This is a touched up version of the GIF89 documentation from CompuServe.
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 [TOC]

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.


Foreword [TOC]

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.


Licensing [TOC]

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


About the Document [TOC]

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.


General Description [TOC]

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.


Version Numbers [TOC]

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 Data Stream. An encoder should use the earliest possible version number that includes all the blocks used in the Data Stream. Within each block section in this document, there is an entry labeled Required Version which specifies the earliest version number that includes the corresponding block. The encoder should make every attempt to use the earliest version number covering all the blocks in the Data Stream; the unnecessary use of later version numbers will hinder processing by some decoders.


The Encoder [TOC]

The Encoder is the program used to create a GIF Data Stream. From raster data and other information, the encoder produces the necessary control and data blocks needed for reproducing the original graphics.

The encoder has the following primary responsibilities.


The Decoder [TOC]

The Decoder is the program used to process a GIF Data Stream. It processes the Data Stream sequentially, parsing the various blocks and sub-blocks, using the control information to set hardware and process parameters and interpreting the data to render the graphics.

The decoder has the following primary responsibilities.


Compliance [TOC]

An encoder or a decoder is said to comply with a given version of the Graphics Interchange Format if and only if it fully conforms with and correctly implements the definition of the standard associated with that version. An encoder or a decoder may be compliant with a given version number and not compliant with some subsequent version.


About Recommendations [TOC]

Each block section in this document contains an entry labeled Recommendation; this section lists a set of recommendations intended to guide and organize the use of the particular blocks. Such recommendations are geared towards making the functions of encoders and decoders more efficient, as well as making optimal use of the communications bandwidth. It is advised that these recommendations be followed.


About Color Tables [TOC]

The GIF format utilizes color tables to render raster-based graphics. A color table can have one of two different scopes: global or local. A Global Color Table is used by all those graphics in the Data Stream which do not have a Local Color Table associated with them. The scope of the Global Color Table is the entire Data Stream. A Local Color Table is always associated with the graphic that immediately follows it; the scope of a Local Color Table is limited to that single graphic. A Local Color Table supersedes a Global Color Table, that is, if a Data Stream contains a Global Color Table, and an image has a Local Color Table associated with it, the decoder must save the Global Color Table, use the Local Color Table to render the image, and then restore the Global Color Table. Both types of color tables are optional, making it possible for a Data Stream to contain numerous graphics without a color table at all. For this reason, it is recommended that the decoder save the last Global Color Table used until another Global Color Table is encountered. In this way, a Data Stream which does not contain either a Global Color Table or a Local Color Table may be processed using the last Global Color Table saved. If a Global Color Table from a previous Stream is used, that table becomes the Global Color Table of the present Stream. This is intended to reduce the overhead incurred by color tables. In particular, it is recommended that an encoder use only one Global Color Table if all the images in related Data Streams can be rendered with the same table. If no color table is available at all, the decoder is free to use a system color table or a table of its own. In that case, the decoder may use a color table with as many colors as its hardware is able to support; it is recommended that such a table have black and white as its first two entries, so that monochrome images can be rendered adequately.

The Definition of the GIF Format allows for a Data Stream to contain only the Header, the Logical Screen Descriptor, a Global Color Table and the GIF Trailer. Such a Data Stream would be used to load a decoder with a Global Color Table, in preparation for subsequent Data Streams without a color table at all.


Blocks, Extensions and Scope [TOC]

Blocks can be classified into three groups : Control, Graphic-Rendering and Special Purpose. Control blocks, such as the Header, the Logical Screen Descriptor, the Graphic Control Extension and the Trailer, contain information used to control the process of the Data Stream or information used in setting hardware parameters. Graphic-Rendering blocks such as the Image Descriptor and the Plain Text Extension contain information and data used to render a graphic on the display device. Special Purpose blocks such as the Comment Extension and the Application Extension are neither used to control the process of the Data Stream nor do they contain information or data used to render a graphic on the display device. With the exception of the Logical Screen Descriptor and the Global Color Table, whose scope is the entire Data Stream, all other Control blocks have a limited scope, restricted to the Graphic-Rendering block that follows them. Special Purpose blocks do not delimit the scope of any Control blocks; Special Purpose blocks are transparent to the decoding process. Graphic-Rendering blocks and extensions are used as scope delimiters for Control blocks and extensions. The labels used to identify labeled blocks fall into three ranges : 0x00-0x7F (0-127) are the Graphic Rendering blocks, excluding the Trailer (0x3B); 0x80-0xF9 (128-249) are the Control blocks; 0xFA-0xFF (250-255) are the Special Purpose blocks. These ranges are defined so that decoders can handle block scope by appropriately identifying block labels, even when the block itself cannot be processed.


Block Sizes [TOC]

The Block Size field in a block, counts the number of bytes remaining in the block, not counting the Block Size field itself, and not counting the Block Terminator, if one is to follow. Blocks other than Data Blocks are intended to be of fixed length; the Block Size field is provided in order to facilitate skipping them, not to allow their size to change in the future. Data blocks and sub-blocks are of variable length to accommodate the amount of data.


Using GIF as an embedded protocol [TOC]

As an embedded protocol, GIF may be part of larger application protocols, within which GIF is used to render graphics. In such a case, the application protocol could define a block within which the GIF Data Stream would be contained. The application program would then invoke a GIF decoder upon encountering a block of type GIF. This approach is recommended in favor of using Application Extensions, which become overhead for all other applications that do not process them. Because a GIF Data Stream must be processed in context, the application must rely on some means of identifying the GIF Data Stream outside of the Stream itself.


Data Sub-blocks [TOC]

Description
Data Sub-blocks are units containing data. They do not have a label, these blocks are processed in the context of control blocks, wherever data blocks are specified in the format. The first byte of the Data sub-block indicates the number of data bytes to follow. A data sub-block may contain from 0 to 255 data bytes. The size of the block does not account for the size byte itself, therefore, the empty sub-block is one whose size field contains 0x00.

Required Version
87a.

Syntax
      7 6 5 4 3 2 1 0        Field Name                    Type
     +---------------+
  0  |               |       Block Size                    Byte
     +---------------+
  1  |               |
     +-             -+
  2  |               |
     +-             -+
  3  |               |
     +-             -+
     |               |       Data Values                   Byte
     +-             -+
 up  |               |
     +-   . . . .   -+
 to  |               |
     +-             -+
     |               |
     +-             -+
255  |               |
     +---------------+
Block Size
Number of bytes in the Data Sub-block; the size must be within 0 and 255 bytes, inclusive.
Data Values
Any 8-bit value. There must be exactly as many Data Values as specified by the Block Size field.

Extensions and Scope
This type of block always occurs as part of a larger unit. It does not have a scope of itself.

Recommendation
None.


Block Terminator [TOC]

Description
This zero-length Data Sub-block is used to terminate a sequence of Data Sub-blocks. It contains a single byte in the position of the Block Size field and does not contain data.

Required Version
87a.

Syntax
      7 6 5 4 3 2 1 0        Field Name                    Type
     +---------------+
  0  |               |       Block Size                    Byte
     +---------------+
Block Size
Number of bytes in the Data Sub-block; this field contains the fixed value 0x00.
Data Values
This block does not contain any data.

Extensions and Scope
This block terminates the immediately preceding sequence of Data Sub-blocks. This block cannot be modified by any extension.

Recommendation
None.


Header [TOC]

Description
The Header identifies the GIF Data Stream in context. The Signature field marks the beginning of the Data Stream, and the Version field identifies the set of capabilities required of a decoder to fully process the Data Stream. This block is REQUIRED; exactly one Header must be present per Data Stream.

Required Version
Not applicable. This block is not subject to a version number. This block must appear at the beginning of every Data Stream.

Syntax
      7 6 5 4 3 2 1 0        Field Name                    Type
     +---------------+
   0 |               |       Signature                     3 Bytes
     +-             -+
   1 |               |
     +-             -+
   2 |               |
     +---------------+
   3 |               |       Version                       3 Bytes
     +-             -+
   4 |               |
     +-             -+
   5 |               |
     +---------------+
Signature
Identifies the GIF Data Stream. This field contains the fixed value 'GIF'.
Version
Version number used to format the data stream. Identifies the minimum set of capabilities necessary to a decoder to fully process the contents of the Data Stream.

Version Numbers as of 10 July 1990 : "87a" - May 1987, "89a" - July 1989

Version numbers are ordered numerically increasing on the first two digits starting with 87 (87,88,...,99,00,...,85,86) and alphabetically increasing on the third character (a,...,z).

Extensions and Scope
The scope of this block is the entire Data Stream. This block cannot be modified by any extension.

Recommendations
Signature:
This field identifies the beginning of the GIF Data Stream; it is not intended to provide a unique signature for the identification of the data. It is recommended that the GIF Data Stream be identified externally by the application. (Refer to Appendix G for on-line identification of the GIF Data Stream.)
Version:
ENCODER : An encoder should use the earliest possible version number that defines all the blocks used in the Data Stream. When two or more Data Streams are combined, the latest of the individual version numbers should be used for the resulting Data Stream. DECODER : A decoder should attempt to process the data stream to the best of its ability; if it encounters a version number which it is not capable of processing fully, it should nevertheless, attempt to process the data stream to the best of its ability, perhaps after warning the user that the data may be incomplete.


Logical Screen Descriptor [TOC]

Description
The Logical Screen Descriptor contains the parameters necessary to define the area of the display device within which the images will be rendered. The coordinates in this block are given with respect to the top-left corner of the virtual screen; they do not necessarily refer to absolute coordinates on the display device. This implies that they could refer to window coordinates in a window-based environment or printer coordinates when a printer is used.

This block is REQUIRED; exactly one Logical Screen Descriptor must be present per Data Stream.

Required Version
Not applicable. This block is not subject to a version number. This block must appear immediately after the Header.

Syntax
      7 6 5 4 3 2 1 0        Field Name                    Type
     +---------------+
  0  |               |       Logical Screen Width          Unsigned
     +-             -+
  1  |               |
     +---------------+
  2  |               |       Logical Screen Height         Unsigned
     +-             -+
  3  |               |
     +---------------+
  4  | |     | |     |       <Packed Fields>               See below
     +---------------+
  5  |               |       Background Color Index        Byte
     +---------------+
  6  |               |       Pixel Aspect Ratio            Byte
     +---------------+

     <Packed Fields>  =      Global Color Table Flag       1 Bit
                             Color Resolution              3 Bits
                             Sort Flag                     1 Bit
                             Size of Global Color Table    3 Bits
Logical Screen Width
Width, in pixels, of the Logical Screen where the images will be rendered in the displaying device.

Logical Screen Height
Height, in pixels, of the Logical Screen where the images will be rendered in the displaying device.

Global Color Table Flag
Flag indicating the presence of a Global Color Table; if the flag is set, the Global Color Table will immediately follow the Logical Screen Descriptor. This flag also selects the interpretation of the Background Color Index; if the flag is set, the value of the Background Color Index field should be used as the table index of the background color. (This field is the most significant bit of the byte.)

Values:
0 - No Global Color Table follows, the Background Color Index field is meaningless.
1 - A Global Color Table will immediately follow, the Background Color Index field is meaningful.

Color Resolution
Number of bits per primary color available to the original image, minus 1. This value represents the size of the entire palette from which the colors in the graphic were selected, not the number of colors actually used in the graphic. For example, if the value in this field is 3, then the palette of the original image had 4 bits per primary color available to create the image. This value should be set to indicate the richness of the original palette, even if not every color from the whole palette is available on the source machine.

Sort Flag
Indicates whether the Global Color Table is sorted. If the flag is set, the Global Color Table is sorted, in order of decreasing importance. Typically, the order would be decreasing frequency, with most frequent color first. This assists a decoder, with fewer available colors, in choosing the best subset of colors; the decoder may use an initial segment of the table to render the graphic.

Values:
0 - Not ordered.
1 - Ordered by decreasing importance, most important color first.

Size of Global Color Table
If the Global Color Table Flag is set to 1, the value in this field is used to calculate the number of bytes contained in the Global Color Table. To determine that actual size of the color table, raise 2 to [the value of the field + 1]. Even if there is no Global Color Table specified, set this field according to the above formula so that decoders can choose the best graphics mode to display the stream in. (This field is made up of the 3 least significant bits of the byte.)

Background Color Index
Index into the Global Color Table for the Background Color. The Background Color is the color used for those pixels on the screen that are not covered by an image. If the Global Color Table Flag is set to (zero), this field should be zero and should be ignored.

Pixel Aspect Ratio
Factor used to compute an approximation of the aspect ratio of the pixel in the original image. If the value of the field is not 0, this approximation of the aspect ratio is computed based on the formula:
            Aspect Ratio = (Pixel Aspect Ratio + 15) / 64
The Pixel Aspect Ratio is defined to be the quotient of the pixel's width over its height. The value range in this field allows specification of the widest pixel of 4:1 to the tallest pixel of 1:4 in increments of 1/64th.

Values:
0 - No aspect ratio information is given.
1..255 - Value used in the computation.

Extensions and Scope
The scope of this block is the entire Data Stream. This block cannot be modified by any extension.

Recommendations
None.


Global Color Table [TOC]

Description
This block contains a color table, which is a sequence of bytes representing red-green-blue color triplets. The Global Color Table is used by images without a Local Color Table and by Plain Text Extensions. Its presence is marked by the Global Color Table Flag being set to 1 in the Logical Screen Descriptor; if present, it immediately follows the Logical Screen Descriptor and contains a number of bytes equal to: 3 x 2^(Size of Global Color Table+1).

This block is OPTIONAL; at most one Global Color Table may be present per Data Stream.

Required Version
87a.

Syntax
      7 6 5 4 3 2 1 0        Field Name                    Type
     +===============+
  0  |               |       Red 0                         Byte
     +-             -+
  1  |               |       Green 0                       Byte
     +-             -+
  2  |               |       Blue 0                        Byte
     +-             -+
  3  |               |       Red 1                         Byte
     +-             -+
     |               |       Green 1                       Byte
     +-             -+
 up  |               |
     +-   . . . .   -+       ...
 to  |               |
     +-             -+
     |               |       Green 255                     Byte
     +-             -+
767  |               |       Blue 255                      Byte
     +===============+
Extensions and Scope
The scope of this block is the entire Data Stream. This block cannot be modified by any extension.

Recommendation
None.


Image Descriptor [TOC]

Description
Each image in the Data Stream is composed of an Image Descriptor, an optional Local Color Table, and the image data. Each image must fit within the boundaries of the Logical Screen, as defined in the Logical Screen Descriptor.

The Image Descriptor contains the parameters necessary to process a table based image. The coordinates given in this block refer to coordinates within the Logical Screen, and are given in pixels. This block is a Graphic-Rendering Block, optionally preceded by one or more Control blocks such as the Graphic Control Extension, and may be optionally followed by a Local Color Table; the Image Descriptor is always followed by the image data.

This block is REQUIRED for an image. Exactly one Image Descriptor must be present per image in the Data Stream. An unlimited number of images may be present per Data Stream.

Required Version
87a.

Syntax
      7 6 5 4 3 2 1 0        Field Name                    Type
     +---------------+
  0  |               |       Image Separator               Byte
     +---------------+
  1  |               |       Image Left Position           Unsigned
     +-             -+
  2  |               |
     +---------------+
  3  |               |       Image Top Position            Unsigned
     +-             -+
  4  |               |
     +---------------+
  5  |               |       Image Width                   Unsigned
     +-             -+
  6  |               |
     +---------------+
  7  |               |       Image Height                  Unsigned
     +-             -+
  8  |               |
     +---------------+
  9  | | | |   |     |       <Packed Fields>               See below
     +---------------+

     <Packed Fields>  =      Local Color Table Flag        1 Bit
                             Interlace Flag                1 Bit
                             Sort Flag                     1 Bit
                             Reserved                      2 Bits
                             Size of Local Color Table     3 Bits
Image Separator
Identifies the beginning of an Image Descriptor. This field contains the fixed value 0x2C.

Image Left Position
Column number, in pixels, of the left edge of the image, with respect to the left edge of the Logical Screen. Leftmost column of the Logical Screen is 0.

Image Top Position
Row number, in pixels, of the top edge of the image with respect to the top edge of the Logical Screen. Top row of the Logical Screen is 0.

Image Width
Width of the image in pixels.

Image Height
Height of the image in pixels.

Local Color Table Flag
Indicates the presence of a Local Color Table immediately following this Image Descriptor. (This field is the most significant bit of the byte.)

Values:
0 - Local Color Table is not present. Use Global Color Table if available.
1 - Local Color Table present, and to follow immediately after this Image Descriptor.

Interlace Flag
Indicates if the image is interlaced. An image is interlaced in a four-pass interlace pattern; see Appendix E for details.

Values:
0 - Image is not interlaced.
1 - Image is interlaced.

Sort Flag
Indicates whether the Local Color Table is sorted. If the flag is set, the Local Color Table is sorted, in order of decreasing importance. Typically, the order would be decreasing frequency, with most frequent color first. This assists a decoder, with fewer available colors, in choosing the best subset of colors; the decoder may use an initial segment of the table to render the graphic.

Values:
0 - Not ordered.
1 - Ordered by decreasing importance, most important color first.

Size of Local Color Table
If the Local Color Table Flag is set to 1, the value in this field is used to calculate the number of bytes contained in the Local Color Table. To determine that actual size of the color table, raise 2 to the value of the field + 1. This value should be 0 if there is no Local Color Table specified. (This field is made up of the 3 least significant bits of the byte.)

Extensions and Scope
The scope of this block is the Table-based Image Data Block that follows it. This block may be modified by the Graphic Control Extension.

Recommendation
None.


Local Color Table [TOC]

Description
This block contains a color table, which is a sequence of bytes representing red-green-blue color triplets. The Local Color Table is used by the image that immediately follows. Its presence is marked by the Local Color Table Flag being set to 1 in the Image Descriptor; if present, the Local Color Table immediately follows the Image Descriptor and contains a number of bytes equal to 3x2^(Size of Local Color Table+1). If present, this color table temporarily becomes the active color table and the following image should be processed using it. This block is OPTIONAL; at most one Local Color Table may be present per Image Descriptor and its scope is the single image associated with the Image Descriptor that precedes it.

Required Version
87a.

Syntax
      7 6 5 4 3 2 1 0        Field Name                    Type
     +===============+
  0  |               |       Red 0                         Byte
     +-             -+
  1  |               |       Green 0                       Byte
     +-             -+
  2  |               |       Blue 0                        Byte
     +-             -+
  3  |               |       Red 1                         Byte
     +-             -+
     |               |       Green 1                       Byte
     +-             -+
 up  |               |
     +-   . . . .   -+       ...
 to  |               |
     +-             -+
     |               |       Green 255                     Byte
     +-             -+
767  |               |       Blue 255                      Byte
     +===============+
Extensions and Scope
The scope of this block is the Table-based Image Data Block that immediately follows it. This block cannot be modified by any extension.

Recommendations
None.


Table Based Image Data [TOC]

Description
The image data for a table based image consists of a sequence of sub-blocks, of size at most 255 bytes each, containing an index into the active color table, for each pixel in the image. Pixel indices are in order of left to right and from top to bottom. Each index must be within the range of the size of the active color table, starting at 0. The sequence of indices is encoded using the LZW Algorithm with variable-length code, as described in Appendix F

Required Version
87a.

Syntax
The image data format is as follows:
      7 6 5 4 3 2 1 0        Field Name                    Type
     +---------------+
     |               |       LZW Minimum Code Size         Byte
     +---------------+

     +===============+
     |               |
     /               /       Image Data                    Data Sub-blocks
     |               |
     +===============+
LZW Minimum Code Size.
This byte determines the initial number of bits used for LZW codes in the image data, as described in Appendix F.

Extensions and Scope
This block has no scope, it contains raster data. Extensions intended to modify a Table-based image must appear before the corresponding Image Descriptor.

Recommendations
None.


Graphic Control Extension [TOC]

Description
The Graphic Control Extension contains parameters used when processing a graphic rendering block. The scope of this extension is the first graphic rendering block to follow. The extension contains only one data sub-block.

This block is OPTIONAL; at most one Graphic Control Extension may precede a graphic rendering block. This is the only limit to the number of Graphic Control Extensions that may be contained in a Data Stream.

Required Version
89a.

Syntax
      7 6 5 4 3 2 1 0        Field Name                    Type
     +---------------+
  0  |               |       Extension Introducer          Byte
     +---------------+
  1  |               |       Graphic Control Label         Byte
     +---------------+

     +---------------+
  0  |               |       Block Size                    Byte
     +---------------+
  1  |     |     | | |       <Packed Fields>               See below
     +---------------+
  2  |               |       Delay Time                    Unsigned
     +-             -+
  3  |               |
     +---------------+
  4  |               |       Transparent Color Index       Byte
     +---------------+

     +---------------+
  0  |               |       Block Terminator              Byte
     +---------------+


      <Packed Fields>  =     Reserved                      3 Bits
                             Disposal Method               3 Bits
                             User Input Flag               1 Bit
                             Transparent Color Flag        1 Bit
Extension Introducer
Identifies the beginning of an extension

Graphic Control Label
Identifies the current block as a Graphic Control Extension. This field contains the fixed value 0xF9.

Block Size
Number of bytes in the block, after the Block Size field and up to but not including the Block Terminator. This field contains the fixed value 4.

Disposal Method
Indicates the way in which the graphic is to be treated after being displayed.

Values:
0 - No disposal specified. The decoder is not required to take any action.
1 - Do not dispose. The graphic is to be left in place.
2 - Restore to background color. The area used by the graphic must be restored to the background color.
3 - Restore to previous. The decoder is required to restore the area overwritten by the graphic with what was there prior to rendering the graphic.
4-7 - To be defined.

User Input Flag
Indicates whether or not user input is expected before continuing. If the flag is set, processing will continue when user input is entered. The nature of the User input is determined by the application (Carriage Return, Mouse Button Click, etc.).

Values:
0 - User input is not expected.
1 - User input is expected.

When a Delay Time is used and the User Input Flag is set, processing will continue when user input is received or when the delay time expires, whichever occurs first.

Transparency Flag
Indicates whether a transparency index is given in the Transparent Index field. (This field is the least significant bit of the byte.)

Values:
0 - Transparent Index is not given.
1 - Transparent Index is given.

Delay Time
If not 0, this field specifies the number of hundredths (1/100) of a second to wait before continuing with the processing of the Data Stream. The clock starts ticking immediately after the graphic is rendered. This field may be used in conjunction with the User Input Flag field.

Transparency Index
The Transparency Index is such that when encountered, the corresponding pixel of the display device is not modified and processing goes on to the next pixel. The index is present if and only if the Transparency Flag is set to 1.

Block Terminator
This zero-length data block marks the end of the Graphic Control Extension.

Extensions and Scope
The scope of this Extension is the graphic rendering block that follows it; it is possible for other extensions to be present between this block and its target. This block can modify the Image Descriptor Block and the Plain Text Extension.

Recommendations
Disposal Method
The mode Restore To Previous is intended to be used in small sections of the graphic; the use of this mode imposes severe demands on the decoder to store the section of the graphic that needs to be saved. For this reason, this mode should be used sparingly. This mode is not intended to save an entire graphic or large areas of a graphic; when this is the case, the encoder should make every attempt to make the sections of the graphic to be restored be separate graphics in the data stream. In the case where a decoder is not capable of saving an area of a graphic marked as Restore To Previous, it is recommended that a decoder restore to the background color.

User Input Flag
When the flag is set, indicating that user input is expected, the decoder may sound the bell (0x07) to alert the user that input is being expected. In the absence of a specified Delay Time, the decoder should wait for user input indefinitely. It is recommended that the encoder not set the User Input Flag without a Delay Time specified.


Comment Extension [TOC]

Description
The Comment Extension contains textual information which is not part of the actual graphics in the GIF Data Stream. It is suitable for including comments about the graphics, credits, descriptions or any other type of non-control and non-graphic data. The Comment Extension may be ignored by the decoder, or it may be saved for later processing; under no circumstances should a Comment Extension disrupt or interfere with the processing of the Data Stream.

This block is OPTIONAL; any number of them may appear in the Data Stream.

Required Version
89a.

Syntax
      7 6 5 4 3 2 1 0        Field Name                    Type
     +---------------+
  0  |               |       Extension Introducer          Byte
     +---------------+
  1  |               |       Comment Label                 Byte
     +---------------+

     +===============+
     |               |
  N  |               |       Comment Data                  Data Sub-blocks
     |               |
     +===============+

     +---------------+
  0  |               |       Block Terminator              Byte
     +---------------+
Extension Introducer
Identifies the beginning of an extension block. This field contains the fixed value 0x21.

Comment Label
Identifies the block as a Comment Extension. This field contains the fixed value 0xFE.

Comment Data
Sequence of sub-blocks, each of size at most 255 bytes and at least 1 byte, with the size in a byte preceding the data. The end of the sequence is marked by the Block Terminator.

Block Terminator
This zero-length data block marks the end of the Comment Extension.

Extensions and Scope
This block does not have scope. This block cannot be modified by any extension.

Recommendations
Data
This block is intended for humans. It should contain text using the 7-bit ASCII character set. This block should not be used to store control information for custom processing.

Position
This block may appear at any point in the Data Stream at which a block can begin; however, it is recommended that Comment Extensions do not interfere with Control or Data blocks; they should be located at the beginning or at the end of the Data Stream to the extent possible.


Plain Text Extension [TOC]

Description
The Plain Text Extension contains textual data and the parameters necessary to render that data as a graphic, in a simple form. The textual data will be encoded with the 7-bit printable ASCII characters. Text data are rendered using a grid of character cells defined by the parameters in the block fields. Each character is rendered in an individual cell. The textual data in this block is to be rendered as mono-spaced characters, one character per cell, with a best fitting font and size. For further information, see the section on Recommendations below. The data characters are taken sequentially from the data portion of the block and rendered within a cell, starting with the upper left cell in the grid and proceeding from left to right and from top to bottom. Text data is rendered until the end of data is reached or the character grid is filled. The Character Grid contains an integral number of cells; in the case that the cell dimensions do not allow for an integral number, fractional cells must be discarded; an encoder must be careful to specify the grid dimensions accurately so that this does not happen. This block requires a Global Color Table to be available; the colors used by this block reference the Global Color Table in the Stream if there is one, or the Global Color Table from a previous Stream, if one was saved. This block is a graphic rendering block, therefore it may be modified by a Graphic Control Extension. This block is OPTIONAL; any number of them may appear in the Data Stream.

Required Version
89a.

Syntax
      7 6 5 4 3 2 1 0        Field Name                    Type
     +---------------+
  0  |               |       Extension Introducer          Byte
     +---------------+
  1  |               |       Plain Text Label              Byte
     +---------------+

     +---------------+
  0  |               |       Block Size                    Byte
     +---------------+
  1  |               |       Text Grid Left Position       Unsigned
     +-             -+
  2  |               |
     +---------------+
  3  |               |       Text Grid Top Position        Unsigned
     +-             -+
  4  |               |
     +---------------+
  5  |               |       Text Grid Width               Unsigned
     +-             -+
  6  |               |
     +---------------+
  7  |               |       Text Grid Height              Unsigned
     +-             -+
  8  |               |
     +---------------+
  9  |               |       Character Cell Width          Byte
     +---------------+
 10  |               |       Character Cell Height         Byte
     +---------------+
 11  |               |       Text Foreground Color Index   Byte
     +---------------+
 12  |               |       Text Background Color Index   Byte
     +---------------+

     +===============+
     |               |
  N  |               |       Plain Text Data               Data Sub-blocks
     |               |
     +===============+

     +---------------+
  0  |               |       Block Terminator              Byte
     +---------------+
Extension Introducer
Identifies the beginning of an extension block. This field contains the fixed value 0x21.

Plain Text Label
Identifies the current block as a Plain Text Extension. This field contains the fixed value 0x01.

Block Size
Number of bytes in the extension, after the Block Size field and up to but not including the beginning of the data portion. This field contains the fixed value 12.

Text Grid Left Position
Column number, in pixels, of the left edge of the text grid, with respect to the left edge of the Logical Screen.

Text Grid Top Position
Row number, in pixels, of the top edge of the text grid, with respect to the top edge of the Logical Screen.

Image Grid Width
Width of the text grid in pixels.

Image Grid Height
Height of the text grid in pixels.

Character Cell Width
Width, in pixels, of each cell in the grid.

Character Cell Height
Height, in pixels, of each cell in the grid.

Text Foreground Color Index
Index into the Global Color Table to be used to render the text foreground.

Text Background Color Index
Index into the Global Color Table to be used to render the text background.

Plain Text Data
Sequence of sub-blocks, each of size at most 255 bytes and at least 1 byte, with the size in a byte preceding the data. The end of the sequence is marked by the Block Terminator.

Block Terminator
This zero-length data block marks the end of the Plain Text Data Blocks.

Extensions and Scope
The scope of this block is the Plain Text Data Block contained in it. This block may be modified by the Graphic Control Extension.

Recommendations
The data in the Plain Text Extension is assumed to be preformatted. The selection of font and size is left to the discretion of the decoder. If characters less than 0x20 or greater than 0xf7 are encountered, it is recommended that the decoder display a Space character (0x20). The encoder should use grid and cell dimensions such that an integral number of cells fit in the grid both horizontally as well as vertically. For broadest compatibility, character cell dimensions should be around 8x8 or 8x16 (width x height); consider an image for unusual sized text.


Application Extension [TOC]

Description
The Application Extension contains application-specific information; it conforms with the extension block syntax, as described below, and its block label is 0xFF.

Required Version
89a.

Syntax
      7 6 5 4 3 2 1 0        Field Name                    Type
     +---------------+
  0  |               |       Extension Introducer          Byte
     +---------------+
  1  |               |       Extension Label               Byte
     +---------------+

     +---------------+
  0  |               |       Block Size                    Byte
     +---------------+
  1  |               |
     +-             -+
  2  |               |
     +-             -+
  3  |               |       Application Identifier        8 Bytes
     +-             -+
  4  |               |
     +-             -+
  5  |               |
     +-             -+
  6  |               |
     +-             -+
  7  |               |
     +-             -+
  8  |               |
     +---------------+
  9  |               |
     +-             -+
 10  |               |       Appl. Authentication Code     3 Bytes
     +-             -+
 11  |               |
     +---------------+

     +===============+
     |               |
     |               |       Application Data              Data Sub-blocks
     |               |
     |               |
     +===============+

     +---------------+
  0  |               |       Block Terminator              Byte
     +---------------+
Extension Introducer
Defines this block as an extension. This field contains the fixed value 0x21.

Application Extension Label
Identifies the block as an Application Extension. This field contains the fixed value 0xFF.

Block Size
Number of bytes in this extension block, following the Block Size field, up to but not including the beginning of the Application Data. This field contains the fixed value 11.

Application Identifier
Sequence of eight printable ASCII characters used to identify the application owning the Application Extension.

Application Authentication Code
Sequence of three bytes used to authenticate the Application Identifier. An Application program may use an algorithm to compute a binary code that uniquely identifies it as the application owning the Application Extension.

Extensions and Scope
This block does not have scope. This block cannot be modified by any extension.

Recommendation
None.


Trailer [TOC]

Description
This block is a single-field block indicating the end of the GIF Data Stream. It contains the fixed value 0x3B.

Required Version
87a.

Syntax
      7 6 5 4 3 2 1 0        Field Name                    Type
     +---------------+
  0  |               |       GIF Trailer                   Byte
     +---------------+
Extensions and Scope
This block does not have scope, it terminates the GIF Data Stream. This block may not be modified by any extension.

Recommendations
None.


Appendix A - Quick Reference Table. [TOC]

Block Name                  Required   Label       Ext.   Vers.
Application Extension       Opt. (*)   0xFF (255)  yes    89a
Comment Extension           Opt. (*)   0xFE (254)  yes    89a
Global Color Table          Opt. (1)   none        no     87a
Graphic Control Extension   Opt. (*)   0xF9 (249)  yes    89a
Header                      Req. (1)   none        no     N/A
Image Descriptor            Opt. (*)   0x2C (044)  no     87a (89a)
Local Color Table           Opt. (*)   none        no     87a
Logical Screen Descriptor   Req. (1)   none        no     87a (89a)
Plain Text Extension        Opt. (*)   0x01 (001)  yes    89a
Trailer                     Req. (1)   0x3B (059)  no     87a

Unlabeled Blocks
Header                      Req. (1)   none        no     N/A
Logical Screen Descriptor   Req. (1)   none        no     87a (89a)
Global Color Table          Opt. (1)   none        no     87a
Local Color Table           Opt. (*)   none        no     87a

Graphic-Rendering Blocks
Plain Text Extension        Opt. (*)   0x01 (001)  yes    89a
Image Descriptor            Opt. (*)   0x2C (044)  no     87a (89a)

Control Blocks
Graphic Control Extension   Opt. (*)   0xF9 (249)  yes    89a

Special Purpose Blocks
Trailer                     Req. (1)   0x3B (059)  no     87a
Comment Extension           Opt. (*)   0xFE (254)  yes    89a
Application Extension   >

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