Indexed color

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In computing, indexed color is a technique to manage digital images' colors in a limited fashion, in order to save computer 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 color lookup table (CLUT) or palette: an array of color specifications. Every element in the array represents a color, indexed by its position within the array. For color information, each image pixel then specifies only its index into the palette. This technique is sometimes referred as pseudocolor or indirect color, as colors are addressed indirectly. Early graphics display systems that used 8-bit indexed color with frame buffers and CLUTs include Shoup's SuperPaint (1973) and the video frame buffer described in 1975 by Kajiya, Sutherland, and Cheadle. These supported a palette of 256 RGB colors. SuperPaint used a shift-register frame buffer, while the Kajiya et al. system used a random-access frame buffer. A few earlier systems used 3-bit color, but typically treated the bits as independent red, green, and blue on/off bits rather than jointly as an index into a CLUT.