Palette (computing): Difference between revisions

{{about|color lookup tables|color schemes used in design software|color scheme|the UI element also known as utility window|Palette window}}

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

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| image1 = RGB 24bits palette sample image.jpg

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| caption1 = Sample image

| image2 = Sample Image RGB Cube.gif

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

In [[computer graphics]], a '''palette''', also called '''color lookup table''' (CLUT), is athe set of [[color]]s available incolors afrom digitalwhich imagingan scenario,image suchcan asbe amade. display orIn asome filesystems, format.<ref>[https://docs.gimp.org/2.10/en/gimp-palette-dialog.html docs gimp 2.10:the palette-dialog]</ref> Usingis afixed smallby colorthe palettehardware isdesign, aand techniquein usedothers toit reduceis memorydynamic, usagetypically inimplemented computervia imaginga applications.'''color Alookup palettetable''' is basically(CLUT), a correspondence table definingin awhich limitedselected subsetcolors offrom a certain [[color space]]'s color reproduction range; colorsare inassigned thean paletteindex, areby thenwhich they can be referenced. usingBy referencing the colors via an index, which takes less information than the one needed to describe the actual colors in saidthe colorspacecolor space, this technique aims to reduce data usage, including processing, transfer bandwidth, RAM usage, and storage. Images in what'swhich calledcolors anare indicated by references to a CLUT are called [[indexed- color]] imageimages.

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]]. ThisUsing meansthis thattechnique, 8 bits per pixel are used to describe the [[luminance]] level in each of the [[RGB channel]]s, therefore 24 bits offully storagedescribe (inthe eithercolor RAMof oreach inpixel. persistent storage)The arefull necessarysystem topalette fullyfor describesuch thehardware colortherefore ofhas each2²⁴ pixelcolors. 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. In(often doingusing so,adaptive eachmethods). Each possible color is assigned an index., Awhich allows each color to be referenced using less information than needed to fully describe the color. commonAn example is the 256-color palette commonly used in the [[GIF]] file format, in which 256 colors are selected from the whole 24 bpp color space, to be used to represent an image (oftenare usingselected adaptivefrom methods)the whole 24 bit color space, each being is assigned an 8 bit index. In thisThis way, while the system can arbitrarilypotentially 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.

[[Microsoft Windows]] applications manage the palette of 4-bit or 8-bit indexed color display devices through specialized functions of the [[Win32 API]]. The applicability of palettes in [[Highcolor]] and [[24-bit color|Truecolor]] display modes becomes questionable. These APIs deals with the so-called ''"system palette''" and with many ''"logical palettes''".

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).

When a given application intends to output colorized graphics and/or images, it can set their own ''"logical palette''", that is, its own private selection of colors (up to 256). It is supposed that every graphic element that the application tries to show on screen employs the colors of its logical palette. Every program can manage freely one or more logical palettes without further expected interference (in advance).

Before the output is effectively made, the program must ''realize'' its logical palette: theThe system tries to match then the ''"logical''" colors with ''"physical''" ones. If an intended color is already present intoin the system palette, the system internally maps both the logical andto the system palette indexes (due tobecause they rarely coincide). If the intended color is not present yet, the system applies an internal algorithm to discard the least -used color in the system palette (generally, someone used by another window in the background) and substitutes it with the new color. Due to there arebeing limited room for colors in the system palette, the algorithm also tries also to remap similar colors together, and will always byavoid avoidingcreating redundant colors.

The final result depends on how many applications are workingtrying withto show their colors on screen colorsat the same time. The foreground window is always favored, so background windows at background may behave in different ways: from become corrupted to quickly redraw themselves. When the system palette changes, the system triggers a specific [[Event-driven programming|event]] to inform every application. When received, a window can quickly redraw itself using a single Win32 API function. But this must be doingdone explicitly in the program code; hence the fact that many programs lackfail into managehandle this event, and their windows will become corrupt in this situation.

An application can force the system palette to be loaded with specific colors and (even in a specific order), ''"tricking''" the system by telling it they are color entries intended for ''animation'' (quick color changes of the colors in the physical palette at specific entries). The system cannot assumewill then assume that everythose hardware palette entryentries isno longer are free for theirits palette color managements, and those entries are excluded from itsmanagement algorithm. The final result dependdepends on the skills of the color -forcing program and the behavior of the other programs (thealthough laststhis problem is the exactlysame as in the regular case), and that of the operating system in itself.

* 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]]