Abstract of INFO-MAC archived Text file 'dev/info/gif-format-gif89a.txt' Uploaded 07/06/1993 83421 bytes Date: Sun, 27 Jun 1993 21:56:58 -0700 From: Alan Coopersmith Subject: GIF 89a format dev/info/gif-format.txt is the 1987 (GIF87a) version of the format description. Enclosed is the July 31, 1990 revision which covers the GIF89a version. Note: GIF & Graphics Interchange Format are trademarks of Compuserve. ------------------------------------------------------------------------------- Cover Sheet for the GIF89a Specification DEFERRED CLEAR CODE IN LZW COMPRESSION There has been confusion about where clear codes can be found in the data stream. As the specification says, they may appear at anytime. There is not a requirement to send a clear code when the string table is full. It is the encoder's decision as to when the table should be cleared. When the table is full, the encoder can chose to use the table as is, making no changes to it until the encoder chooses to clear it. The encoder during this time sends out codes that are of the maximum Code Size. As we can see from the above, when the decoder's table is full, it must not change the table until a clear code is received. The Code Size is that of the maximum Code Size. Processing other than this is done normally. Because of a large base of decoders that do not handle the decompression in this manner, we ask developers of GIF encoding software to NOT implement this feature until at least January 1991 and later if they see that their particular market is not ready for it. This will give developers of GIF decoding software time to implement this feature and to get it into the hands of their clients before the decoders start "breaking" on the new GIF's. It is not required that encoders change their software to take advantage of the deferred clear code, but it is for decoders. APPLICATION EXTENSION BLOCK - APPLICATION IDENTIFIER There will be a Courtesy Directory file located on CompuServe in the PICS forum. This directory will contain Application Identifiers for Application Extension Blocks that have been used by developers of GIF applications. This file is intended to help keep developers that wish to create Application Extension Blocks from using the same Application Identifiers. This is not an official directory; it is for voluntary participation only and does not guarantee that someone will not use the same identifier.