G I F (tm) Graphics Interchange Format (tm) A standard defining a mechanism for the storage and transmission of raster-based graphics information June 15, 1987 (c) CompuServe Incorporated, 1987 All rights reserved While this document is copyrighted, the information contained within is made available for use in computer software without royalties, or licensing restrictions. 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 Page 2 Graphics Interchange Format (GIF) Specification Table of Contents INTRODUCTION . . . . . . . . . . . . . . . . . page 3 GENERAL FILE FORMAT . . . . . . . . . . . . . page 3 GIF SIGNATURE . . . . . . . . . . . . . . . . page 4 SCREEN DESCRIPTOR . . . . . . . . . . . . . . page 4 GLOBAL COLOR MAP . . . . . . . . . . . . . . . page 5 IMAGE DESCRIPTOR . . . . . . . . . . . . . . . page 6 LOCAL COLOR MAP . . . . . . . . . . . . . . . page 7 RASTER DATA . . . . . . . . . . . . . . . . . page 7 GIF TERMINATOR . . . . . . . . . . . . . . . . page 8 GIF EXTENSION BLOCKS . . . . . . . . . . . . . page 8 APPENDIX A - GLOSSARY . . . . . . . . . . . . page 9 APPENDIX B - INTERACTIVE SEQUENCES . . . . . . page 10 APPENDIX C - IMAGE PACKAGING & COMPRESSION . . page 12 APPENDIX D - MULTIPLE IMAGE PROCESSING . . . . page 15 Graphics Interchange Format (GIF) Page 3 Specification INTRODUCTION 'GIF' (tm) is CompuServe's standard for defining generalized color raster images. This 'Graphics Interchange Format' (tm) allows high-quality, high-resolution graphics to be displayed on a variety of graphics hardware and is intended as an exchange and display mechanism for graphics images. The image format described in this document is designed to support current and future image technology and will in addition serve as a basis for future CompuServe graphics products. The main focus of this document is to provide the technical information necessary for a programmer to implement GIF encoders and decoders. As such, some assumptions are made as to terminology relavent to graphics and programming in general. The first section of this document describes the GIF data format and its components and applies to all GIF decoders, either as standalone programs or as part of a communications package. Appendix B is a section relavent to decoders that are part of a communications software package and describes the protocol requirements for entering and exiting GIF mode, and responding to host interrogations. A glossary in Appendix A defines some of the terminology used in this document. Appendix C gives a detailed explanation of how the graphics image itself is packaged as a series of data bytes. Graphics Interchange Format Data Definition GENERAL FILE FORMAT +-----------------------+ | +-------------------+ | | | GIF Signature | | | +-------------------+ | | +-------------------+ | | | Screen Descriptor | | | +-------------------+ | | +-------------------+ | | | Global Color Map | | | +-------------------+ | . . . . . . | +-------------------+ | ---+ | | Image Descriptor | | | | +-------------------+ | | | +-------------------+ | | | | Local Color Map | | |- Repeated 1 to n times | +-------------------+ | | | +-------------------+ | | | | Raster Data | | | | +-------------------+ | ---+ . . . . . . |- GIF Terminator -| +-----------------------+ Graphics Interchange Format (GIF) Page 4 Specification GIF SIGNATURE The following GIF Signature identifies the data following as a valid GIF image stream. It consists of the following six characters: G I F 8 7 a The last three characters '87a' may be viewed as a version number for this particular GIF definition and will be used in general as a reference in documents regarding GIF that address any version dependencies. SCREEN DESCRIPTOR The Screen Descriptor describes the overall parameters for all GIF images following. It defines the overall dimensions of the image space or logical screen required, the existance of color mapping information, background screen color, and color depth information. This information is stored in a series of 8-bit bytes as described below. bits 7 6 5 4 3 2 1 0 Byte # +---------------+ | | 1 +-Screen Width -+ Raster width in pixels (LSB first) | | 2 +---------------+ | | 3 +-Screen Height-+ Raster height in pixels (LSB first) | | 4 +-+-----+-+-----+ M = 1, Global color map follows Descriptor |M| cr |0|pixel| 5 cr+1 = # bits of color resolution +-+-----+-+-----+ pixel+1 = # bits/pixel in image | background | 6 background=Color index of screen background +---------------+ (color is defined from the Global color |0 0 0 0 0 0 0 0| 7 map or default map if none specified) +---------------+ The logical screen width and height can both be larger than the physical display. How images larger than the physical display are handled is implementation dependent and can take advantage of hardware characteristics (e.g. Macintosh scrolling windows). Otherwise images can be clipped to the edges of the display. The value of 'pixel' also defines the maximum number of colors within an image. The range of values for 'pixel' is 0 to 7 which represents 1 to 8 bits. This translates to a range of 2 (B & W) to 256 colors. Bit 3 of word 5 is reserved for future definition and must be zero. Graphics Interchange Format (GIF) Page 5 Specification GLOBAL COLOR MAP The Global Color Map is optional but recommended for images where accurate color rendition is desired. The existence of this color map is indicated in the 'M' field of byte 5 of the Screen Descriptor. A color map can also be associated with each image in a GIF file as described later. However this global map will normally be used because of hardware restrictions in equipment available today. In the individual Image Descriptors the 'M' flag will normally be zero. If the Global Color Map is present, it's definition immediately follows the Screen Descriptor. The number of color map entries following a Screen Descriptor is equal to 2**(# bits per pixel), where each entry consists of three byte values representing the relative intensities of red, green and blue respectively. The structure of the Color Map block is: bits 7 6 5 4 3 2 1 0 Byte # +---------------+ | red intensity | 1 Red value for color index 0 +---------------+ |green intensity| 2 Green value for color index 0 +---------------+ | blue intensity| 3 Blue value for color index 0 +---------------+ | red intensity | 4 Red value for color index 1 +---------------+ |green intensity| 5 Green value for color index 1 +---------------+ | blue intensity| 6 Blue value for color index 1 +---------------+ : : (Continues for remaining colors) Each image pixel value received will be displayed according to its closest match with an available color of the display based on this color map. The color components represent a fractional intensity value from none (0) to full (255). White would be represented as (255,255,255), black as (0,0,0) and medium yellow as (180,180,0). For display, if the device supports fewer than 8 bits per color component, the higher order bits of each component are used. In the creation of a GIF color map entry with hardware supporting fewer than 8 bits per component, the component values for the hardware should be converted to the 8-bit format with the following calculation: <map_value> = <component_value>*255/(2**<nbits> -1) This assures accurate translation of colors for all displays. In the cases of creating GIF images from hardware without color palette capability, a fixed palette should be created based on the available display colors for that hardware. If no Global Color Map is indicated, a default color map is generated ...
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