gif89.txt

giflib-4.1.6.tar.gz,最新的GIF 解码库

    Cover Sheet for the GIF89a Specification


    DEFERRED CLEAR CODE IN LZW COMPRESSION

    There has been confusion about where clear codes can be found in the
    data stream.  As the specification says, they may appear at anytime.  There
    is not a requirement to send a clear code when the string table is full.

    It is the encoder's decision as to when the table should be cleared.  When
    the table is full, the encoder can chose to use the table as is, making no
    changes to it until the encoder chooses to clear it.  The encoder during
    this time sends out codes that are of the maximum Code Size.

    As we can see from the above, when the decoder's table is full, it must
    not change the table until a clear code is received.  The Code Size is that
    of the maximum Code Size.  Processing other than this is done normally.

    Because of a large base of decoders that do not handle the decompression in
    this manner, we ask developers of GIF encoding software to NOT implement
    this feature until at least January 1991 and later if they see that their
    particular market is not ready for it.  This will give developers of GIF
    decoding software time to implement this feature and to get it into the
    hands of their clients before the decoders start "breaking" on the new
    GIF's.  It is not required that encoders change their software to take
    advantage of the deferred clear code, but it is for decoders.

    APPLICATION EXTENSION BLOCK - APPLICATION IDENTIFIER

    There will be a Courtesy Directory file located on CompuServe in the PICS
    forum.  This directory will contain Application Identifiers for Application
    Extension Blocks that have been used by developers of GIF applications.
    This file is intended to help keep developers that wish to create
    Application Extension Blocks from using the same Application Identifiers.
    This is not an official directory; it is for voluntary participation only
    and does not guarantee that someone will not use the same identifier.

    E-Mail can be sent to Larry Wood (forum manager of PICS) indicating the
    request for inclusion in this file with an identifier.









































                        GRAPHICS INTERCHANGE FORMAT(sm)

                                  Version 89a

                            (c)1987,1988,1989,1990

                                   Copyright
                            CompuServe Incorporated
                                Columbus, Ohio






























CompuServe Incorporated                           Graphics Interchange Format
Document Date : 31 July 1990                            Programming Reference










                               Table of Contents

Disclaimer.................................................................  1

Foreword...................................................................  1

Licensing..................................................................  1

About the Document.........................................................  2

General Description........................................................  2

Version Numbers............................................................  2

The Encoder................................................................  3

The Decoder................................................................  3

Compliance.................................................................  3

About Recommendations......................................................  4

About Color Tables.........................................................  4

Blocks, Extensions and Scope...............................................  4

Block Sizes................................................................  5

Using GIF as an embedded protocol..........................................  5

Data Sub-blocks............................................................  5

Block Terminator...........................................................  6

Header.....................................................................  7

Logical Screen Descriptor..................................................  8

Global Color Table......................................................... 10

Image Descriptor........................................................... 11

Local Color Table.......................................................... 13

Table Based Image Data..................................................... 14

Graphic Control Extension.................................................. 15

Comment Extension.......................................................... 17

Plain Text Extension....................................................... 18

Application Extension...................................................... 21

Trailer.................................................................... 23











Quick Reference Table...................................................... 24

GIF Grammar................................................................ 25

Glossary................................................................... 27

Conventions................................................................ 28

Interlaced Images.......................................................... 29

Variable-Length-Code LZW Compression....................................... 30

On-line Capabilities Dialogue.............................................. 33


















































                                                                         1


1. Disclaimer.

The information provided herein is subject to change without notice. In no
event will CompuServe Incorporated be liable for damages, including any loss of
revenue, loss of profits or other incidental or consequential damages arising
out of the use or inability to use the information; CompuServe Incorporated
makes no claim as to the suitability of the information.


2. Foreword.

This document defines the Graphics Interchange Format(sm). The specification
given here defines version 89a, which is an extension of version 87a.

The Graphics Interchange Format(sm) as specified here should be considered
complete; any deviation from it should be considered invalid, including but not
limited to, the use of reserved or undefined fields within control or data
blocks, the inclusion of extraneous data within or between blocks, the use of
methods or algorithms not specifically listed as part of the format, etc. In
general, any and all deviations, extensions or modifications not specified in
this document should be considered to be in violation of the format and should
be avoided.


3. Licensing.

The Graphics Interchange Format(c) is the copyright property of CompuServe
Incorporated. Only CompuServe Incorporated is authorized to define, redefine,
enhance, alter, modify or change in any way the definition of the format.

CompuServe Incorporated hereby grants a limited, non-exclusive, royalty-free
license for the use of the Graphics Interchange Format(sm) in computer
software; computer software utilizing GIF(sm) must acknowledge ownership of the
Graphics Interchange Format and its Service Mark by CompuServe Incorporated, in
User and Technical Documentation. Computer software utilizing GIF, which is
distributed or may be distributed without User or Technical Documentation must
display to the screen or printer a message acknowledging ownership of the
Graphics Interchange Format and the Service Mark by CompuServe Incorporated; in
this case, the acknowledgement may be displayed in an opening screen or leading
banner, or a closing screen or trailing banner. A message such as the following
may be used:

      "The Graphics Interchange Format(c) is the Copyright property of
      CompuServe Incorporated. GIF(sm) is a Service Mark property of
      CompuServe Incorporated."

For further information, please contact :

      CompuServe Incorporated
      Graphics Technology Department
      5000 Arlington Center Boulevard
      Columbus, Ohio  43220
      U. S. A.

CompuServe Incorporated maintains a mailing list with all those individuals and
organizations who wish to receive copies of this document when it is corrected







                                                                         2


or revised. This service is offered free of charge; please provide us with your
mailing address.


4. About the Document.

This document describes in detail the definition of the Graphics Interchange
Format.  This document is intended as a programming reference; it is
recommended that the entire document be read carefully before programming,
because of the interdependence of the various parts. There is an individual
section for each of the Format blocks. Within each section, the sub-section
labeled Required Version refers to the version number that an encoder will have
to use if the corresponding block is used in the Data Stream. Within each
section, a diagram describes the individual fields in the block; the diagrams
are drawn vertically; top bytes in the diagram appear first in the Data Stream.
Bits within a byte are drawn most significant on the left end.  Multi-byte
numeric fields are ordered Least Significant Byte first. Numeric constants are
represented as Hexadecimal numbers, preceded by "0x".  Bit fields within a byte
are described in order from most significant bits to least significant bits.


5. General Description.

The Graphics Interchange Format(sm) defines a protocol intended for the on-line
transmission and interchange of raster graphic data in a way that is
independent of the hardware used in their creation or display.

The Graphics Interchange Format is defined in terms of blocks and sub-blocks
which contain relevant parameters and data used in the reproduction of a
graphic. A GIF Data Stream is a sequence of protocol blocks and sub-blocks
representing a collection of graphics. In general, the graphics in a Data
Stream are assumed to be related to some degree, and to share some control
information; it is recommended that encoders attempt to group together related
graphics in order to minimize hardware changes during processing and to
minimize control information overhead. For the same reason, unrelated graphics
or graphics which require resetting hardware parameters should be encoded
separately to the extent possible.

A Data Stream may originate locally, as when read from a file, or it may
originate remotely, as when transmitted over a data communications line. The
Format is defined with the assumption that an error-free Transport Level
Protocol is used for communications; the Format makes no provisions for
error-detection and error-correction.

The GIF Data Stream must be interpreted in context, that is, the application
program must rely on information external to the Data Stream to invoke the
decoder process.


6. Version Numbers.

The version number in the Header of a Data Stream is intended to identify the
minimum set of capabilities required of a decoder in order to fully process the