 | This article needs additional citations for verification .(December 2014) |
An RGB color space is one of many specific additive colorimetric color spaces based on the RGB color model. [1] [2]
RGB color spaces are commonly found describing the physical output of display devices such as television screens and computer monitors, but some RGB color spaces use imaginary primaries and can not be displayed directly.
The normal human eye contains three types of color-sensitive cone cells. Each cell is responsive to light of either long, medium, or short wavelengths, which we generally categorize as red, green, and blue. Taken together, the responses of these cone cells are called the Tristimulus values, and the combination of their responses is processed into the psychological effect of color vision.
An RGB color space is defined by:
An RGB color space uses primaries based on the RGB color model. Mixing of the three primaries in different proportions then creates the perception of colors other than the primaries. Applying Grassmann's law of light additivity, the range of colors that can be produced are those enclosed within the triangle on the chromaticity diagram defined using the primaries as vertices. The TRC and white point further define the possible colors, creating a volume of encodable colors enclosed within the 3D-triangle. [3]
The primary colors are usually specified in terms of their xyY chromaticity coordinates, though the uʹ,vʹ coordinates from the UCS chromaticity diagram may be used. Both xyY and uʹ,vʹ are derived from the CIE 1931 color space, a device independent space also known as XYZ which covers the full gamut of human-perceptible colors visible to the CIE 2° standard observer.
RGB color spaces are well-suited to describing the electronic display of color, such as computer monitors and color television. These devices often reproduce colours using an array of red, green, and blue phosphors agitated by a cathode ray tube (CRT), or an array of red, green, and blue LCDs lit by a backlight, and are therefore naturally described by an additive color model with RGB primaries.
Early examples of RGB color spaces came with the adoption of the NTSC color television standard in 1953 across North America, followed by PAL and SECAM covering the rest of the world. These early RGB spaces were defined in part by the phosphor used by CRTs in use at the time, and the gamma of the electron beam. While these color spaces reproduced the intended colors using additive red, green, and blue primaries, the broadcast signal itself was encoded from RGB components to a composite signal such as YIQ, and decoded back by the receiver into RGB signals for display.
HDTV uses the BT.709 color space, later repurposed for computer monitors as sRGB. Both use the same color primaries and white point, but different transfer functions, as HDTV is intended for a dark living room while sRGB is intended for a brighter office environment.[ citation needed ] The gamut of these spaces is limited, covering only 35.9% of the CIE 1931 gamut. [4] While this allows the use of a limited bit depth without causing color banding, and therefore reduces transmission bandwidth, it also prevents the encoding of deeply saturated colors that might be available in an alternate color spaces. Some RGB color spaces such as Adobe RGB and ProPhoto intended for the creation, rather than transmission, of images are designed with expanded gamuts to address this issue, however this does not mean the larger space has 'more colors". The numerical quantity of colors is related to bit depth and not the size or shape of the gamut. A large space with a low bit depth can be detrimental to the gamut density and result in high errors[ further explanation needed ].
More recent color spaces such as Rec. 2020 for UHD-TVs define an extremely large gamut covering 63.3% of the CIE 1931 space. [5] This standard is not currently realisable with current LCD technology, and alternative architectures such as quantum dot [6] or OLED [7] based devices are currently in development.
| Color space | Reference Standard | Year | White point | Primaries | Display | Transfer function parameters | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Red | Green | Blue | γ | α | β | δ | βδ | ||||||||
| xʀ | yʀ | xɢ | yɢ | xʙ | yʙ | EOTF | a + 1 | K0/φ = Et | φ | K0 | |||||
| NTSC-J | Based on NTSC(M) | 1987 | D93 | 0.63 | 0.34 | 0.31 | 0.595 | 0.155 | 0.07 | 2.5 | |||||
| NTSC, MUSE | SMPTE RP 145 (C), 170M, 240M | 1987 | D65 | 20/9 | 1.1115 | 0.0057 | 4 | 0.0228 | |||||||
| Apple RGB | (Apple Computer) | 0.625 | 0.28 | 1.8 | |||||||||||
| PAL / SECAM | EBU 3213-E, BT.470/601 (B/G) | 1970 | 0.64 | 0.33 | 0.29 | 0.60 | 0.15 | 0.06 | 2.8 | 14/5 | |||||
| sRGB | IEC 61966-2-1 | 1996, 1999 | 0.30 | 2.2 | 12/5 | 1.055 | 0.0031308 | 12.92 | 0.04045 | ||||||
| scRGB | IEC 61966-2-2 | 2003 | |||||||||||||
| HDTV | ITU-R BT.709 | 1999 | 2.4 | 20/9 | 1.099 | 0.004 | 4.5 | 0.018 | |||||||
| Adobe RGB | (Adobe) | 1998 | 0.21 | 0.71 | 2.2 | 563/256 | |||||||||
| M.A.C. | ITU-R BO.650-2 [8] | 1985 | 0.67 | 0.14 | 0.08 | 2.8 | |||||||||
| NTSC-FCC | ITU-R BT.470/601 (M) | 1953 | C | 2.5 | 11/5 | ||||||||||
| PAL-M | ITU-R BT.470-6 [9] | 1972 | 2.2 | ||||||||||||
| eciRGB | ISO 22028-4 | 2008, 2012 | D50 | 1.8 | 3 | 1.16 | 0.008856 | 9.033 | 0.08 | ||||||
| DCI-P3 | SMPTE RP 431-2 | 2011 | 6300K | 0.68 | 0.32 | 0.265 | 0.69 | 0.15 | 0.06 | 2.6 | 13/5 | ||||
| Display P3 | SMPTE EG 432-1 | 2010 | D65 | ~2.2 | 12/5 | 1.055 | 0.0031308 | 12.92 | 0.04045 | ||||||
| UHDTV | ITU-R BT.2020, BT.2100 | 2012, 2016 | 0.708 | 0.292 | 0.170 | 0.797 | 0.131 | 0.046 | 2.4 | 1.0993 | 0.018054 | 4.5 | 0.081243 | ||
| Wide Gamut | (Adobe) | D50 | 0.7347 | 0.2653 | 0.1152 | 0.8264 | 0.1566 | 0.0177 | 2.2 | 563/256 | |||||
| RIMM | ISO 22028-3 | 2006, 2012 | 0.7347 | 0.2653 | 0.1596 | 0.8404 | 0.0366 | 0.0001 | 2.222 | 20/9 | 1.099 | 0.0018 | 5.5 | 0.099 | |
| ProPhoto (ROMM) | ISO 22028-2 | 2006, 2013 | 0.734699 | 0.265301 | 0.159597 | 0.840403 | 0.036598 | 000105 | 1.8 | 9/5 | 1 | 0.001953125 | 16 | 0.031248 | |
| CIE RGB | CIE 1931 color space | 1931 | E | 0.73474284 | 0.26525716 | 0.27377903 | 0.7174777 | 0.16655563 | 0.00891073 | ||||||
| CIE XYZ | 1 | 0 | 0 | 1 | 0 | 0 | 1 | ||||||||
The CIE 1931 color space standard defines both the CIE RGB space, which is an RGB color space with monochromatic primaries, and the CIE XYZ color space, which is functionally similar to a linear RGB color space, however the primaries are not physically realizable, thus are not described as red, green, and blue.
M.A.C. is not to be confused with MacOS. Here, M.A.C.refers to Multiplexed Analogue Components.
The RGB color model is an additive color model in which the red, green and blue primary colors of light are added together in various ways to reproduce a broad array of colors. The name of the model comes from the initials of the three additive primary colors, red, green, and blue.
A set of primary colors or primary colours consists of colorants or colored lights that can be mixed in varying amounts to produce a gamut of colors. This is the essential method used to create the perception of a broad range of colors in, e.g., electronic displays, color printing, and paintings. Perceptions associated with a given combination of primary colors can be predicted by an appropriate mixing model that reflects the physics of how light interacts with physical media, and ultimately the retina.
Y′UV, also written YUV, is the color model found in the PAL analogue color TV standard. A color is described as a Y′ component (luma) and two chroma components U and V. The prime symbol (') denotes that the luma is calculated from gamma-corrected RGB input and that it is different from true luminance. Today, the term YUV is commonly used in the computer industry to describe colorspaces that are encoded using YCbCr.
The CIELAB color space, also referred to as L*a*b*, is a color space defined by the International Commission on Illumination in 1976. It expresses color as three values: L* for perceptual lightness and a* and b* for the four unique colors of human vision: red, green, blue and yellow. CIELAB was intended as a perceptually uniform space, where a given numerical change corresponds to a similar perceived change in color. While the LAB space is not truly perceptually uniform, it nevertheless is useful in industry for detecting small differences in color.
In color reproduction, including computer graphics and photography, the gamut, or color gamut, is a certain complete subset of colors. The most common usage refers to the subset of colors that can be accurately represented in a given circumstance, such as within a given color space or by a certain output device.
The RGB chromaticity space, two dimensions of the normalized RGB space, is a chromaticity space, a two-dimensional color space in which there is no intensity information.
sRGB is a standard RGB color space that HP and Microsoft created cooperatively in 1996 to use on monitors, printers, and the World Wide Web. It was subsequently standardized by the International Electrotechnical Commission (IEC) as IEC 61966-2-1:1999. sRGB is the current defined standard colorspace for the web, and it is usually the assumed colorspace for images that are neither tagged for a colorspace nor have an embedded color profile.
The Adobe RGB (1998) color space or opRGB is a color space developed by Adobe Inc. in 1998. It was designed to encompass most of the colors achievable on CMYK color printers, but by using RGB primary colors on a device such as a computer display. The Adobe RGB (1998) color space encompasses roughly 50% of the visible colors specified by the CIELAB color space – improving upon the gamut of the sRGB color space, primarily in cyan-green hues. It was subsequently standardized by the IEC as IEC 61966-2-5:1999 with a name opRGB and is used in HDMI.
A color model is an abstract mathematical model describing the way colors can be represented as tuples of numbers, typically as three or four values or color components. When this model is associated with a precise description of how the components are to be interpreted, taking account of visual perception, the resulting set of colors is called "color space."
The CIE 1931 color spaces are the first defined quantitative links between distributions of wavelengths in the electromagnetic visible spectrum, and physiologically perceived colors in human color vision. The mathematical relationships that define these color spaces are essential tools for color management, important when dealing with color inks, illuminated displays, and recording devices such as digital cameras. The system was designed in 1931 by the "Commission Internationale de l'éclairage", known in English as the International Commission on Illumination.
Relative luminance follows the photometric definition of luminance including spectral weighting for human vision, but while luminance is a measure of light in units such as , Relative luminance values are normalized as 0.0 to 1.0, with 1.0 being a theoretical perfect reflector of 100% reference white. Like the photometric definition, it is related to the luminous flux density in a particular direction, which is radiant flux density weighted by the luminous efficiency function y(λ) of the CIE Standard Observer.
xvYCC or extended-gamut YCbCr is a color space that can be used in the video electronics of television sets to support a gamut 1.8 times as large as that of the sRGB color space. xvYCC was proposed by Sony, specified by the IEC in October 2005 and published in January 2006 as IEC 61966-2-4. xvYCC extends the ITU-R BT.709 tone curve by defining over-ranged values. xvYCC-encoded video retains the same color primaries and white point as BT.709, and uses either a BT.601 or BT.709 RGB-to-YCC conversion matrix and encoding. This allows it to travel through existing digital limited range YCC data paths, and any colors within the normal gamut will be compatible. It works by allowing negative RGB inputs and expanding the output chroma. These are used to encode more saturated colors by using a greater part of the RGB values that can be encoded in the YCbCr signal compared with those used in Broadcast Safe Level. The extra-gamut colors can then be displayed by a device whose underlying technology is not limited by the standard primaries.
Rec. 709, also known as Rec.709, BT.709, and ITU 709, is a standard developed by ITU-R for image encoding and signal characteristics of high-definition television.
A color space is a specific organization of colors. In combination with color profiling supported by various physical devices, it supports reproducible representations of color – whether such representation entails an analog or a digital representation. A color space may be arbitrary, i.e. with physically realized colors assigned to a set of physical color swatches with corresponding assigned color names, or structured with mathematical rigor. A "color space" is a useful conceptual tool for understanding the color capabilities of a particular device or digital file. When trying to reproduce color on another device, color spaces can show whether shadow/highlight detail and color saturation can be retained, and by how much either will be compromised.
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 resolutions, frame rates with progressive scan, bit depths, color primaries, RGB and luma-chroma color representations, chroma subsamplings, and an opto-electronic transfer function. The first version of Rec. 2020 was posted on the International Telecommunication Union (ITU) website on August 23, 2012, and two further editions have been published since then.
The Academy Color Encoding System (ACES) is a color image encoding system created under the auspices of the Academy of Motion Picture Arts and Sciences. ACES is characterised by a color accurate workflow, with "seamless interchange of high quality motion picture images regardless of source".
A color appearance model (CAM) is a mathematical model that seeks to describe the perceptual aspects of human color vision, i.e. viewing conditions under which the appearance of a color does not tally with the corresponding physical measurement of the stimulus source.
DCI-P3 is an RGB color space first defined in 2005 as part of the Digital Cinema Initiative, to be used for digital theatrical motion picture distribution (DCDM). Display P3 is a variant developed by Apple Inc. for wide-gamut displays.
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 functions instead of the traditional "gamma" previously used for SDR-TV.
