.GIF File Format

Introduction

GIF87a is the first version of GIF, published by CompuServe in 1987 so that pictures could be shared across the different computers that used its online service. A file holds a logical screen and one or more images placed on it, each with up to 256 colours from a palette and compressed without loss using LZW.

It has no transparency, no frame timing and no comments; those came with GIF89a. Programs still write GIF87a files when a picture needs none of the later additions.

Structure

The file starts with a 6-byte header, GIF87a, and a 7-byte logical screen descriptor giving the canvas size (little-endian), the colour resolution and whether a global colour table follows. Then come the images, each introduced by 2C, and the trailer 3B that ends the file.

Each image has a 10-byte descriptor giving its position and size on the screen, an optional local colour table, the LZW minimum code size and the compressed data in sub-blocks: a length byte of up to 255 followed by that many bytes, ending with a zero-length sub-block.

Telling it apart

The file starts with GIF87a (47 49 46 38 37 61). A GIF89a file starts with GIF89a and may hold extension blocks, introduced by 21, between its images.

Variants

• Graphics Interchange Format (GIF87a) – the first GIF: palette images on a logical screen, with no extensions
• Graphics Interchange Format (GIF89a) – the current GIF, with extensions for transparency, timing and animation

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