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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) GIF Revision 1.0, 5/88 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) | +------------+-------+------------------------