Colour look-up table

A colour look-up table (CLUT) is a mechanism used to transform a range of input colours into another range of colours. It can be a hardware device built into an imaging system or a software function built into an image processing application. The hardware colour look-up table will convert the logical colour (pseudo-colour) numbers stored in each pixel of video memory into physical colours, normally represented as RGB triplets, that can be displayed on a computer monitor. The colour palette is simply a block of fast RAM which is addressed by the logical colour and whose output is split into the red, green, and blue levels which drive the actual display (e.g., a CRT or cathode ray tube).

A CLUT is characterized by:

A common example would be a palette of 256 colours (e.g. VGA hardware); that is, the number of entries is 256, and thus each entry is addressed by an 8-bit pixel value. The 8 bits is known as colour depth, bit depth or bits per pixel (bpp). Each colour can be chosen from a full palette, typically with a total of 16.7 million colours; that is, the width of each entry is 24 bits, 8 bits per channel, which means combinations of 256 levels for each of the red, green, and blue components: 256 × 256 × 256 = 16,777,216 colours. Another common use case was low bit depth elements (e.g. 4bpp per element, with multiple palettes) composited into a high colour frame buffer (e.g. in the PlayStation 2).

An abstract, graphic example would be:

width

memory

Changes to the palette affect the whole screen at once and can be used to produce special effects which would be much slower to produce by updating pixels.

This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.

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