Graphic Interchange Format - GIF


The Graphics Interchange Format (GIF) lays down the protocol that is required for the transmission of graphical data that is not dependent on the hardware used in the display. The GIF is defined in terms of blocks and sub-blocks; these contain values and data that are used when the image is reproduced. The graphics in the data stream are assumed to be related to some degree and to share control information. The GIF format has been designed to run over a trouble free communications link as there is no error checking or error correction built into it.

Within the header of the data stream is the version number, this is used to identify itself to the decoder so that it is able to process the data stream correctly. The decoder is used to process a GIF data stream. The processing of this data is done sequentially; using the information to set the hardware and process values, and reading the data to display the image. It is the decoder’s responsibility to process each image in the data stream in sequence, and without a delay, and to set the hardware values as closely as possible to the control data contained in the stream of data. The GIF employs colour tables to produce graphics; a colour table has a global or local table. A global table is used for the graphics in a data stream which are not in a local colour table. The local colour table is confined to that single graphic.

  Bit depth Memory planes Colour map Calibration Compression RGB images
GIF 1,4,8,24 Yes Yes No LZW In file
  1. The blocks that are used are a collection of bytes which form a protocol unit.
  2. The GIF data stream is composed of blocks and sub blocks representing images and graphics, together with control information to render them on a display device. All control and data blocks in the data stream must follow the header.
  3. The GIF decoder is the program that decodes the GIF data stream and the encoder performs the opposite function, capturing and formatting the image and graphic data.
  4. A GIF image has a maximum of 256 colours.

A description of the Graphic Interchange Format file format, there are other file format descriptions in the image formats section.

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