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.