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install.doc How to congure 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.

lelist.doc Road map of IJG les.

coderules.doc Coding style rules --- please read

if you contribute code.

Please read at least the les install.doc and usage.doc.

Useful information can also be found in the JPEG FAQ

(Frequently Asked Questions) article. See ARCHIVE

LOCATIONS below to nd 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 les (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 dened in the

standard.

We provide a set of library routines for reading and writing

JPEG image les, plus two sample applications “cjpeg”

and “djpeg”, which use the library to perform conversion

between JPEG and some other popular image le

formats.

The library is intended to be reused in other applications.

In order to support le 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

le 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 les.

The emphasis in designing this software has been on

achieving portability and exibility, 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

nd 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 tness for a particular

purpose. This software is provided “AS IS”, and you, its

user, assume the entire risk as to its quality and accuracy.

Lane.

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 le must be included,

with this copyright and no-warranty notice unaltered;

and any additions, deletions, or changes to the original

les 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 unmodied 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 specically 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 le

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 conguration script “congure” was produced

with GNU Autoconf.

It is copyright by the Free Software Foundation but is

freely distributable.

The same holds for its supporting scripts (cong.

guess, cong.sub, ltcong, 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 les.

To avoid entanglement with the Unisys LZW patent, GIF

reading support has been removed altogether, and the

GIF writer has been simplied to produce “uncompressed

GIFs”. This technique does not use the LZW algorithm; the

resulting GIF les 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 le containing a revised version of Wallace’s

article is available at ftp://ftp.uu.net/graphics/jpeg/

wallace.ps.gz. The le (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 certied ofcial 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 specication, 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 dened

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 le 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

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