Incolor reproduction andcolorimetry, agamut, orcolor gamut/ˈɡæmət/, is aconvex set containing thecolors that can be accurately represented, i.e. reproduced by anoutput device (e.g. printer or display) or measured by aninput device (e.g. camera orvisual system). Devices with a larger gamut can represent more colors. Similarly, gamut may also refer to the colors within a definedcolor space, which is not linked to a specific device. A trichromatic gamut is often visualized as acolor triangle. A less common usage defines gamut as the subset of colors contained within an image, scene or video.
The termgamut was adopted from the field of music, where the medieval Latin expression "gamma ut" meant the lowest tone of the G scale and, in time, came to imply the entire range of musical notes of which musical melodies are composed.Shakespeare's use of the term inThe Taming of the Shrew is sometimes attributed to the author / musicianThomas Morley.[1] In the 1850s, the term was applied to a range of colors or hue, for example byThomas de Quincey, who wrote "Porphyry, I have heard, runs through as large a gamut of hues as marble."[2]
The gamut of a device or process is that portion of thecolor space that can be represented, or reproduced. Generally, the color gamut is specified in thehue–saturationplane, as a system can usually produce colors over a wideintensity range within its color gamut.Device gamuts must usereal primaries (those that can be represented by a physicalspectral power distribution) and therefore are alwaysincomplete (smaller than the human visible gamut). No gamut defined by a finite number ofprimaries can represent the entire human visible gamut. Three primaries are necessary for representing an approximation of the human visible gamut. More primaries can be used to increase the size of the gamut. For example, while painting withred, yellow and blue pigments is sufficient for modeling color vision, adding further pigments (e.g. orange or green) can increase the size of the gamut, allowing the reproduction of more saturated colors.
While processing a digital image, the most convenient color model used is the RGB model. Printing the image requires transforming the image from the original RGB color model to the printer'sCMYK color model. During this process, the colors from the RGB model which are out of gamut must be somehow converted to approximate values within the CMYK model. Simply trimming only the colors which are out of gamut to the closest colors in the destination space wouldburn the image. There are several algorithms approximating this transformation, but none of them can be truly perfect, since those colors are simply out of the target device's capabilities. This is why identifying the colors in an image that are out of gamut in the target color space as soon as possible during processing is critical for the quality of the final product. It is also important to remember that there are colors inside the CMYK gamut that are outside the most commonly used RGB color spaces, such assRGB andAdobe RGB.
Color management is the process of ensuring consistent and accurate colors across devices with different gamuts. Color management handles the transformations between color gamuts and canonical color spaces to ensure that colors are represented equally on different devices. A device's gamut is defined by a color profile, usually theICC profile, which relates the gamut to a standardizedcolor space and allows for calibration of the device. Transforming from one gamut to a smaller gamut loses information asout-of-gamut colors are projected on to the smaller gamut and transforming back to the larger gamut does not regain this lost information.
Colorimetry is the measurement of color, generally in a way that mimics humancolor perception.[3] Input devices such as digital cameras or scanners are made to mimic trichromatic human color perception and are based on three sensors elements with different spectral sensitivities, ideally aligned approximately with thespectral sensitivities of humanphotopsins. In this sense, they have a similar gamut to the human visual system. However, most of these devices violate the Luther condition and are not intended to be truly colorimetric, with the exception oftristimulus colorimeters. Higher-dimension input devices, such asmultispectral imagers,hyperspectral imagers orspectrometers, capture color at a much larger gamut, dimensionally, than the human visible gamut. To be perceived by humans, the images must first be down-dimensionalized and treated withfalse color.
The extent of color that can be detected by the average human, approximated by thestandard observer, is thevisible (or visual) gamut. The chromaticities present in the visible gamut are usually visualized in theCIE 1931 chromaticity diagram, where thespectral locus (curved edge) represents themonochromatic (single-wavelength) orspectral colors. As current displays have a smaller gamut than the visible gamut, the colors that are out-of-gamut are reproduced as colors inside the display's gamut. Device gamuts are generally depicted in reference to the visible gamut. The standard observer represents a typical human, but colorblindness leads to a reduced visible gamut.
 | Gamuts are commonly represented as areas within theCIE 1931 chromaticity diagram. This ignores the intensity/brightness dimension of the gamut, which is not depicted. Gamuts defined by three primaries are visualized ascolor triangles. |
The sRGB gamut projected into theCIE xyY color space.x andy are the horizontal axes and represent chromaticity.Y is the vertical axis and represents (linear)luminance. | Gamuts may also be represented in 3D space as acolor solid, which includes a visualization of thedynamic range of the device. |
sRGB gamut inCIE 1931 XYZ color space Optimal color solid in CIE 1931 XYZ color space, withD65 white point | The pictures show the gamuts of the sRGB color space (left), which is approximately the one that mostcomputer monitors andTVs have; and the theoretical gamut of surfaces (optimal color solid, or Rösch-MacAdam color solid) (underD65 illumination) (right). The left diagram shows that the shape of the RGB gamut is a triangle between red, green, and blue at lower luminosities; a triangle between cyan, magenta, and yellow at higher luminosities, and a single white point at maximum luminosity. The exact positions of the apexes depends on the emission spectra of thephosphors in the display, and on the ratio between the maximum luminosities of the three phosphors (i.e., the color balance orwhite point). |
Optimal colors are the most chromatic colors that surfaces can have*. Thecolor solid bounded by the set of all optimal colors is called the optimal color solid orRösch–MacAdam color solid.[4] For now, we are unable to produce objects with such colors, at least not without recurring to more complex physical phenomena.
*(with classical reflection. Phenomena like fluorescence or structural color may produce objects whose color lies outside the optimal color solid)
Thereflectance spectrum of a color is the amount of light of each wavelength that it reflects, in proportion to a given maximum, which has the value of 1 (100%). If the reflectance spectrum of a color is 0 (0%) or 1 (100%) across the entire visible spectrum, and it has no more than two transitions between 0 and 1, or 1 and 0, then it is an optimal color. With the current state of technology, we are unable to produce any material or pigment with these properties.[5]
Thus four types of "optimal color" spectra are possible:
The first type produces colors that are similar to thespectral colors and follow roughly the horseshoe-shaped portion of theCIE xy chromaticity diagram (thespectral locus), but are, in surfaces, morechromatic, although lessspectrally pure. The second type produces colors that are similar to (but, in surfaces, more chromatic and less spectrally pure than) the colors on the straight line in the CIE xy chromaticity diagram (theline of purples), leading tomagenta or purple-like colors.
In optimal color solids, the colors of the visible spectrum are theoretically black, because their reflectance spectrum is 1 (100%) in only one wavelength, and 0 in all of the other infinite visible wavelengths that there are, meaning that they have a lightness of 0 with respect to white, and will also have 0 chroma, but, of course, 100% of spectral purity. In short: In optimal color solids, spectral colors are equivalent to black (0 lightness, 0 chroma), but have full spectral purity (they are located in the horseshoe-shaped spectral locus of the chromaticiy diagram).[6]
In linear color spaces that contain all colors visible by humans, such asLMS orCIE 1931 XYZ, the set ofhalf-lines that start at the origin (black, (0, 0, 0)) and pass through all the points that represent the colors of the visible spectrum, and the portion of a plane that passes through the violet half-line and the red half-line (both ends of the visible spectrum), generate the "spectrum cone". The black point (coordinates (0, 0, 0)) of the optimal color solid (and only the black point) is tangent to the "spectrum cone", and the white point ((1, 1, 1)) (only the white point) is tangent to the "inverted spectrum cone", with the "inverted spectrum cone" beingsymmetrical to the "spectrum cone" with respect to the middle gray point ((0.5, 0.5, 0.5)). This means that, in linear color spaces, the optimal color solid is centrally symmetric.[6]
In most color spaces, the surface of the optimal color solid is smooth, except for two points (black and white); and two sharp edges: the "warm" edge, which goes from black, to red, to orange, to yellow, to white; and the "cold" edge, which goes from black, to blue, tocyan, to white. This is due to the following: If the portion of the reflectance spectrum of a color is spectral red (which is located at one end of the spectrum), it will be seen as black. If the size of the portion of total or reflectance is increased, now covering from the red end of the spectrum to the yellow wavelengths, it will be seen as red. If the portion is expanded even more, covering the green wavelengths, it will be seen as orange or yellow. If it is expanded even more, it will cover more wavelengths than the yellowsemichrome does, approaching white, until it is reached when the full spectrum is reflected. The described process is called "cumulation". Cumulation can be started at either end of the visible spectrum (we just described cumulation starting from the red end of the spectrum, generating the "warm" sharp edge), cumulation starting at the violet end of the spectrum will generate the "cold" sharp edge.[6]
Each hue has a maximum chroma point, semichrome, or full color; objects cannot have a color of that hue with a higher chroma. They are the most chromatic, vibrant colors that objects can have. They were calledsemichromes orfull colors by the German chemist and philosopherWilhelm Ostwald in the early 20th century.[6][7]
If B is the complementary wavelength of wavelength A, then the straight line that connects A and B passes through the achromatic axis in a linear color space, such as LMS or CIE 1931 XYZ. If the reflectance spectrum of a color is 1 (100%) for all the wavelengths between A and B, and 0 for all the wavelengths of the otherhalf of the color space, then that color is a maximum chroma color, semichrome, or full color (this is the explanation to why they were calledsemichromes). Thus, maximum chroma colors are a type of optimal color.[6][7]
As explained, full colors are far from being monochromatic. If the spectral purity of a maximum chroma color is increased, itschroma decreases, because it will approach the visible spectrum, ergo, it will approach black.[6]
In perceptually uniform color spaces, the lightness of the full colors varies from around 30% in the violetish blue hues, to around 90% in theyellowish hues. The chroma of each maximum chroma point also varies depending on the hue; in optimal color solids plotted in perceptually uniform color spaces, semichromes like red, green, blue, violet, andmagenta have a high chroma, while semichromes like yellow, orange, andcyan have a slightly lower chroma.
Incolor spheres and theHSL color space, the maximum chroma colors are located around the equator at the periphery of the color sphere. This makes color solids with a spherical shape inherently non-perceptually uniform, since they imply that all full colors have alightness of 50%, when, as humans perceive them, there are full colors with a lightness from around 30% to around 90%. A perceptually uniform color solid has an irregular shape.[8][9]
In the beginning of the 20th century, industrial demands for a controllable way to describe colors and the new possibility to measure light spectra initiated intense research on mathematical descriptions of colors.
The idea of optimal colors was introduced by the Baltic German chemistWilhelm Ostwald.Erwin Schrödinger showed in his 1919 articleTheorie der Pigmente von größter Leuchtkraft (Theory of Pigments with Highest Luminosity)[5] that the most-saturated colors that can be created with a given total reflectivity are generated by surfaces having either zero or full reflectance at any given wavelength, and the reflectivity spectrum must have at most two transitions between zero and full.
Schrödinger's work was further developed byDavid MacAdam andSiegfried Rösch [Wikidata].[10] MacAdam was the first person to calculate precise coordinates of selected points on the boundary of the optimal color solid in the CIE 1931 color space for lightness levels from Y = 10 to 95 in steps of 10 units. This enabled him to draw the optimal color solid at an acceptable degree of precision. Because of his achievement, the boundary of the optimal color solid is called theMacAdam limit (1935).
On modern computers, it is possible to calculate an optimal color solid with great precision in seconds. Usually, only the MacAdam limits (the optimal colors, the boundary of the Optimal color solid) are computed, because all the other (non-optimal) possible surface colors exist inside the boundary.
Light sources used as primaries in an additive color reproduction system need to be bright, so they are generally not close to monochromatic. That is, the color gamut of most variable-color light sources can be understood as a result of difficulties producing puremonochromatic (singlewavelength) light. The best technological source of monochromatic light is thelaser, which can be rather expensive and impractical for many systems. However, asoptoelectronic technology matures, single-longitudinal-mode diode lasers are becoming less expensive, and many applications can already profit from this; such asRaman spectroscopy,holography,biomedical research,fluorescence,reprographics,interferometry,semiconductor inspection, remote detection,optical data storage, image recording,spectral analysis,printing, point-to-point free-space communications, andfiber optic communications.[11][12][13][14]
Systems that use additive color processes usually have a color gamut which is roughly aconvex polygon (or a slightly concave shape) in aperceptually uniformhue-chroma plane (not to be confused with the chromaticity diagram). The vertices of the polygon are the most chromatic colors that the system can produce.
In subtractive color systems, the color gamut is more often an irregular, rounded region.
The gamut of a CMYK color space is, ideally, the same as that for an RGB one. In practice, due to the way raster-printed colors interact with each other and the paper and due to their non-ideal absorption spectra, the gamut has rounded corners.
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Following is a list of representative color systems more-or-less ordered from large to small color gamut:
TheUltra HD Forum defines a wide color gamut (WCG) as a color gamut wider than that of BT.709 (Rec. 709).[19]Color spaces with WCGs include:
The print gamut achieved by usingcyan, magenta, yellow, and black inks is sometimes a limitation, for example when printing colors of corporate logos. Therefore, some methods of color printing use additional ink colors to achieve a larger gamut. For example, some use green, orange, and violet inks to increase the achievable saturation of hues near those. These method are variously called heptatone color printing, extended gamut printing, and 7-color printing, etc.[22][23]
gamut-of-hues 0-1856.
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