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

An RGB color space is any additive color space based on the RGB color model.[1][2] An RGB color space is defined by chromaticity coordinates of the red, green, and blue additive primaries, the white point which is usually a standard illuminant, and the transfer function which is also known as the tone response curve (TRC) or gamma. Applying Grassmann's law of light additivity, a colorspace so defined can produce colors which are enclosed within the 2D triangle on the chromaticity diagram defined by those primary coordinates. The TRC and white point further define the possible colors, creating a volume in a 3D shape that never exceeds the triangular bounds.[3]

The primary colors are often 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 uses the 2° standard observer, an averaging of experimental data defining the limits of the human color gamut.

Introduction

RGB-Cube
RGB-Cube

The normal human eye contains three type of photosensitive cells called cones, which are sensitive to wavelengths of light that we generally categorize as red, green, and blue.

An RGB color space uses illuminated primaries which are chosen to stimulate each cone type as independently as possible. In this way, mixing the three lights in different proportions can stimulate the cones in the eye and create a color perception.

Applications

One million colors in RGB space, visible in full-size image.
One million colors in RGB space, visible in full-size image.

Color television was the first practical use of an RGB color space for presenting images. NTSC was adopted in 1953 in North America. Other parts of the world adopted either PAL or SECAM. While these RGB color spaces created colors using additive red, green, and blue primaries, the broadcast signal was encoded from RGB components to a composite signal such as YIQ.

These early RGB spaces were defined in part by the phosphor used inside the CRT and the gamma of the electron beam. Today, LCD is among the most common technologies in use.

An LCD display can be thought of as a grid of millions of tiny red, green, and blue lights, each with their own dimmer control. The gamut of the display depends on how far apart the red, green, and blue color coordinates are on the chromaticity diagram, the nature of the TRC and the signal encoding of the image.

HDTV uses an RGB color space known as BT.709. A common standard for computer monitors is sRGB which uses the same color primaries and white point as BT.709, but the transfer function is different as HDTV is intended for a dark living room, and sRGB is intended for a brighter office environment.

At one time, sRGB was the most commonly used color space for computer displays, and having all devices use the same calibrated color space is useful so that color conversions do not need to take place before images are displayed. sRGB's limited gamut fits well into an 8-bit image, but also leaves out deeply saturated colors that might be available in an alternate RGB color spaces.

Some color spaces defined for this purpose with RGB primaries are and Adobe RGB (which has a larger gamut), and ProPhoto, which is even larger in terms of gamut volume. However this does not mean the larger space has 'more colors," as 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.

RGB color space specifications

RGB color spaces
Color space Reference Standard Year White point Primaries Display

gamma

Transfer function parameters
Red Green Blue γ α β δ βδ
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[4] 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[5] 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 0.735 0.265 0.115 0.826 0.157 0.018 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 1.8 9/5 1 0.001953125 16 0.031248
CIE RGB CIE 1931 color space 1931 E 0.2738 0.7174 0.1666 0.0089
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.

See also

References

  1. ^ Saini, Harvinder Singh; Sayal, Rishi; Buyya, Rajkumar; Aliseri, Govardhan (2020). Innovations in Computer Science and Engineering. Singapore: Springer Singapore. p. 235. ISBN 9789811520433.
  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. ^ https://extranet.itu.int/brdocsearch/R-REC/R-REC-BO/R-REC-BO.650/R-REC-BO.650-2-199203-I/R-REC-BO.650-2-199203-I!!PDF-E.pdf#page=18[bare URL PDF]
  5. ^ https://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.470-6-199811-S!!PDF-E.pdf#page=2[bare URL PDF]