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Graphics Interchange Format
Rotating earth (large).gif
A rotating globe in GIF format. Posterization is noticeable in the blue gradient areas due to the restricted palette.
File extension: .gif
MIME type: image/gif
Type code: GIF
Uniform Type Identifier: com.compuserve.gif
Magic: GIF87a/GIF89a
Developed by: CompuServe
Type of format: Raster graphics image format

The Graphics Interchange Format (often shortened to GIF) is a file format for images.

It is a raster graphics format; that is the image has a fixed size. CompuServe developed and introduced the format in 1987. Today, it is widely used on the World Wide Web.The format supports up to 8 bits per pixel, or 256 colors. It also supports animations. Each frame of an animation can have its own set of 256 colors. GIF is not used to show photographs very much, because having only 256 colors is limiting for such a task.

To reduce their size, images are compressed using Lempel–Ziv–Welch compression. Lempel-Ziv-Welch, or LZW is a lossless compression. No information is lost when compressing or uncompressing. There were some problems with the patent of the LZW compression. That patent was not held by CompuServe, but by Unisys. These issues led to the development of another format, called PNG. This is no longer a problem, because the patents have expired.

Animated GIF

Newtons cradle animation book 2
GIF can be used to display animation, as in this image of Newton's Cradle.

Basic animation was added to the GIF89a spec via the Graphics Control Extension (GCE), which allows various images (frames) in the file to be painted with time delays, forming a video clip. An animated GIF file comprises a number of frames that are displayed in succession, each introduced by its own GCE, which gives the time delay to wait after the frame is drawn. Global information at the start of the file applies by default to all frames. The data is stream-oriented, so the file-offset of the start of each GCE depends on the length of preceding data. Within each frame the LZW-coded image data is arranged in sub-blocks of up to 255 bytes; the size of each sub-block is declared by the byte that precedes it.

By default, however, an animation displays the sequence of frames only once, stopping when the last frame is displayed.

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