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Color depth, also known asbit depth, is either the number ofbits used toindicate the color of a singlepixel, or the number of bits used for each color component of a single pixel. When referring to a pixel, the concept can be defined asbits per pixel (bpp). When referring to a color component, the concept can be defined asbits per component,bits per channel,bits per color (all three abbreviatedbpc), and alsobits per pixel component,bits per color channel orbits per sample.[1][2][3] Modern standards tend to use bits per component,[1][2][4][5] but historical lower-depth systems used bits per pixel more often.
Color depth is only one aspect of color representation, expressing the precision with which the amount of each primary can be expressed; the other aspect is how broad a range of colors can be expressed (thegamut). The definition of both color precision and gamut is accomplished with a color encoding specification which assigns a digital code value to a location in acolor space.
The number of bits of resolved intensity in a color channel is also known asradiometric resolution, especially in the context ofsatellite images.[6]
With the relatively low color depth, the stored value is typically a number representing the index into a color map orpalette (a form ofvector quantization). The colors available in the palette itself may be fixed by the hardware or modifiable by software. Modifiable palettes are sometimes referred to aspseudocolor palettes.
Old graphics chips, particularly those used inhome computers andvideo game consoles, often have the ability to use a different palette persprites andtiles in order to increase the maximum number of simultaneously displayed colors, while minimizing use of then-expensive memory (and bandwidth). For example, in theZX Spectrum the picture is stored in a two-color format, but these two colors can be separately defined for each rectangular block of 8×8 pixels.
The palette itself has a color depth (number of bits per entry). While the bestVGA systems only offered an 18-bit (262,144 color) palette[7][8][9][10] from which colors could be chosen, all color Macintosh video hardware offered a 24-bit (16 million color) palette. 24-bit palettes are nearly universal on any recent hardware or file format using them.
If instead the color can be directly figured out from the pixel values, it is "direct color". Palettes were rarely used for depths greater than 12 bits per pixel, as the memory consumed by the palette would exceed the necessary memory for direct color on every pixel.
2 colors, often black and white direct color. Sometimes 1 meant black and 0 meant white, the inverse of modern standards. Most of the first graphics displays were of this type, theX Window System was developed for such displays, and this was assumed for a3M computer. In the late 1980s there were professional displays with resolutions up to 300 dpi (the same as a contemporary laser printer) but color proved more popular.
4 colors, usually from a selection of fixed palettes. Gray-scale earlyNeXTstation, color Macintoshes, Atari ST medium resolution.
8 colors, almost always all combinations of full-intensity red, green, and blue. Many early home computers with TV displays, including theZX Spectrum andBBC Micro.
16 colors, usually from a selection of fixed palettes. Used by IBMCGA (at the lowest resolution),EGA, and by the least common denominatorVGA standard at higher resolution. Color Macintoshes, Atari ST low resolution,Commodore 64, andAmstrad CPCs also supported 4-bit color.
32 colors from a programmable palette, used by theOriginal Amiga chipset.
64 colors. Used by theMaster System, Enhanced Graphics Adapter, GIME for TRS-80 Color Computer 3, Pebble Time smartwatch (64 color e-paper display), andParallax Propeller using the reference VGA circuit.
256 colors, usually from a fully-programmable palette: Most early color Unix workstations,Super VGA, colorMacintosh,Atari TT,Amiga AGA chipset,Falcon030,Acorn Archimedes. Both X and Windows provided elaborate systems to try to allow each program to select its own palette, often resulting in incorrect colors in any window other than the one with focus.
Some systems placed a color cube in the palette for a direct-color system (and so all programs would use the same palette). Usually fewer levels of blue were provided than others, since the normal human eye is less sensitive to the blue component than to either red or green (two thirds of the eye's receptors process the longer wavelengths[11]).Popular sizes were:
4,096 colors, usually from a fully-programmable palette (though it was often set to a 16×16×16 color cube). SomeSilicon Graphics systems, ColorNeXTstation systems, andAmiga systems inHAM mode have this color depth.
RGBA4444, a related 16 bpp representation providing the color cube and 16 levels of transparency, is a commontexture format in mobile graphics.
In high-color systems, two bytes (16 bits) are stored for each pixel. Most often, each component (R, G, and B) is assigned 5 bits, plus one unused bit (or used for a mask channel or to switch to indexed color); this allows 32,768 colors to be represented. However, an alternate assignment which reassigns the unused bit to the G channel allows 65,536 colors to be represented, but without transparency.[12] These color depths are sometimes used in small devices with a color display, such as mobile phones, and are sometimes considered sufficient to display photographic images.[13] Among the first hardware to use the standard were theSharpX68000 and IBM'sExtended Graphics Array (XGA).
The term "high color" has recently been used to mean color depths greater than 24 bits.
Almost all of the least expensive LCDs (such as typicaltwisted nematic types) provide 18-bit color (64×64×64 = 262,144 combinations) to achieve faster color transition times, and use eitherdithering orframe rate control to approximate 24-bit-per-pixel true color,[14] or throw away 6 bits of color information entirely. More expensive LCDs (typicallyIPS) can display 24-bit color depth or greater.
24 bits almost always use 8 bits each of R, G, and B (8 bpc). As of 2018, 24-bit color depth is used by virtually every computer and phone display[15] and the vast majority ofimage storage formats. Almost all cases of 32 bits per pixel assigns 24 bits to the color, and the remaining 8 are thealpha channel or unused.
224 gives 16,777,216 color variations. The human eye can discriminate up to ten million colors,[16] and since thegamut of a display is smaller than the range of human vision, this means this should cover that range with more detail than can be perceived. However, displays do not evenly distribute the colors in human perception space, so humans can see the changes between some adjacent colors ascolor banding.Monochromatic images set all three channels to the same value, resulting in only 256 different colors; some software attempts to dither the gray level into the color channels to increase this, although in modern software this is more often used forsubpixel rendering to increase the space resolution on LCD screens where the colors have slightly different positions.
TheDVD-Video andBlu-ray Disc standards support a bit depth of 8 bits per color inYCbCr with 4:2:0chroma subsampling.[17][18] YCbCr can be losslessly converted to RGB.
macOS andMac OS refer to 24-bit color as "millions" of colors.
The termtrue color is sometimes used to mean what this article callsdirect color.[19] It is also often used to refer to all color depths greater than or equal to 24 bits.
Deep color consists of a billion or more colors.[20] 230 is 1,073,741,824. Usually this is 10 bits each of red, green, and blue (10 bpc). If analpha channel of the same size is added then each pixel takes 40 bits.
Some earlier systems placed three 10-bit channels in a 32-bitword, with 2 bits unused (or used as a 4-levelalpha channel); theCineon file format, for example, used this. SomeSGI systems had 10- (or more) bitdigital-to-analog converters for the video signal and could be set up to interpret data stored this way for display.BMP files define this as one of its formats, and it is called "HiColor" byMicrosoft.
Video cards with 10 bits per component started coming to market in the late 1990s. An early example was theRadius ThunderPower card for the Macintosh, which included extensions forQuickDraw andAdobe Photoshop plugins to support editing 30-bit images.[21] Some vendors call their 24-bit color depth withFRC panels 30-bit panels; however, true deep color displays have 10-bit or more color depth without FRC.
TheHDMI 1.3 specification defines a bit depth of 30 bits (as well as 36 and 48 bit depths).[22]In that regard, theNvidia Quadro graphics cards manufactured after 2006 support 30-bit deep color[23] and Pascal or later GeForce and Titan cards when paired with the Studio Driver[24] as do some models of theRadeon HD 5900 series such as the HD 5970.[25][26] TheATI FireGL V7350graphics card supports 40- and 64-bit pixels (30 and 48 bit color depth with an alpha channel).[27]
TheDisplayPort specification also supports color depths greater than 24 bpp in version 1.3 through "VESA Display Stream Compression, which uses a visuallylossless low-latency algorithm based on predictive DPCM and YCoCg-R color space and allows increased resolutions and color depths and reduced power consumption."[28]
AtWinHEC 2008, Microsoft announced that color depths of 30 bits and 48 bits would be supported inWindows 7, along with the wide color gamutscRGB.[29][30]
High Efficiency Video Coding (HEVC or H.265) defines the Main 10 profile, which allows for 8 or 10 bits per sample with 4:2:0chroma subsampling.[2][4][5][31][32] The Main 10 profile was added at the October 2012 HEVC meeting based on proposal JCTVC-K0109 which proposed that a 10-bit profile be added to HEVC for consumer applications.[5] The proposal stated that this was to allow for improved video quality and to support theRec. 2020 color space that will be used byUHDTV.[5] The second version of HEVC has five profiles that allow for a bit depth of 8 bits to 16 bits per sample.[33]
As of 2020, some smartphones have started using 30-bit color depth, such as theOnePlus 8 Pro,Oppo Find X2 & Find X2 Pro,Sony Xperia 1 II,Xiaomi Mi 10 Ultra,Motorola Edge+,ROG Phone 3 andSharp Aquos Zero 2.[citation needed]
Using 12 bits per color channel produces 36 bits, 68,719,476,736 colors. If an alpha channel of the same size is added then there are 48 bits per pixel.
Using 16 bits per color channel produces 48 bits, 281,474,976,710,656 colors. If an alpha channel of the same size is added then there are 64 bits per pixel.
Image editing software such asAdobe Photoshop started using 16 bits per channel fairly early in order to reduce the quantization on intermediate results (i.e. if an operation is divided by 4 and then multiplied by 4, it would lose the bottom 2 bits of 8-bit data, but if 16 bits were used it would lose none of the 8-bit data).
Some systems started using those bits for numbers outside the 0–1 range rather than for increasing the resolution. Numbers greater than 1 were for colors brighter than the display could show, as inhigh-dynamic-range imaging (HDRI). Negative numbers can increase the gamut to cover all possible colors, and for storing the results of filtering operations with negative filter coefficients. ThePixar Image Computer used 12 bits to store numbers in the range [-1.5, 2.5), with 2 bits for the integer portion and 10 for the fraction. TheCineon imaging system used 10-bit professional video displays with the video hardware adjusted so that a value of 95 was black and 685 was white.[34] The amplified signal tended to reduce the lifetime of the CRT.
More bits also encouraged the storage of light as linear values, where the number directly corresponds to the amount of light emitted. Linear levels makes calculation of computer graphics much easier. However, linear color results in disproportionately more samples near white and fewer near black, so the quality of 16-bit linear is about equal to 12-bitsRGB.
Floating point numbers can represent linear light levels spacing the samples semi-logarithmically. Floating point representations also allow for drastically larger dynamic ranges as well as negative values. Most systems first supported 32-bit per channelsingle-precision, which far exceeded the accuracy required for most applications. In 1999,Industrial Light & Magic released theopen standard image file formatOpenEXR which supported 16-bit-per-channelhalf-precision floating-point numbers. At values near 1.0, half precision floating point values have only the precision of an 11-bit integer value, leading some graphics professionals to reject half-precision in situations where the extended dynamic range is not needed.[citation needed]
Virtually all television displays and computer displays form images by varying the strength of just threeprimary colors: red, green, and blue. For example, bright yellow is formed by roughly equal red and green contributions, with no blue contribution.
For storing and manipulating images, alternative ways of expanding the traditional triangle exist: One can convert image coding to use fictitious primaries, that are not physically possible but that have the effect of extending the triangle to enclose a much larger color gamut. An equivalent, simpler change is to allow negative numbers in color channels, so that the represented colors can extend out of the color triangle formed by the primaries. However these only extend the colors that can be represented in the image encoding; neither trick extends thegamut of colors that can actually be rendered on a display device.
Supplementary colors can widen thecolor gamut of a display, since it is no longer limited to the interior of a triangle formed by threeprimaries at its corners, e.g. theCIE 1931 color space. Recent technologies such asTexas Instruments'sBrilliantColor augment the typical red, green, and blue channels with up to three other primaries: cyan, magenta, and yellow.[35] Cyan would be indicated by negative values in the red channel, magenta by negative values in the green channel, and yellow by negative values in the blue channel, validating the use of otherwise fictitious negative numbers in the color channels.
Mitsubishi andSamsung (among others) useBrilliantColor in some of their TV sets to extend the range of displayable colors.[citation needed]TheSharp Aquos line of televisions has introducedQuattron technology, which augments the usual RGB pixel components with a yellow subpixel. However, formats and media that allow or make use of the extended colorgamut are at present extremely rare.[citation needed]
Because humans are overwhelminglytrichromats ordichromats[b] one might suppose that adding a fourth "primary" color could provide no practical benefit. However humans can see a broaderrange of colors than a mixture of three colored lights can display. The deficit of colors is particularly noticeable in saturated shades of bluish green (shown as the left upper grey part of the horseshoe in the diagram) of RGB displays: Most humans can see more vivid blue-greens than any color video screen can display.
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