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 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:

• The number of entries in the palette: determines the maximum number of colours which can appear on screen simultaneously (a subset of the wider full palette, which is to be understood as the total number of colours that a given system is able to generate or manage, e.g. the full RGB colour palette).

• The width of each entry in the palette: determines the number of colours which the wider full palette can represent.

A common example would be a palette of 256 colours; 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.

An abstract, graphic example would be:

	Image		Image Palette (CLUT)		Full RGB Palette
Properties	Dimensions	Colour depth	Number of entries (logical colours)	Entry width	Number of entries (physical colours)
	8 × 6 pixels	2 bits per pixel →	(→ 2 bits) 4	3 bits →	(→ 3-bit RGB) 8
Size in memory	8 × 6 × 2 = 96 bits		4 × 3 = 12 bits		N/A
Numeric entries	00 00 00 00 00 00 00 00 00 00 00 00 01 01 00 00 00 00 00 01 01 01 01 00 10 10 10 10 10 10 10 10 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11		00 011 01 110 10 001 11 010		000 000 001   010   011   100   101   110   111
Coloured entries	00 00 00 00 00 00 00 00		00 011 01   10   11

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.

See also

• Palette (computing)
• Lookup table
• Truecolor
• Indexed color (Colors and palettes)
• Color depth (Indexed color)

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.