Gamut

In color reproduction, including computer graphics and photography, the gamut, or color gamut (pronounced /ˈgæmət/), is a certain complete subset of colors. The most common usage refers to the subset of colors which can be accurately represented in a given circumstance, such as within a given color space or by a certain output device. Another sense, less frequently used but not less correct, refers to the complete set of colors found within an image at a given time. In this context, digitizing a photograph, converting a digitized image to a different color space, or outputting it to a given medium using a certain output device generally alters its gamut, in the sense that some of the colors in the original are lost in the process.

In color theory, the gamut of a device or process is that portion of the visible color space that can be represented, detected, or reproduced. Generally, the color gamut is specified in the hue-saturation plane, as many systems can produce colors with a wide range intensity within their color gamut; in addition, for subtractive color systems, such as printing, the range of intensity available in the system is for the most part meaningless outside the context of its illumination.

When certain colors cannot be displayed within a particular color model, those colors are said to be out of gamut. For example, pure red which is contained in the RGB color model gamut is out of gamut in the CMYK model.

A device which is able to reproduce the entire visible color space is somewhat of a holy grail in the engineering of color displays and printing processes. While modern techniques allow increasingly good approximations, the complexity of these systems often makes them impractical. What is "good enough" is dictated by the limitations of human perception.

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 space to the printer's CMYK color space. During this process, the colors from the RGB which are out of gamut must be somehow converted to approximate values within the CMYK space gamut. Simply trimming only the colors which are out of gamut to the closest colors in the destination space would burn 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 which are out of gamut in the target color space as soon as possible during processing is critical for the quality of the final product.

Owners of digital capture devices (i.e. digital cameras) should note that the generic traditional photography and film term for the complete range of colors which can be captured on a particular medium is dynamic range. This term traditionally refers to the maximum range of brightness (or "lightness", as in the HLS color space) which can be captured on a particular medium. That's because traditional film is more "honest" than its digital counterpart, in that it performs more or less the same in capturing all three primary colors (the digital capture devices generally try to "cheat" on the user by storing as little color information as needed in order to satisfy its given sale characteristics, while also saving on sensors and storage space; this "cheating" is possible due to the characteristics of the human retina). The practical result is that differentiating between films means differentiating between their capability to record different brightness ranges, as opposed to fully defined gamuts, due to their uniformity in recording all colors in the spectrum at a given brightness. It is also worth mentioning that specific films do differentiate between colors, in that they induce a given tint or highlight given colors, thus acting as photographic filters in effect. This behavior is expected (and often desired) by experienced traditional photographers who choose one film over another based on these characteristics.

Representation of gamuts

Gamuts are commonly represented as areas in the CIE 1931 chromaticity diagram as shown at right, with the curved edge representing the monochromatic colors. Gamut areas typically have triangular shapes because most color reproduction is done with three primaries.

However, the accessible gamut depends on the brightness; a full gamut must therefore be represented in 3D space, as below:

The pictures at left show the gamuts of RGB color space (top), such as on computer monitors, and of reflective colors in nature (bottom). The cone drawn in grey corresponds roughly to the CIE diagram at right, with the added dimension of brightness.

The axes in these diagrams are the responses of the short-wavelength, middle-wavelength, and long-wavelength cones in the human eye. The other letters indicate black, red, green, blue, cyan, magenta, yellow, and white colors. (Note: These pictures are not exactly to scale.)

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 the phosphors in the computer monitor, and on the ratio between the maximum luminosities of the three phosphors (i.e., the color balance).

The gamut of the CMYK color space is, ideally, approximately the same as that for RGB, with slightly different apexes, depending on both the exact properties of the dyes and the light source. 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 is smaller and has rounded corners.

The gamut of reflective colors in nature has a similar, though more rounded, shape. An object that reflects only a narrow band of wavelengths will have a color close to the edge of the CIE diagram, but it will have a very low luminosity at the same time. At higher luminosities, the accessible area in the CIE diagram becomes smaller and smaller, up to a single point of white, where all wavelengths are reflected exactly 100 per cent. The exact coordinates of white are of course determined by the color of the light source.

Limitations of color representation

The color gamut of most systems can be understood as a result of difficulties producing pure monochromatic (single wavelength) light. The best technological source of (nearly) monochromatic light is the laser, which is expensive and impractical for many systems (as laser technology improves and becomes more inexpensive, this may no longer be the case). Other than lasers, most systems represent highly saturated colors with a more or less crude approximation, which includes light with a range of wavelengths besides the desired color. This may be more pronounced for some hues than others.

Systems which use additive color processes usually have a color gamut which is roughly a convex polygon in the hue-saturation plane. The vertices of the polygon are the most saturated colors the system can produce. In subtractive color systems, the color gamut is more often an irregular region.

Comparison of various systems

Following is a list of representative color systems more or less ordered from large to small color gamut:

• Laser video projector uses 3 lasers to produce the broadest gamut available in practical display equipment today, derived from the fact that lasers produce truly monochromatic primaries. The systems work either by scanning the entire picture a dot at a time and modulating the laser directly at high frequency, much like the electron beams in a CRT, or by optically spreading and then modulating the laser and scanning a line at a time, the line itself being modulated in much the same way as in a DLP.
• Photographic film is one of the best systems available for detecting and reproducing color. Movie goers are familiar with the difference in color quality between the film projections seen in theaters and the home video versions. This is because the color gamut of film far exceeds that of television.
• Laser light shows use lasers to produce very nearly monochromatic light, allowing colors far more saturated than those produced by other systems. However, mixing hues to produce less saturated colors is difficult. In addition, such systems are complex, expensive, and ill-suited to general video display.
• CRT and similar video displays have a roughly triangular color gamut which covers a significant portion of the visible color space. In CRTs, the limitations are due to the phosphors in the screen which produce red, green, and blue light. Besides the limitations of the device itself, for displaying realistic images, such displays rely on the quality of color sensors, such as those in digital cameras and scanners. Sony has recently introduced a four-color RGBE (RGB plus "emerald") color sensor system which may eventually lead to high end video displays with an even larger color gamut.
• Liquid crystal display (LCD) screens filter the light emitted by a backlight. The gamut of an LCD screen is therefore limited to the emitted spectrum of the backlight. Typical LCD screens use fluorescent bulbs for backlights. LCD Screens with certain LED backlights yield a more comprehensive gamut than CRTs.
• Television uses a CRT display (usually), but does not take full advantage of its color display properties, due to the limitations of broadcasting. HDTV is far better, but still somewhat less than, for example, computer displays using the same display technology.
• Paint mixing, both artistic and for commercial applications, achieves a reasonably large color gamut by starting with a larger palette than the red, green, and blue of CRTs or cyan, magenta, and yellow of printing. Paint may reproduce some highly saturated colors that can not be reproduced well by CRTs (particularly violet), but overall the color gamut is smaller.
• Printing typically uses the CMYK color space (cyan, magenta, yellow, and black). A very few printing processes do not include black; however, those processes (with the exception of dye-sublimation printers) are poor at representing low saturation, low intensity colors. Efforts have been made to expand the gamut of the printing process by adding inks of non-primary colors; these are typically orange and green (see Hexachrome) or light cyan and light magenta. Spot color inks of a very specific color are also sometimes used.
• A monochrome display's color gamut is a one-dimensional curve in color space.

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