GIF87a

GIF87a is the original version of the Graphical Interchange Format (GIF), introduced in 1987 by CompuServe. Files encoded in this version begin with the ASCII header GIF87a, which identifies the format to software. The design aimed to provide a simple, portable method for storing indexed-color images on early networks and systems.

A GIF87a file comprises a logical screen descriptor that specifies the canvas width and height, and flags such as whether a global color table is present. If a global color table exists, it is included as a sequence of up to 256 RGB triples. Image data is organized into one or more image blocks. Each image block contains an image descriptor, which may reference a local color table, followed by pixel data compressed with the LZW algorithm. The compressed data is stored in a series of sub-blocks, ending with a zero-length block.

Color indexing and compression: The image pixels are represented as indices into the active color table, and the LZW compression method is used to reduce file size. The palette in GIF87a is limited to 256 colors per image, though multiple frames or tables may be used in more complex arrangements.

Animation and transparency: GIF87a does not define mechanisms for animation or transparency. Those features were introduced later with the GIF89a revision, which added the Graphic Control Extension, the Netscape looping extension, and other enhancements. Despite this, GIF87a files can be interpreted by software that supports newer GIF specifications.

Legacy and impact: GIF87a established a widely adopted, compact format for simple graphics. The use of LZW compression contributed to patent-related licensing concerns in the 1990s, influencing the development of alternative formats such as PNG. Nevertheless, GIF remains compatible with modern decoders and continues to be used for basic images and simple animations in its later form.