ryanb@microsoft.UUCP (Danger Mouse) (08/10/90)
(Kirk Johnson) writes: >on 1 august 1990, lwv27@CAS.BITNET (Larry W. Virden) wrote: > Compuserve has made available in their Graphics Support Forum a > new GIF89a document which details the current standards. Yup, and I quickly snarfed it. >i am interested in getting a copy of the new GIF standard. >unfortunately, i do not have access to compuserve. i'm sure >there are others that are in the same boat. Undoubtedly. I actually got it through someone else who has access. I also figure there are quite a number of people who would want it but wouldn't have access to ftp and the like. I've cut it up into three parts. Snarf, then concatenate. (Standard Disclaimer applies, etc.) Part 1 of 3 - Cut here --- Cut here --- Cut here --- Cut here --- Cut here --- Cut here --- Cover Sheet for the GIF89a Specification DEFERRED CLEAR CODE IN LZW COMPRESSION There has been confusion about where clear codes can be found in the data stream. As the specification says, they may appear at anytime. There is not a requirement to send a clear code when the string table is full. It is the encoder's decision as to when the table should be cleared. When the table is full, the encoder can chose to use the table as is, making no changes to it until the encoder chooses to clear it. The encoder during this time sends out codes that are of the maximum Code Size. As we can see from the above, when the decoder's table is full, it must not change the table until a clear code is received. The Code Size is that of the maximum Code Size. Processing other than this is done normally. Because of a large base of decoders that do not handle the decompression in this manner, we ask developers of GIF encoding software to NOT implement this feature until at least January 1991 and later if they see that their particular market is not ready for it. This will give developers of GIF decoding software time to implement this feature and to get it into the hands of their clients before the decoders start "breaking" on the new GIF's. It is not required that encoders change their software to take advantage of the deferred clear code, but it is for decoders. APPLICATION EXTENSION BLOCK - APPLICATION IDENTIFIER There will be a Courtesy Directory file located on CompuServe in the PICS forum. This directory will contain Application Identifiers for Application Extension Blocks that have been used by developers of GIF applications. This file is intended to help keep developers that wish to create Application Extension Blocks from using the same Application Identifiers. This is not an official directory; it is for voluntary participation only and does not guarantee that someone will not use the same identifier. E-Mail can be sent to Larry Wood (forum manager of PICS) indicating the request for inclusion in this file with an identifier. GRAPHICS INTERCHANGE FORMAT(sm) Version 89a (c)1987,1988,1989,1990 Copyright CompuServe Incorporated Columbus, Ohio CompuServe Incorporated Graphics Interchange Format Document Date : 31 July 1990 Programming Reference Table of Contents Disclaimer................................................................. 1 Foreword................................................................... 1 Licensing.................................................................. 1 About the Document......................................................... 2 General Description........................................................ 2 Version Numbers............................................................ 2 The Encoder................................................................ 3 The Decoder................................................................ 3 Compliance................................................................. 3 About Recommendations...................................................... 4 About Color Tables......................................................... 4 Blocks, Extensions and Scope............................................... 4 Block Sizes................................................................ 5 Using GIF as an embedded protocol.......................................... 5 Data Sub-blocks............................................................ 5 Block Terminator........................................................... 6 Header..................................................................... 7 Logical Screen Descriptor.................................................. 8 Global Color Table......................................................... 10 Image Descriptor........................................................... 11 Local Color Table.......................................................... 13 Table Based Image Data..................................................... 14 Graphic Control Extension.................................................. 15 Comment Extension.......................................................... 17 Plain Text Extension....................................................... 18 Application Extension...................................................... 21 Trailer.................................................................... 23 Quick Reference Table...................................................... 24 GIF Grammar................................................................ 25 Glossary................................................................... 27 Conventions................................................................ 28 Interlaced Images.......................................................... 29 Variable-Length-Code LZW Compression....................................... 30 On-line Capabilities Dialogue.............................................. 33 1 1. Disclaimer. The information provided herein is subject to change without notice. In no event will CompuServe Incorporated be liable for damages, including any loss of revenue, loss of profits or other incidental or consequential damages arising out of the use or inability to use the information; CompuServe Incorporated makes no claim as to the suitability of the information. 2. Foreword. This document defines the Graphics Interchange Format(sm). The specification given here defines version 89a, which is an extension of version 87a. The Graphics Interchange Format(sm) as specified here should be considered complete; any deviation from it should be considered invalid, including but not limited to, the use of reserved or undefined fields within control or data blocks, the inclusion of extraneous data within or between blocks, the use of methods or algorithms not specifically listed as part of the format, etc. In general, any and all deviations, extensions or modifications not specified in this document should be considered to be in violation of the format and should be avoided. 3. Licensing. The Graphics Interchange Format(c) is the copyright property of CompuServe Incorporated. Only CompuServe Incorporated is authorized to define, redefine, enhance, alter, modify or change in any way the definition of the format. CompuServe Incorporated hereby grants a limited, non-exclusive, royalty-free license for the use of the Graphics Interchange Format(sm) in computer software; computer software utilizing GIF(sm) must acknowledge ownership of the Graphics Interchange Format and its Service Mark by CompuServe Incorporated, in User and Technical Documentation. Computer software utilizing GIF, which is distributed or may be distributed without User or Technical Documentation must display to the screen or printer a message acknowledging ownership of the Graphics Interchange Format and the Service Mark by CompuServe Incorporated; in this case, the acknowledgement may be displayed in an opening screen or leading banner, or a closing screen or trailing banner. A message such as the following may be used: "The Graphics Interchange Format(c) is the Copyright property of CompuServe Incorporated. GIF(sm) is a Service Mark property of CompuServe Incorporated." For further information, please contact : CompuServe Incorporated Graphics Technology Department 5000 Arlington Center Boulevard Columbus, Ohio 43220 U. S. A. CompuServe Incorporated maintains a mailing list with all those individuals and organizations who wish to receive copies of this document when it is corrected 2 or revised. This service is offered free of charge; please provide us with your mailing address. 4. About the Document. This document describes in detail the definition of the Graphics Interchange Format. This document is intended as a programming reference; it is recommended that the entire document be read carefully before programming, because of the interdependence of the various parts. There is an individual section for each of the Format blocks. Within each section, the sub-section labeled Required Version refers to the version number that an encoder will have to use if the corresponding block is used in the Data Stream. Within each section, a diagram describes the individual fields in the block; the diagrams are drawn vertically; top bytes in the diagram appear first in the Data Stream. Bits within a byte are drawn most significant on the left end. Multi-byte numeric fields are ordered Least Significant Byte first. Numeric constants are represented as Hexadecimal numbers, preceded by "0x". Bit fields within a byte are described in order from most significant bits to least significant bits. 5. General Description. The Graphics Interchange Format(sm) defines a protocol intended for the on-line transmission and interchange of raster graphic data in a way that is independent of the hardware used in their creation or display. The Graphics Interchange Format is defined in terms of blocks and sub-blocks which contain relevant parameters and data used in the reproduction of a graphic. A GIF Data Stream is a sequence of protocol blocks and sub-blocks representing a collection of graphics. In general, the graphics in a Data Stream are assumed to be related to some degree, and to share some control information; it is recommended that encoders attempt to group together related graphics in order to minimize hardware changes during processing and to minimize control information overhead. For the same reason, unrelated graphics or graphics which require resetting hardware parameters should be encoded separately to the extent possible. A Data Stream may originate locally, as when read from a file, or it may originate remotely, as when transmitted over a data communications line. The Format is defined with the assumption that an error-free Transport Level Protocol is used for communications; the Format makes no provisions for error-detection and error-correction. The GIF Data Stream must be interpreted in context, that is, the application program must rely on information external to the Data Stream to invoke the decoder process. 6. Version Numbers. The version number in the Header of a Data Stream is intended to identify the minimum set of capabilities required of a decoder in order to fully process the 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 3 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. 7. The Encoder. 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. - Include in the Data Stream all the necessary information to reproduce the graphics. - Insure that a Data Stream is labeled with the earliest possible Version Number that will cover the definition of all the blocks in it; this is to ensure that the largest number of decoders can process the Data Stream. - Ensure encoding of the graphics in such a way that the decoding process is optimized. Avoid redundant information as much as possible. - To the extent possible, avoid grouping graphics which might require resetting hardware parameters during the decoding process. - Set to zero (off) each of the bits of each and every field designated as reserved. Note that some fields in the Logical Screen Descriptor and the Image Descriptor were reserved under Version 87a, but are used under version 89a. 8. The Decoder. 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. - Process each graphic in the Data Stream in sequence, without delays other than those specified in the control information. - Set its hardware parameters to fit, as closely as possible, the control information contained in the Data Stream. 9. Compliance. 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 4 encoder or a decoder may be compliant with a given version number and not compliant with some subsequent version. 10. About Recommendations. 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. 11. About Color Tables. 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. 12. Blocks, Extensions and Scope. 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 5 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. 13. Block Sizes. 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. 14. Using GIF as an embedded protocol. 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. 15. Data Sub-blocks. a. 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. b. Required Version. 87a. 6 c. 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 | | +---------------+ i) Block Size - Number of bytes in the Data Sub-block; the size must be within 0 and 255 bytes, inclusive. ii) Data Values - Any 8-bit value. There must be exactly as many Data Values as specified by the Block Size field. d. Extensions and Scope. This type of block always occurs as part of a larger unit. It does not have a scope of itself. e. Recommendation. None. 16. Block Terminator. a. 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. b. Required Version. 87a. c. Syntax. 7 6 5 4 3 2 1 0 Field Name Type +---------------+ 0 | | Block Size Byte +---------------+ i) Block Size - Number of bytes in the Data Sub-block; this field contains the fixed value 0x00. ii) Data Values - This block does not contain any data. 7 d. Extensions and Scope. This block terminates the immediately preceding sequence of Data Sub-blocks. This block cannot be modified by any extension. e. Recommendation. None. 17. Header. a. 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. b. Required Version. Not applicable. This block is not subject to a version number. This block must appear at the beginning of every Data Stream. c. Syntax. 7 6 5 4 3 2 1 0 Field Name Type +---------------+ 0 | | Signature 3 Bytes +- -+ 1 | | +- -+ 2 | | +---------------+ 3 | | Version 3 Bytes +- -+ 4 | | +- -+ 5 | | +---------------+ i) Signature - Identifies the GIF Data Stream. This field contains the fixed value 'GIF'. ii) 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). iii) Extensions and Scope. The scope of this block is the entire Data Stream. This block cannot be modified by any extension. 8 d. Recommendations. i) 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.) ii) 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. 18. Logical Screen Descriptor. a. 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. b. Required Version. Not applicable. This block is not subject to a version number. This block must appear immediately after the Header. c. 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 +---------------+ 9 <Packed Fields> = Global Color Table Flag 1 Bit Color Resolution 3 Bits Sort Flag 1 Bit Size of Global Color Table 3 Bits i) Logical Screen Width - Width, in pixels, of the Logical Screen where the images will be rendered in the displaying device. ii) Logical Screen Height - Height, in pixels, of the Logical Screen where the images will be rendered in the displaying device. iii) 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. - Cut here --- Cut here --- Cut here --- Cut here --- Cut here --- Cut here ---