Graphics Interchange Format (GIF)

Image

The Graphics Interchange Format (GIF) is a bitmap image format that was developed by a team at the online services provider CompuServe led by American computer scientist Steve Wilhite on 15 June 1987. It has since come into widespread usage on the World Wide Web due to its wide support and portability between applications and operating systems.

The format supports up to 8 bits per pixel for each image, allowing a single image to reference its own palette of up to 256 different colors chosen from the 24-bit RGB color space. It also supports animations and allows a separate palette of up to 256 colors for each frame. These palette limitations make GIF less suitable for reproducing color photographs and other images with color gradients, but well-suited for simpler images such as graphics or logos with solid areas of color. Unlike video, the GIF file format does not support audio.

GIF images are compressed using the Lempel–Ziv–Welch (LZW) lossless data compression technique to reduce the file size without degrading the visual quality. This compression technique was patented in 1985. Controversy over the licensing agreement between the software patent holder, Unisys, and CompuServe in 1994 spurred the development of the Portable Network Graphics (PNG) standard. By 2004 all the relevant patents had expired.

Usage

  • GIFs are suitable for sharp-edged line art with a limited number of colors, such as logos. This takes advantage of the format's lossless compression, which favors flat areas of uniform color with well defined edges.
  • GIFs may be used to store low-color sprite data for games.
  • GIFs can be used for small animations and low-resolution video clips.
  • GIFs can be used as a reaction when messaging online, used to convey emotion and feelings, alternative to using words
  • Popular on social media platforms such as Tumblr, Facebook and Twitter.

File format

Conceptually, a GIF file describes a fixed-sized graphical area (the "logical screen") populated with zero or more "images". Many GIF files have a single image that fills the entire logical screen. Others divide the logical screen into separate sub-images. The images may also function as animation frames in an animated GIF file, but again these need not fill the entire logical screen.

GIF files start with a fixed-length header ("GIF87a" or "GIF89a") giving the version, followed by a fixed-length Logical Screen Descriptor giving the pixel dimensions and other characteristics of the logical screen. The screen descriptor may also specify the presence and size of a Global Color Table, which follows next if present.

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