.GIF File Format

Introduction

GIF89a is the version of GIF that CompuServe published in 1989 and revised in 1990. It keeps everything in GIF87a and adds extension blocks. The graphic control extension gives each frame a delay, a disposal method and a transparent colour, which is what makes animated and transparent GIFs possible. Comment, plain text and application extensions carry the rest.

Nearly every animated GIF on the web is a GIF89a file.

Structure

A GIF starts with a 6-byte header, GIF89a, and a 7-byte logical screen descriptor giving the canvas size (little-endian) and whether a global colour table follows. The rest of the file is a series of blocks, each introduced by one byte: 21 for an extension, 2C for an image and 3B for the trailer that ends the file.

An image block has a 10-byte descriptor, an optional local colour table, the LZW minimum code size and the compressed data. Image data and extension data are written as sub-blocks: a length byte of up to 255 followed by that many bytes, ending with a zero-length sub-block. A graphic control extension before an image sets its delay and transparent colour, and the NETSCAPE2.0 application extension sets the loop count.

Telling it apart

The file starts with GIF89a (47 49 46 38 39 61). A GIF87a file differs only in those last three characters, and has no extension blocks.

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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