (also known as JPEG, together with ITU-T SG16). |
| |
| |
| 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. |
| FILE FORMAT WARS Software *not* to get. |
| TO DO Plans for future IJG releases. |
| |
| Other documentation files in the distribution are: |
| |
| User documentation: |
| usage.txt Usage instructions for cjpeg, djpeg, jpegtran, |
| rdjpgcom, and wrjpgcom. |
| *.1 Unix-style man pages for programs (same info as usage.txt). |
| wizard.txt Advanced usage instructions for JPEG wizards only. |
| change.log Version-to-version change highlights. |
| Programmer and internal documentation: |
| libjpeg.txt How to use the JPEG library in your own programs. |
| example.txt Sample code for calling the JPEG library. |
| structure.txt Overview of the JPEG library's internal structure. |
| coderules.txt Coding style rules --- please read if you contribute code. |
| |
| Please read at least usage.txt. Some 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 encoding, decoding, |
| and transcoding. JPEG (pronounced "jay-peg") is a standardized compression |
| method for full-color and grayscale images. JPEG's strong suit is compressing |
| photographic images or other types of images that have smooth color and |
| brightness transitions between neighboring pixels. Images with sharp lines or |
| other abrupt features may not compress well with JPEG, and a higher JPEG |
| quality may have to be used to avoid visible compression artifacts with such |
| images. |
| |
| JPEG is lossy, meaning that the output pixels are not necessarily identical to |
| the input pixels. However, on photographic content and other "smooth" images, |
| very good compression ratios can be obtained with no visible compression |
| artifacts, and extremely high compression ratios are possible if you are |
| willing to sacrifice image quality (by reducing the "quality" setting in the |
| compressor.) |
| |
| 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. |
| 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) 1991-2016, Thomas G. Lane, Guido Vollbeding. |
| 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. |
| |
| |
| The IJG distribution formerly included code to read and write GIF files. |
| To avoid entanglement with the Unisys LZW patent (now expired), 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 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 PDF file containing a revised version of Wallace's article is |
| available at http://www.ijg.org/files/Wallace.JPEG.pdf. 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 currently available 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). |
| |
| The original JPEG 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. |
| |
| 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. JFIF version 1 has been adopted as ISO/IEC 10918-5 (05/2013) and |
| Recommendation ITU-T T.871 (05/2011): Information technology - Digital |
| compression and coding of continuous-tone still images: JPEG File Interchange |
| Format (JFIF). It is available as a free download in PDF file format from |
| https://www.iso.org/standard/54989.html and http://www.itu.int/rec/T-REC-T.871. |
| A PDF file of the older JFIF 1.02 specification is available at |
| http://www.w3.org/Graphics/JPEG/jfif3.pdf. |
| |
| The TIFF 6.0 file format specification can be obtained from |
| http://mirrors.ctan.org/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 |
| http://www.ijg.org/files/. 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. |
| |
| |
| ARCHIVE LOCATIONS |
| ================= |
| |
| The "official" archive site for this software is www.ijg.org. |
| The most recent released version can always be found there in |
| directory "files". |
| |
| The JPEG FAQ (Frequently Asked Questions) article is a source of some |
| general information about JPEG. It is available at |
| http://www.faqs.org/faqs/jpeg-faq. |
| |
| |
| FILE FORMAT COMPATIBILITY |
| ========================= |
| |
| This software implements ITU T.81 | ISO/IEC 10918 with some extensions from |
| ITU T.871 | ISO/IEC 10918-5 (JPEG File Interchange Format-- see REFERENCES). |
| Informally, the term "JPEG image" or "JPEG file" most often refers to JFIF or |
| a subset thereof, but there are other formats containing the name "JPEG" that |
| are incompatible with the DCT-based JPEG standard or with JFIF (for instance, |
| JPEG 2000 and JPEG XR). This software therefore does not support these |
| formats. Indeed, one of the original reasons for developing this free software |
| was to help force convergence on a common, interoperable format standard for |
| JPEG files. |
| |
| JFIF is a minimal or "low end" representation. TIFF/JPEG (TIFF revision 6.0 as |
| modified by TIFF Technical Note #2) can be used for "high end" applications |
| that need to record a lot of additional data about an image. |
| |
| |
| TO DO |
| ===== |
| |
| Please send bug reports, offers of help, etc. to jpeg-info@jpegclub.org. |