Indexed color
In computing, indexed color is a technique to manage digital images' colors in a limited fashion, in order to save computer memory and file storage, while speeding up display refresh and file transfers. It is a form of vector quantization compression.
When an image is encoded in this way, color information is not directly carried by the image pixel data, but is stored in a separate piece of data called a palette: an array of color elements. Every element in the array represents a color, indexed by its position within the array. The individual entries are sometimes known as color registers. The image pixels do not contain the full specification of its color, but only its index in the palette. This technique is sometimes referred as pseudocolor or indirect color, as colors are addressed indirectly.
Perhaps the first device that supported palette colors was a random-access frame buffer, described in 1975 by Kajiya, Sutherland and Cheadle. This supported a palette of 256 36-bit RGB colors.
The palette itself stores a limited number of distinct colors; 4, 16 or 256 are the most common cases. These limits are often imposed by the target architecture's display adapter hardware, so it is not a coincidence that those numbers are exact powers of two (the binary code): 22 = 4, 24 = 16 and 28 = 256. While 256 values can be fit into a single 8-bit byte (and then a single indexed color pixel also occupies a single byte), pixel indices with 16 (4-bit, a nibble) or fewer colors can be packed together into a single byte (two nibbles per byte, if 16 colors are employed, or four 2-bit pixels per byte if using 4 colors). Sometimes, 1-bit (2-color) values can be used, and then up to eight pixels can be packed into a single byte; such images are considered binary images (sometimes referred as a bitmap or bilevel image) and not an indexed color image.
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