In color reproduction, including
computer graphics
Computer graphics deals with generating images with the aid of computers. Today, computer graphics is a core technology in digital photography, film, video games, cell phone and computer displays, and many specialized applications. A great de ...
and
photography
Photography is the art, application, and practice of creating durable images by recording light, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It is emplo ...
, the gamut, or color gamut , is a certain ''complete subset'' of
color
Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are associ ...
s. 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
An output device is any piece of computer hardware equipment which converts information into a human-perceptible form or, historically, into a physical machine-readable form for use with other non-computerized equipment. It can be text, graphics, ...
.
Another sense, less frequently used but still 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.
Introduction
The term ''gamut'' was adopted from the field of music, where in middle age Latin "gamut" meant the entire range of musical notes of which musical melodies are composed;
Shakespeare
William Shakespeare ( 26 April 1564 – 23 April 1616) was an English playwright, poet and actor. He is widely regarded as the greatest writer in the English language and the world's pre-eminent dramatist. He is often called England's nation ...
's use of the term in ''
The Taming of the Shrew
''The Taming of the Shrew'' is a comedy by William Shakespeare, believed to have been written between 1590 and 1592. The play begins with a framing device, often referred to as the induction, in which a mischievous nobleman tricks a drunk ...
'' is sometimes attributed to the author/musician
Thomas Morley. In the 1850s, the term was applied to a range of colors or hue, for example by
Thomas De Quincey, who wrote "
Porphyry, I have heard, runs through as large a gamut of hues as marble."
In
color theory, the gamut of a device or process is that portion of the
color space that can be represented, or reproduced. Generally, the color gamut is specified in the
hue–
saturation plane, as a system can usually produce colors over a wide
intensity range within its color gamut; for a
subtractive color system (such as used in
printing), the range of intensity available in the system is for the most part meaningless without considering system-specific properties (such as the
illumination of the ink).
When certain colors cannot be expressed within a particular color model, those colors are said to be ''out of gamut''.
A device that can reproduce the entire visible color space is an unrealized goal within the
engineering
Engineering is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad range of more speciali ...
of
color displays and printing processes. Modern techniques allow increasingly good approximations, but the complexity of these systems often makes them impractical.
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.
Representation of gamuts
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 percent; the exact coordinates of white are determined by the color of the light source.
Limitations of color representation
Surfaces
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 chemist
Wilhelm Ostwald.
Erwin Schrödinger showed in his 1919 article ' (Theory of Pigments with Highest Luminosity)
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.
Thus two types of "optimal color" spectra are possible: Either the transition goes from zero at both ends of the spectrum to one in the middle, as shown in the image at right, or it goes from one at the ends to zero in the middle. The first type produces colors that are similar to the spectral colors and follow roughly the horseshoe-shaped portion of the
CIE xy chromaticity diagram, but are generally less saturated. The second type produces colors that are similar to (but generally less saturated than) the colors on the straight line in the CIE xy chromaticity diagram, leading to magenta-like colors.
Schrödinger's work was further developed by
David MacAdam and .
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 the ''MacAdam limit''.
On modern computers, it is possible to calculate an optimal color solid with great precision in seconds or minutes. The MacAdam limit, on which the most saturated (or "optimal") colors reside, shows that colors that are near monochromatic colors can only be achieved at very low luminance levels, except for yellows, because a mixture of the wavelengths from the long straight-line portion of the spectral locus between green and red will combine to make a color very close to a monochromatic yellow.
Light sources
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 pure
monochromatic (single
wavelength) light. The best technological source of monochromatic light is the
laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The ...
, which can be rather expensive and impractical for many systems. However, as
optoelectronic technology matures, single-longitudinal-mode diode lasers are becoming less expensive, and many applications can already profit from this; such as Raman 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, and fiber optic communications.
Systems that 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:
* A
laser video projector uses three 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 projector. Lasers can also be used as a light source for a DLP projector. More than three lasers can be combined to increase the gamut range, a technique sometimes used in
holography.
*
Digital Light Processing or DLP technology is a trademarked technology from Texas Instruments. The DLP chip contains a rectangular array of up to 2 million hinge-mounted microscopic mirrors. Each of the micromirrors measures less than one-fifth the width of a human hair. A DLP chip's micromirror tilts either toward the light source in a DLP projection system (ON) or away from it (OFF). This creates a light or dark pixel on the projection surface. Current DLP projectors use a quickly rotating wheel with transparent colored "pie slices" to present each color frame successively. One rotation shows the complete image.
*
Photographic film can reproduce a larger color gamut than typical television, computer, or
home video systems.
*
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.
*
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 cold-cathode fluorescent bulbs (
CCFLs) for backlights. LCD Screens with certain
LED or wide-gamut CCFL backlights yield a more comprehensive gamut than CRTs. However, some LCD technologies vary the color presented by viewing angle.
In Plane Switching
IPS (in-plane switching) is a screen technology for liquid-crystal displays (LCDs). In IPS, a layer of liquid crystals is sandwiched between two glass surfaces. The liquid crystal molecules are aligned parallel to those surfaces in predetermined ...
or
Patterned vertical alignment screens have a wider span of colors than
Twisted Nematic
The twisted nematic effect (''TN-effect'') was a main technology breakthrough that made LCDs practical. Unlike earlier displays, TN-cells did not require a current to flow for operation and used low operating voltages suitable for use with batteri ...
.
*
Television
Television, sometimes shortened to TV, is a telecommunication medium for transmitting moving images and sound. The term can refer to a television set, or the medium of television transmission. Television is a mass medium for advertising, ...
normally uses a CRT, LCD, LED or
plasma display
A plasma display panel (PDP) is a type of flat panel display that uses small cells containing plasma: ionized gas that responds to electric fields. Plasma televisions were the first large (over 32 inches diagonal) flat panel displays to be rele ...
, but does not take full advantage of its color display properties, due to the limitations of
broadcasting. The common color profile for TV is based on ITU standard
Rec. 601
ITU-R Recommendation BT.601, more commonly known by the abbreviations Rec. 601 or BT.601 (or its former name CCIR 601) is a standard originally issued in 1982 by the CCIR (an organization, which has since been renamed as the Internatio ...
.
HDTV is less restrictive and uses a slightly improved color profile based on ITU standard
Rec. 709. Still somewhat less than, for example, computer displays using the same display technology. This is due to the use of a limited subset of RGB in broadcasting (values from 16-235), versus full RGB in computer displays, where all bits from 0 to 255 are used.
*
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 cannot 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). 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 Hexachrome is a discontinued six-color printing process designed by Pantone Inc. In addition to custom CMYK inks, Hexachrome uses orange and green inks to expand the color gamut for better color reproduction. It is therefore also known as a CMYKOG p ...
) or light cyan and light magenta (see
CcMmYK color model
CcMmYK, sometimes referred to as CMYKLcLm or CMYKcm, is a six-color printing process used in some inkjet printers optimized for photo printing. It complements the more common four color CMYK process, which stands for Cyan, Magenta, Yellow and K ...
).
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.
Wide color gamut
The
Ultra HD Forum
Ultra HD Forum is an organization whose goal is to help solve the real world hurdles in deploying Ultra HD video and thus to help promote UHD deployment. The Ultra HD Forum will help navigate amongst the standards related to high dynamic range (H ...
defines a wide color gamut (WCG) as a color gamut wider than that of BT.709 (
Rec. 709).
Color spaces with WCGs include:
*
Rec. 2020
ITU-R Recommendation BT.2020, more commonly known by the abbreviations Rec. 2020 or BT.2020, defines various aspects of ultra-high-definition television (UHDTV) with standard dynamic range (SDR) and wide color gamut (WCG), including picture ...
– ITU-R Recommendation for
UHDTV
*
Rec. 2100
ITU-R Recommendation BT.2100, more commonly known by the abbreviations Rec. 2100 or BT.2100, introduced high-dynamic-range television (HDR-TV) by recommending the use of the perceptual quantizer (PQ) or hybrid log–gamma (HLG) transfer fun ...
– ITU-R Recommendation for
HDR-TV (same
chromaticity of
color primaries and
white point as
Rec. 2020
ITU-R Recommendation BT.2020, more commonly known by the abbreviations Rec. 2020 or BT.2020, defines various aspects of ultra-high-definition television (UHDTV) with standard dynamic range (SDR) and wide color gamut (WCG), including picture ...
)
*
DCI-P3
*
Adobe RGB color space
Extended-gamut printing
The print gamut achieved by using cyan, 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.
References
External links
Using the Chromaticity Diagram for Color Gamut Evaluationby Bruce Lindbloom.
book by Jan Morovic.
Quantifying Color Gamutby William D. Kappele
* Stanford University CS 17
explaining color gamut mapping.
{{color topics
Color space
Color