A colour look-up table 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

Image processing

In electrical engineering and computer science, image processing is any form of signal processing for which the input is an image, such as a photograph or video frame; the output of image processing may be either an image or, a set of characteristics or parameters related to the image...

 application. The hardware colour look-up table will convert the logical colour (pseudo-colour) numbers stored in each pixel

Pixel

In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....

 of video memory

Video memory

Video memory is a term generally used in computers to describe some form of writable memory, usually RAM, dedicated to the purpose of holding the information necessary for a graphics card to drive a display device...

 into physical colours, normally represented as RGB

RGB color model

The RGB color model is an additive color model in which red, green, and blue light is added together in various ways to reproduce a broad array of colors...

 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

Cathode ray tube

The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...

 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

8-bit

The first widely adopted 8-bit microprocessor was the Intel 8080, being used in many hobbyist computers of the late 1970s and early 1980s, often running the CP/M operating system. The Zilog Z80 and the Motorola 6800 were also used in similar computers...

 pixel value. The 8 bits is known as colour depth

Color depth

In computer graphics, color depth or bit depth is the number of bits used to represent the color of a single pixel in a bitmapped image or video frame buffer. This concept is also known as bits per pixel , particularly when specified along with the number of bits used...

, 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		EWLINE 000 000 001   010   011   100   101   110   111
Coloured entries	class="wikitable">
00	00	00	00	00	00	00	00

class="wikitable"> 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)
  Palette (computing)
  In computer graphics, a palette is either a given, finite set of colors for the management of digital images , or a small on-screen graphical element for choosing from a limited set of choices, not necessarily colors .Depending on the context In computer graphics, a palette is either a given,...
  
  
• Lookup table
  Lookup table
  In computer science, a lookup table is a data structure, usually an array or associative array, often used to replace a runtime computation with a simpler array indexing operation. The savings in terms of processing time can be significant, since retrieving a value from memory is often faster than...
  
  
• Truecolor
• Indexed color (Colors and palettes)
• Color depth (Indexed color)