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Other Bitmap File Data Structures

Besides headers, footers, and palettes, bitmap files may contain additional data structures, which are usually added to aid manipulation of the image data by the rendering application.

A file format which allows more than one image in the file needs to provide some method of identifying the start of each image. Thus, an image offset table, sometimes called an image file index or page table, may be used to store the offset values of the start of each image from the beginning of the file.

A scan-line table may be provided for locating the start of each image scan line in the pixel data. This can be useful if the image data is compressed and pixel data corresponding to individual scan lines must be accessed randomly; the pixels in the image data cannot be counted until the image data is decoded. Scan-line tables contain one entry per image scan line. Variants of this idea include strip location tables (one entry per group of scan lines) and tile location tables (one entry for each rectangular subarea of the image).

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This page is taken from the Encyclopedia of Graphics File Formats and is licensed by O'Reilly under the Creative Common/Attribution license.