











                               G I F (tm)

                    Graphics Interchange Format (tm)



                           A standard for the
                        storage and transmission
                  of raster-based graphics information





                * * * * PRELIMINARY REVIEW DRAFT * * * *




                          Software Version 87a

                          Document Revision 1.0

                                May 1988


                (c) 1987, 1988 by CompuServe Incorporated
                           All rights reserved



        "GIF" and "Graphics Interchange Format" are trademarks of
                        CompuServe Incorporated,
                          an H&R Block Company

                       5000 Arlington Centre Blvd.
                          Columbus, Ohio 43220
                             (614) 457-8600























































 Disclaimer

 This document contains the latest information available at the time of
 its creation.  However, CompuServe reserves the right to modify the
 services described herein at any time, with or without written
 notification.


























                                       i

                               Table of Contents


 Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   ii

 1.0  Introduction to GIF  . . . . . . . . . . . . . . . . . . . . .    1

      1.1  Changes to GIF  . . . . . . . . . . . . . . . . . . . . .    2

      1.2  Availability of GIF . . . . . . . . . . . . . . . . . . .    3

      1.3  Naming Files which Contain GIF Data . . . . . . . . . . .    4

      1.4  GIF Development Area  . . . . . . . . . . . . . . . . . .    5

 2.0  Format of GIF Data . . . . . . . . . . . . . . . . . . . . . .    6

      2.1  Signature/Version Block . . . . . . . . . . . . . . . . .    8

      2.2  Logical Screen Descriptor Block . . . . . . . . . . . . .   10

      2.3  Global Color Map Block  . . . . . . . . . . . . . . . . .   14

      2.4  Extension Block . . . . . . . . . . . . . . . . . . . . .   16

           2.4.1  Comment Extension Block  . . . . . . . . . . . . .   18

           2.4.2  Image Control Extension Block  . . . . . . . . . .   20

      2.5  Image Descriptor Block  . . . . . . . . . . . . . . . . .   23

           2.5.1  Interlaced Image Processing  . . . . . . . . . . .   27

      2.6  Local Color Map Block . . . . . . . . . . . . . . . . . .   29

      2.7  Raster Data Block . . . . . . . . . . . . . . . . . . . .   30

      2.8  Terminator Block  . . . . . . . . . . . . . . . . . . . .   32

 3.0  Encoding and Decoding  . . . . . . . . . . . . . . . . . . . .   33

      3.1  Compressing and Expanding Pixel Data  . . . . . . . . . .   35

      3.2  Byte Packaging  . . . . . . . . . . . . . . . . . . . . .   37

      3.3  Display Rendering . . . . . . . . . . . . . . . . . . . .   38

 4.0  Transmitting and Displaying GIF Data . . . . . . . . . . . . .   39

      4.1  GIF Enquiry Escape Sequence and Response  . . . . . . . .   40

      4.2  GIF Invocation Escape Sequences . . . . . . . . . . . . .   45

      4.3  Terminating Display of an Image . . . . . . . . . . . . .   46

      4.4  User Interface to Decoders  . . . . . . . . . . . . . . .   47

 Appendix:  Compression and Expansion Algorithm  . . . . . . . . .    A-1


 GIF                                                   Revision 1.0, 5/88





                                      ii

 Preface
 ------------------------------------------------------------------------
 ------------------------------------------------------------------------

 Objectives      The objectives of this document are to:

                   . describe the Graphics Interchange Format (GIF)

                   . describe encoding and decoding 

                   . provide guidelines for application programs which
                     transmit and display GIF data

                 The document describes version 87a of GIF.

                 --------------------------------------------------------

 Intended        The intended audience of this document is graphics
 audience        programmers who write application programs which use GIF
                 data.

                 --------------------------------------------------------

 Organization    A table of contents is provided to aid in locating
                 specific information within the document.

                 The document contains an introduction to GIF which is
                 followed by detailed information about GIF.  This
                 detailed information is divided into three sections:

                   . Format of GIF data

                   . Encoding and decoding

                   . Transmitting and displaying GIF data

                 The compression and expansion algorithm applied to pixel
                 data is included in the Appendix at the end of the
                 document.

                 --------------------------------------------------------

 Conventions     The following conventions are used in this document:

                   . all numbers are decimal unless otherwise indicated

                   . a hexadecimal number is immediately followed by a
                     lowercase "h"

                     Examples:  0h, 2Ch, FFh, A014h

                 --------------------------------------------------------






                                                              (continued)

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                                      iii

 Preface, continued
 ------------------------------------------------------------------------
 ------------------------------------------------------------------------

 Conventions       . exponentiation is represented in the standard
 (continued)         mathematical superscripted fashion
                                                                 (n+1)
                     Example:  2 to the (n+1) power is shown as 2

                   . bits within a byte are numbered from right to left,
                     starting with 0; bit 0 is the least significant bit
                     and bit 7 is the most significant bit

                   . American Standard Code for Information Interchange
                     (ASCII) character codes are often listed in
                     parentheses after special characters (for example,
                     ^) and character sequences which must be entered
                     exactly as shown; this is done, in part, to avoid
                     confusion when the document is transmitted
                     electronically and special characters in the
                     resulting file do not appear as they did in the
                     original document

                 --------------------------------------------------------




































 GIF                                                   Revision 1.0, 5/88





                                       1

 1.0  Introduction to GIF
 ------------------------------------------------------------------------
 ------------------------------------------------------------------------

 What is GIF?    GIF is the acronym for Graphics Interchange Format.  GIF
                 is a format which is used to store and transmit raster-
                 based color and monochrome images in an efficient,
                 hardware-independent manner.

                 --------------------------------------------------------

 History of GIF  GIF was developed by CompuServe Incorporated to address
                 the need for a general color image definition and to
                 facilitate the exchange of images between dissimilar
                 computer hardware.  

                 --------------------------------------------------------

 Features        To encourage its use in a wide variety of applications,
                 an explicit design goal of GIF is to provide a flexible
                 and comprehensive set of features.  These features
                 include:

                   . arbitrary image dimensions up to 64K pixels

                   . as many as 256 colors in a given image

                   . image colors chosen from a palette of over 16
                     million colors 

                   . hardware-independent format

                   . reduced file sizes by use of advanced data
                     compression techniques

                   . efficient decoding method which allows online
                     viewing of images via networks

                                   . hardware-independent format not as efficient as a
                     hardware-specific format

                   . transmission of GIF data requires full 8-bit
                     transmission capability

                   . follows raster-based, as opposed to object-based,
                     graphics philosophy

                   . quality of image display subject to limitations of
                     display hardware

                 --------------------------------------------------------





 GIF                                                   Revision 1.0, 5/88





                                       2

 1.1  Changes to GIF
 ------------------------------------------------------------------------
 ------------------------------------------------------------------------

 Controlling     Changes to the GIF definition are under the control of
 changes         CompuServe and will remain so until GIF is proposed or
                 adopted as an official industry standard.  Changes are
                 expected every 12-18 months to address the needs of the
                 evolving graphics marketplace.

                 --------------------------------------------------------

 User            Users are discouraged from making ad hoc changes to the
 enhancements    GIF definition, not only to avoid the "multiple-variant"
                 syndrome other public domain computer standards suffer
                 from, but also to keep in the spirit of machine and
                 application independence central to the GIF effort.

                 --------------------------------------------------------

 Proposing       Users are encouraged to submit their proposals for
 changes         changes to the GIF definition.  Each proposal should
                 outline the change and the reasoning behind it. 
                 Proposals may be submitted via U.S. mail to:

                      Manager of Graphics Technology
                      CompuServe Incorporated
                      5000 Arlington Centre Boulevard
                      Columbus, Ohio  43220

                 Proposals may also be submitted in the GIF development
                 area of the CompuServe Information Service (CIS) (see
                 section 1.4, page 5).

                 --------------------------------------------------------

























 GIF                                                   Revision 1.0, 5/88





                                       3

 1.2  Availability of GIF
 ------------------------------------------------------------------------
 ------------------------------------------------------------------------

 Availability    GIF is available for use in any application program
 of GIF          without royalties or licensing restrictions.  CompuServe
                 does ask, however, that any such program include a
                 trademark statement attributing ownership of GIF to
                 CompuServe Incorporated.

                 --------------------------------------------------------

 Support for     GIF support is available in the GIF development area of
 GIF             the CompuServe Information Service (CIS) (see section
                 1.4, page 5).

                 --------------------------------------------------------











































 GIF                                                   Revision 1.0, 5/88





                                       4

 1.3  Naming Files which Contain GIF Data
 ------------------------------------------------------------------------
 ------------------------------------------------------------------------

 Overview        Guidelines for naming files which contain GIF data are
                 provided in this section.  Use of these guidelines is
                 suggested; however, their use is not necessary for
                 conformance with the GIF standard.

                 --------------------------------------------------------

 In MS-DOS and   Use ".GIF" as the extension for a file which contains
 similar         GIF data when in MS-DOS or a similar operating system.
 operating
 systems
                 --------------------------------------------------------

 With the Apple  Classify a file which contains GIF data as type "GIFf"
 Macintosh       when using software for the Apple Macintosh which reads
                 or writes such files.  Doing this helps to prevent
                 problems when using communications programs to transfer
                 files.

                 No official GIF icon has been defined to represent files
                 which contain GIF data.  Any icon used to represent such 
                 files should include the characters "GIF".

                 --------------------------------------------------------

 With other      When naming files which contain GIF data, use a unique
 operating       and consistent file identification within the structure
 systems         of the local file identification conventions.

                 --------------------------------------------------------


























 GIF                                                   Revision 1.0, 5/88





                                       5

 1.4  GIF Development Area
 ------------------------------------------------------------------------
 ------------------------------------------------------------------------

 What is the     The GIF development area is an area within the
 GIF             CompuServe Information Service (CIS) where CompuServe
 development     provides GIF support and incentives for its use by
 area?           independent software developers.  The GIF development
                 area is located in the Pictures Support Forum.

                 --------------------------------------------------------

 Benefits of     Benefits of joining the GIF development area include:
 joining the
 area              . free access to the area

                   . access to GIF source code modules

                   . access to GIF utilities and working programs

                   . support from CompuServe associates

                   . support from other GIF developers

                   . periodic electronic conferences with CompuServe
                     associates and other GIF developers

                 --------------------------------------------------------

 Requirements    Members of the GIF development area must:

                   . be CIS subscribers

                   . adhere to the published GIF standard

                   . include, in any application program developed using
                     GIF, a trademark statement attributing ownership of
                     GIF to CompuServe Incorporated 

                 --------------------------------------------------------

 How to join     To become a member of the GIF development area:

                  1.  Access the Pictures Support Forum (direct access
                      may be obtained by typing the CIS command "GO
                      PICS").

                  2.  Join the Pictures Support Forum by following the
                      instructions provided in that forum.

                  3.  Enter library 1 of the Pictures Support Forum
                      (General/Utilities).

                  4.  Read the file "DEVGIF.TXT" and follow its
                      instructions for joining the GIF development area.

                 Once your application has been approved, access will be
                 granted to the GIF development area.
                 --------------------------------------------------------

 GIF                                                   Revision 1.0, 5/88





                                       6

 2.0  Format of GIF Data 
 ------------------------------------------------------------------------
 ------------------------------------------------------------------------

 Overview        On a display surface, an image is a rectangular array of
                 pixels.  This pixel array may be stored in a file as a
                 series of pixels.  GIF defines a format for storing
                 pixels and the information needed to interpret and
                 display the pixels.

                 A file may be comprised entirely of GIF data;
                 conversely, GIF data may be one component of a much
                 larger file.

                 --------------------------------------------------------

 Pixels          With GIF, an image is stored as a series of pixels. 
                 These pixels serve as index values into a color map. 
                 The color map defines the color assigned to each pixel.

                 Pixels are stored without any indication of position
                 within the image. 

                 --------------------------------------------------------

 Color maps      A color map is a series of red, green, and blue
                 intensity values that defines the individual colors
                 which may appear in an image.  Pixels serve as index
                 values into the color map.  

                 GIF data may include a global color map; this global
                 map is used for each image when no local color map is
                 associated with the image.  A local color map is a
                 color map that is used for only one image.  When no
                 global or local color map is present, pixels cannot be
                 mapped to color definitions, and interpretation of
                 pixels is indeterminate.

                 --------------------------------------------------------

 Format of GIF   GIF data is comprised of functional data blocks, some of
 data            which may be repeated, others of which are optional. 
                 These blocks are shown on the following page and are
                 described in detail in the following sections.

                 --------------------------------------------------------












                                                              (continued)

 GIF                                                   Revision 1.0, 5/88





                                       7

 2.0  Format of GIF Data, continued
 ------------------------------------------------------------------------
 ------------------------------------------------------------------------

 Format of GIF                          GIF Data
 data
 (continued)     +---------------------------------+
                 |     Signature/Version Block     |
                 +---------------------------------+ -+
                 | Logical Screen Descriptor Block |  | Logical Screen
                 +---------------------------------+  | Block
                 |Global Color Map Block (optional)|  |
                 +---------------------------------+ -+
                 |   Extension Block (optional)    | -- may be repeated
                 +---------------------------------+ -+
                 |     Image Descriptor Block      |  |
                 +---------------------------------+  | Image Block;
                 |Local Color Map Block (optional) |  | may be repeated
                 +---------------------------------+  |
                 |      Raster Data Block(s)       |  |
                 +---------------------------------+ -+
                 |   Extension Block (optional)    | -- may be repeated
                 +---------------------------------+
                 |        Terminator Block         |
                 +---------------------------------+

                 --------------------------------------------------------

 Data types      Data type refers to the structure of a data segment and
                 determines how that data segment is interpreted.  The
                 data types used in GIF data are:

                 +---------+--------------------------------------------+
                 |Data Type|                Description                 |
                 +---------+--------------------------------------------+
                 |byte     |8-bit unsigned value; may represent charac- |
                 |         |ters or numeric values between 0 and 255    |
                 +---------+--------------------------------------------+
                 |field    |2 to 7 contiguous bits within a given byte; |
                 |         |interpretation depends on the quantity      |
                 |         |represented                                 |
                 +---------+--------------------------------------------+
                 |flag     |1-bit value which represents a logical true |
                 |         |(1) or false (0)                            |
                 +---------+--------------------------------------------+
                 |integer  |16-bit unsigned integer value; always stored|
                 |         |least significant byte first                |
                 +---------+--------------------------------------------+

                 --------------------------------------------------------










 GIF                                                   Revision 1.0, 5/88





                                       8

 2.1  Signature/Version Block
 ------------------------------------------------------------------------
 ------------------------------------------------------------------------

 Overview        The Signature/Version Block:

                   . identifies a block of data as GIF data

                   . indicates the version of GIF used to define the
                     data

                 --------------------------------------------------------

 Use of the      Decoding software may use the Signature/Version Block to
 Signature/      determine the version of GIF used to define the data.  
 Version Block   This information may be needed by the decoding software
                 to determine if it is able to process the data; if the
                 decoding software was built from a version of GIF which
                 is older than that used to define the data, the
                 decoding software may not be able to adequately support
                 features of the newer definition.

                 --------------------------------------------------------

 Existence       The Signature/Version Block is required.  It must be the
                 first block in the GIF data.

                 --------------------------------------------------------

 Size            The Signature/Version Block is six bytes long.

                 --------------------------------------------------------

 Structure of    The structure of the Signature/Version Block is
 the Signature/  illustrated below.
 Version Block
                               Bits

                   7   6   5   4   3   2   1   0    Byte #
                 +---+---+---+---+---+---+---+---+
                 |                               |    1
                 +-                             -+
                 |           Signature           |    2
                 +-                             -+
                 |                               |    3
                 +-------------------------------+
                 |                               |    4
                 +-                             -+
                 |            Version            |    5
                 +-                             -+
                 |                               |    6
                 +-------------------------------+ 

                 --------------------------------------------------------




                                                              (continued)

 GIF                                                   Revision 1.0, 5/88





                                       9

 2.1  Signature/Version Block, continued
 ------------------------------------------------------------------------
 ------------------------------------------------------------------------

 Structure of    where:
 the
 Signature/      +------------+-------+---------------------------------+
 Version Block   |    Item    | Data  |           Description           |
 (continued)     |            | Type  |                                 |
                 +------------+-------+---------------------------------+
                 |Signature   |bytes  |ASCII text characters "GIF"      |
                 |            |       |(47h 49h 46h); identifies the    |
                 |            |       |block as GIF data                |
                 +------------+-------+---------------------------------+
                 |Version     |bytes  |3-byte string of ASCII characters|
                 |            |       |which identifies the version of  |
                 |            |       |GIF used to define the data; the |
                 |            |       |first two characters are the     |
                 |            |       |year of the definition and the   |
                 |            |       |last character is the lowercase  |
                 |            |       |alphabetic version sequence      |
                 |            |       |number; the current version      |
                 |            |       |is 87a (38h 37h 61h)             |
                 +------------+-------+---------------------------------+

                 --------------------------------------------------------


































 GIF                                                   Revision 1.0, 5/88





                                      10

 2.2  Logical Screen Descriptor Block
 ------------------------------------------------------------------------
 ------------------------------------------------------------------------

 Overview        The Logical Screen Descriptor Block:

                   . defines the logical screen

                   . provides information about the global color map

                   . provides information about image pixels

                   . specifies the background color

                   . specifies the pixel aspect ratio

                 --------------------------------------------------------

 What is the     The logical screen is the area required to contain the
 logical         images in a block of GIF data.  The dimensions of the
 screen?         logical screen are specified in the Logical Screen
                 Descriptor Block.  

                 Note that the logical screen may be larger than the
                 display surface.

                 --------------------------------------------------------

 What is pixel   Pixel aspect ratio is the ratio of a pixel's height as
 aspect ratio?   compared to its width, and it serves as an indication of
                 the pixel's shape.  A pixel aspect ratio of 1.0 defines
                 a square pixel; a ratio of 2.0 defines a pixel which is
                 twice as high as it is wide.

                 --------------------------------------------------------

 Existence       The Logical Screen Descriptor Block is required.  It
                 must immediately follow the Signature/Version Block.

                 --------------------------------------------------------

 Size            The Logical Screen Descriptor Block is seven bytes long.

                 --------------------------------------------------------














                                                              (continued)

 GIF                                                   Revision 1.0, 5/88





                                      11

 2.2  Logical Screen Descriptor Block, continued
 ------------------------------------------------------------------------
 ------------------------------------------------------------------------

 Structure of    The structure of the Logical Screen Descriptor Block is
 the Logical     illustrated below.
 Screen
 Descriptor                    Bits
 Block
                   7   6   5   4   3   2   1   0    Byte #
                 +---+---+---+---+---+---+---+---+
                 |                               |    1
                 +-        Screen Width         -+
                 |                               |    2
                 +-------------------------------+
                 |                               |    3
                 +-        Screen Height        -+
                 |                               |    4
                 +---+-----------+---+-----------+
                 |Map| Color Res |R1 |Pixel Size |    5
                 +---+-----------+---+-----------+
                 |    Background Color Index     |    6
                 +---+---------------------------+
                 |Srt|    Pixel Aspect Ratio     |    7
                 +---+---------------------------+ 

                 where:

                 +------------+-------+---------------------------------+
                 |    Item    | Data  |           Description           |
                 |            | Type  |                                 |
                 +------------+-------+---------------------------------+
                 |Screen Width|integer|width, in pixels, of the logical |
                 |            |       |screen                           |
                 |            |       |                                 |
                 |            |       |Value:  1 to 65535               |
                 +------------+-------+---------------------------------+
                 |Screen      |integer|height, in pixels, of the logical|
                 |Height      |       |screen                           |
                 |            |       |                                 |
                 |            |       |Value:  1 to 65535               |
                 +------------+-------+---------------------------------+
                 |Map         |flag   |if set, a Global Color Map Block |
                 |            |       |follows the Logical Screen       |
                 |            |       |Descriptor Block; otherwise, an  |
                 |            |       |Extension Block or an Image      |
                 |            |       |Descriptor Block follows         |
                 +------------+-------+---------------------------------+

                 --------------------------------------------------------








                                                              (continued)

 GIF                                                   Revision 1.0, 5/88





                                      12

 2.2  Logical Screen Descriptor Block, continued
 ------------------------------------------------------------------------
 ------------------------------------------------------------------------

 Structure of    +------------+-------+---------------------------------+
 the Logical     |    Item    | Data  |           Description           |
 Screen          |            | Type  |                                 |
 Descriptor      +------------+-------+---------------------------------+
 Block           |Color Res   |field  |defines the number of bits per   |
 (continued)     |            |       |color component which the GIF    |
                 |            |       |encoding process had available   |
                 |            |       |for defining colors in the global|
                 |            |       |color map; is an indication of   |
                 |            |       |the richness of the palette from |
                 |            |       |which colors in the global color |
                 |            |       |map were chosen                  |
                 |            |       |                                 |
                 |            |       |Field value:  0 to 7             |
                 |            |       |Item value:   1 to 8 bits per    |
                 |            |       |  red, green, and blue color     |
                 |            |       |  component                      |
                 |            |       |                                 |
                 |            |       |Number of colors in the palette: |
                 |            |       |   (3 x item value)              |
                 |            |       |  2                              |
                 |            |       |  (8 to 16,000,000+)             |
                 +------------+-------+---------------------------------+
                 |R1          |n/a    |reserved; must be 0              |
                 +------------+-------+---------------------------------+
                 |Pixel Size  |field  |number of bits in each pixel of  |
                 |            |       |the images in the GIF data;      |
                 |            |       |determines the number of colors  |
                 |            |       |defined in the global color map  |
                 |            |       |(note:  does not indicate the    |
                 |            |       |number of colors actually used in|
                 |            |       |an image)                        |
                 |            |       |                                 |
                 |            |       |Field value:  0 to 7             |
                 |            |       |Item value:   1 to 8 bits per    |
                 |            |       |  pixel                          |
                 |            |       |                                 |
                 |            |       |Pixel value:                     |
                 |            |       |        (item value)             |
                 |            |       |  0 to 2             - 1         |
                 |            |       |  (maximum value of 255)         |
                 |            |       |                                 |
                 |            |       |Number of colors defined in the  |
                 |            |       |global color map:                |
                 |            |       |        (item value)             |
                 |            |       |  2 to 2                         |
                 |            |       |  (maximum value of 256)         |
                 +------------+-------+---------------------------------+
                 |Background  |byte   |pixel for the screen background  |
                 |Color Index |       |                                 |
                 +------------+-------+---------------------------------+

                 --------------------------------------------------------

                                                              (continued)

 GIF                                                   Revision 1.0, 5/88





                                      13

 2.2  Logical Screen Descriptor Block, continued
 ------------------------------------------------------------------------
 ------------------------------------------------------------------------

 Structure of    +------------+-------+---------------------------------+
 the Logical     |    Item    | Data  |           Description           |
 Screen          |            | Type  |                                 |
 Descriptor      +------------+-------+---------------------------------+
 Block           |Srt         |flag   |if set, the global color map is  |
 (continued)     |            |       |defined in sorted order, most    |
                 |            |       |used color first; otherwise, no  |
                 |            |       |assumption may be made about the |
                 |            |       |order of color definitions in the|
                 |            |       |global color map                 |
                 +------------+-------+---------------------------------+
                 |Pixel Aspect|field  |height to width ratio of each    |
                 |Ratio       |       |pixel of the source image; if 0  |
                 |            |       |no aspect ratio is specified     |
                 |            |       |                                 |
                 |            |       |Field value:  1 to 127           |
                 |            |       |Item value:                      |
                 |            |       |  (field value + 31) / 64        |
                 |            |       |Item value range:                |
                 |            |       |  32/64 to 158/64 (0.5 to 2.47)  |
                 +------------+-------+---------------------------------+

                 --------------------------------------------------------

































 GIF                                                   Revision 1.0, 5/88





                                      14

 2.3  Global Color Map Block
 ------------------------------------------------------------------------
 ------------------------------------------------------------------------

 Overview        The Global Color Map Block contains the global color
                 map.  The global color map is a series of red, green,
                 and blue intensity values that defines the individual
                 colors which may be used in an image.  The global color
                 map is used for each image in the GIF data unless a
                 local color map is defined for that image.  

                 --------------------------------------------------------

 Existence       The Global Color Map Block is optional.  If present, it
                 must immediately follow the Logical Screen Descriptor
                 Block and the MAP flag of the Logical Screen Descriptor
                 Block must be set.  

                 --------------------------------------------------------

 Size            The size, in bytes, of the Global Color Map Block is
                 three times the number of colors defined within.  The
                 number of colors defined may range from 2 to 256 and is
                 determined from the number of bits per pixel:

                                (number of bits per pixel)
                               2

                 The number of bits per pixel is defined in the PIXEL
                 SIZE field of the Logical Screen Descriptor Block.

                 --------------------------------------------------------

 Structure of    The structure of the Global Color Map Block is
 the Global      illustrated below.
 Color Map
 Block                         Bits

                   7   6   5   4   3   2   1   0    Byte #
                 +---+---+---+---+---+---+---+---+
                 |     Color 0 Red Intensity     |    1
                 +-------------------------------+
                 |    Color 0 Green Intensity    |    2
                 +-------------------------------+
                 |    Color 0 Blue Intensity     |    3
                 +-------------------------------+
                 |     Color 1 Red Intensity     |    4
                 +-------------------------------+
                 |    Color 1 Green Intensity    |    5
                 +-------------------------------+
                 |    Color 1 Blue Intensity     |    6
                 +-------------------------------+ 
                 .                               .  repeat, as needed,
                 .                               .  for remaining colors
                 .                               .
                 +-------------------------------+

                 --------------------------------------------------------
                                                              (continued)

 GIF                                                   Revision 1.0, 5/88





                                      15

 2.3  Global Color Map Block, continued
 ------------------------------------------------------------------------
 ------------------------------------------------------------------------

 Structure of    where:
 the Global
 Color Map       +------------+-------+---------------------------------+
 Block           |    Item    | Data  |           Description           |
 (continued)     |            | Type  |                                 |
                 +------------+-------+---------------------------------+
                 |Color n     |byte   |indicates the amount of red      |
                 |Red         |       |component color n contains       |
                 |Intensity   |       |                                 |
                 |            |       |Item value range:  0 (no red)    |
                 |            |       |  to 255 (maximum red)           |
                 +------------+-------+------------------------