Palette (computing): Difference between revisions

{{short description|In computer graphics, a finite set of available colors}}

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| caption2 = The palette used in the image, shown rotating about the [[RGB color space]].

In [[computer graphics]], a '''palette''' is the set of available colors from which an image can be made. In some systems, alsothe calledpalette is fixed by the hardware design, and in others it is dynamic, typically implemented via a '''color lookup table''' (CLUT), is a correspondence table in which selected colors from a certain [[color space]]'s color reproduction range are assigned an index, by which they can be referenced. By referencing the colors via an index, which takes less information than the one needed to describe the actual colors in saidthe color space, this technique aims to reduce data usage, be it asincluding processing payload, transfer bandwidth, RAM usage, or persistentand storage. Images in which colors are indicated by references to a CLUT are called [[indexed color]] images.

As of 2019, the most common image colorspace in computing[[graphics card]]s is the [[RGB color model]] with 8 bits per pixel [[color depth]]. Using this technique, 8 bits per pixel are used to describe the [[luminance]] level in each of the [[RGB channel]]s, therefore 24 bits of storage are necessary to fully describe the color of each pixel. The full system palette for such hardware therefore has 2²⁴ colors. The objective of the usage of smaller palettes via CLUTs is to lower thisthe storagenumber requirementof bits per pixel by reducing the set of possible colors that are to be handled at once (often using adaptive methods),. assigning eachEach possible color is assigned an index, which allows each color to be referenced using less information than needed to fully describe the color. A commonAn example is the 256-color palette commonly used in the [[GIF]] file format, in which 256 colors to be used to represent an image are selected from the whole 24 bppbit color space, each being assigned an 8 bit index. This way, while the system can potentially reproduce any color in the RGB color space (as long as the 256 color restriction allows), the storage requirement per pixel is lowered from 24 to 8 bits per pixel.

[[File:Palette sizes.ogv|right|thumb|300px|An adaptive color palette expanding from 2 colors to 256 colors, demonstrating how the image changes (click to see animation).]]

This is done by selecting colors in such way that the master palette comprises a full [[RGB color space]] "in miniature", limiting the possible levels that the red, green, and blue components may have. This kind of arrangement is sometimes referred to as a ''uniform palette''.<ref>[http://tex.imm.uran.ru/alchemy.pdf Handmade Software's ''Image Alchemy'' User's Manual.]</ref> The normal [[human eye]] has sensibility to the three primary colors in different degrees: the more to the green, the less to the blue. So RGB arrangements can take advantage of this by assigning more levels for the green component and lessfewer to the blue.

When using [[indexed color]] techniques, real life images are represented with better fidelity to the [[24-bit color|truecolor]] original one by using [[List of software palettes#Adaptive palettes|adaptive palette]]s (sometimes spelledtermed ''adaptative palettes''), in which the colors are selected or [[color quantization|quantized]] through some algorithm directly from the original image (by picking the most frequent colors). This way, and with further [[dithering]], the indexed color image can nearly match the original.

Although related (due to they are used for the same purposes), [[Image mask|image bit masks]] and [[alpha channel]]s are techniques which do not involve the use of palettes nor transparent color at all, but off-image added extra binary data layers.

The "system palette" is a copy in [[Random-access memory|RAM]] of the color display's hardware registers, primarily a physical palette, and it is a unique, shared common resource of the system. At boot, it is loaded with the [[List of software palettes#Microsoft Windows and IBM OS/2 default 16-color palettecolor_palette|default system palette]] (mainly a "master palette" which works well enough with most programs).

* Julio Sanchez and Maria P. Canton (2003). ''[https://books.google.com/books?id=gZ5SDTpMR-MC&pg=PA756&dq=palette+intitle:%22The+PC+Graphics+Handbook%22&lrpg=&as_brr=0&ei=uaPHR7aKHqbutAOohely&sig=4UrkgNS_n3TkvwC1DQ5x91wzcv4PA756 The PC Graphics Handbook.]'' CRC Press. {{ISBN|0-8493-1678-2}}.

[[Category:ComputingComputer output devices]]