Colour look-up table

From Wikipedia, the free encyclopedia
  (Redirected from Colour Look Up Table)
Jump to: navigation, search

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[edit]

External References[edit]

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.