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RGB color spaces

RGB color spaces is a category of additive colorimetric color spaces[1] specifying part of its absolute color space definition using the RGB color model.[2]

1931 CIE chromaticity diagram showing some RGB color spaces as defined by their chromaticity triangles

RGB color spaces are commonly found describing the mapping of the RGB color model to human perceivable color, but some RGB color spaces use imaginary (non-real-world) primaries and thus can not be displayed directly.

Like any color space, while the specifications in this category use the RGB color model to describe their space, it is not mandatory to use that model to signal pixel color values. Broadcast TV color spaces like NTSC, PAL, Rec. 709, Rec. 2020 additionally describe a translation from RGB to YCbCr and that is how they are usually signalled for transmission, but an image can be stored as either RGB or YCbCr. This demonstrates using the singular term "RGB color space" can be misleading, since a chosen color space or signalled colour can be described by any appropriate color model. However the singular can be seen in specifications where storage signalled as RGB is its intended use.

Definition

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RGB cube

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.

RGB use in color space definitions employ primaries (and often a white point) based on the RGB color model, to map to real world color. 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.[3]

The primary colors are usually mapped to 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.

Applications

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See also: List of Color Spaces
 
One million colors in RGB space, visible in full-size image

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 specifications employing the RGB color model

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Color space Reference Standard Primary Color Model for Signalling Year White point Color Primaries Display

gamma

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) YCbCr 1987 D93 0.63 0.34 0.31 0.595 0.155 0.07 Curved
NTSC, MUSE SMPTE RP 145 (C), 170M, 240M YCbCr 1987 D65 ⁠20/9⁠ 1.1115 0.0057 4 0.0228
Apple RGB (Apple Computer) RGB 0.625 0.28 1.8
PAL / SECAM EBU 3213-E, BT.470/601 (B/G) YCbCr 1970 0.64 0.33 0.29 0.60 0.15 0.06 Curved ⁠14/5⁠
sRGB IEC 61966-2-1 RGB 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 YCbCr 1999 Curved ⁠20/9⁠ 1.099 0.004 4.5 0.018
Adobe RGB (Adobe) RGB 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) YCbCr 1953 C 2.2[9] ⁠11/5⁠
PAL-M ITU-R BT.470-6[10] YCbCr 1972
eciRGB ISO 22028-4 2008, 2012 D50 1.8 3 1.16 0.008856 9.033 0.08
DCI-P3 SMPTE RP 431-2 YCbCr 2011 6300K 0.68 0.32 0.265 0.69 0.15 0.06 2.6 ⁠13/5⁠
Display P3 SMPTE EG 432-1 RGB 2010 D65 ~2.2 ⁠12/5⁠ 1.055 0.0031308 12.92 0.04045
UHDTV ITU-R BT.2020, BT.2100 YCbCr 2012, 2016 0.708 0.292 0.170 0.797 0.131 0.046 Curved 1.0993 0.018054 4.5 0.081243
Wide Gamut (Adobe) RGB 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 a 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.

See also

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References

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  1. ^ "colorimetric colour space (definition)". France: International Commission on Illumination (CIE). Retrieved 8 October 2023.
  2. ^ Pascale, Danny. "A Review of RGB color spaces...from xyY to R'G'B'" (PDF). Retrieved 20 October 2021.
  3. ^ Hunt, R. W. G (2004). The Reproduction of Colour (6th ed.). Chichester UK: Wiley–IS&T Series in Imaging Science and Technology. ISBN 0-470-02425-9.
  4. ^ Yamashita, Takayuki; Nishida, Yukihiro; Emoto, Masaki; Ohmura, Kohei; Masaoka, Kenichiro; Masuda, Hiroyasu; Sugawara, Masayuki. "Super Hi-Vision as Next-Generation Television and Its Video Parameters". Information Display. Archived from the original on 2018-02-10.
  5. ^ Baker, Simon (19 February 2014). "The Pointer's Gamut - The Coverage of Real Surface Colors by RGB Color Spaces and Wide Gamut Displays". TFTCentral. Retrieved 13 January 2023.
  6. ^ Chen, Haiwei; He, Juan; Wu, Shin-Tson (September 2017). "Recent Advances on Quantum-Dot-Enhanced Liquid-Crystal Displays". IEEE Journal of Selected Topics in Quantum Electronics. 23 (5): 1–11. Bibcode:2017IJSTQ..2349466C. doi:10.1109/JSTQE.2017.2649466. S2CID 1400159.
  7. ^ Huang, Yuge; Hsiang, En-Lin; Deng, Ming-Yang; Wu, Shin-Tson (18 June 2020). "Mini-LED, Micro-LED and OLED displays: present status and future perspectives". Light: Science & Applications. 9 (1): 105. Bibcode:2020LSA.....9..105H. doi:10.1038/s41377-020-0341-9. PMC 7303200. PMID 32577221. S2CID 235470310.
  8. ^ "RECOMMENDATION ITU-R BO.650-2 *,** | Standards for conventional television systems for satellite broadcasting in the channels defined by Appendix 30 of the Radio Regulations" (PDF).
  9. ^ 47 CFR § 73.682 (20) (iv)
  10. ^ "Recommendation ITU-R BT.470-6 | Conventional Television Systems" (PDF).
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