Difference between revisions of "GIF"
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Revision as of 15:32, 27 May 2011
The Graphics Interchange Format (GIF) (SM) format is a lossless image format. GIF images use internal LZW compression to reduce file size. CompuServe created this format, which is a bitmap image format allowing 256 different colors to be selected from a 24-bit color palette (RGB). GIF also allows for animations by sequencing through multiple GIF image data inside a single file.
"The Graphics Interchange Format(c) is the Copyright property of CompuServe Incorporated. GIF(sm) is a Service Mark property of CompuServe Incorporated."
GIF files consist of a header, image data, optional metadata, and a footer. The header consists of a signature and a version, each 3 bytes long. The signature is 47 49 46 (hex) / GIF (text). The versions are either 38 37 61 or 38 39 61 (hex) / 87a or 89a (text) respectively. The footer or trailer (as identified in the format specification) is usually 3B (hex).
Common file extensions are .gif and .GIF
GIF89a files can contain metadata in text format. GIF metadata is contained in sections identified as a Comment Extension, a Plain Text Extension, and an Application Extension. All extension sections begin with the Extension Introducer 21 (hex).
Comment Extensions are optional and more than one may be present. They were designed to allow including comments about the graphic, credits, descriptions or other types of non-control/non-graphic data. The beginning of this block has the Extension Introducer and a Comment Label FE (hex). Comment data has a sequence of sub-blocks between 1 and 255 bytes in length, with the size in a byte before the data. Comment Extensions should appear either before or after the control and graphic data blocks.
Plain Text Extensions are optional and more than one may be present. They were designed to allow rendering of textual data as a graphic. The beginning of this block has the Extension Introducer and a Comment Label 01 (hex). Plain text data has a sequence of sub-blocks between 1 and 255 bytes in length, with the size in a byte before the data.
Application Extensions are optional. They were designed to allow applications to insert application specific data inside a GIF. The beginning of this block has the Extension Introducer and an Application Extension Label FF (hex).