RGB color space

Short description: Any additive color space based on the RGB color model

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. An RGB color space is defined by chromaticity coordinates of the red, green, and blue additive primaries, the whitepoint 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 whitepoint further define the possible colors, creating a volume in a 3D shape that never exceeds the triangular bounds.

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

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.

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 [math]\displaystyle{ \Delta E }[/math] errors.

RGB color space specifications

RGB color spaces

Color space	Standard	Year	Gamut	White point	Primaries						Transfer function parameters
					Red		Green		Blue		α	β	γ	δ	βδ
					xʀ	yʀ	xɢ	yɢ	xʙ	yʙ	a + 1	K0/φ = Et		φ	K0
ISO RGB			Limited	floating	floating
Extended ISO RGB			Unlimited (signed)
scRGB	IEC 61966-2-2	2003		D65	0.64	0.33	0.30	0.60	0.15	0.06
sRGB	IEC 61966-2-1	1990, 1996	CRT								1.055	0.0031308	12/5	12.92	0.04045
HDTV	ITU-R BT.709	1999									1.099	0.004	20/9	4.5	0.018
Adobe RGB 98		1998					0.21	0.71			1	0	563/256	1	0
PAL / SECAM	EBU 3213-E, ITU-R BT.470/601 (B/G)	1970					0.29	0.60			1	0	14/5	1	0
Apple RGB					0.625	0.34	0.28	0.595	0.155	0.07
NTSC	SMPTE RP 145 (C), 170M, 240M	1987			0.63		0.31				1.1115	0.0057	20/9	4	0.0228
NTSC-J		1987		D93
NTSC (FCC)	ITU-R BT.470/601 (M)	1953		C	0.67	0.33	0.21	0.71	0.14	0.08	1	0	11/5	1	0
eciRGB	ISO 22028-4	1999 (v1), 2007, 2012	Wide	D50							1.16	0.008856	3	9.033	0.08
DCI-P3	SMPTE RP 431-2	2011		6300K	0.68	0.32	0.265	0.69	0.15	0.06	1	0	13/5	1	0
Display P3	SMPTE EG 432-1	2010		D65							1.055	0.0031308	12/5	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	1.0993	0.018054		4.5	0.081243
Adobe Wide Gamut RGB				D50	0.735	0.265	0.115	0.826	0.157	0.018	1	0	563/256	1	0
RIMM	ISO 22028-3	2006, 2012			0.7347	0.2653	0.1596	0.8404	0.0366	0.0001	1.099	0.0018	20/9	5.5	0.099
ROMM RGB, ProPhoto RGB	ISO 22028-2	2006, 2013									1	0.001953125	9/5	16	0.031248
CIE RGB		1931		E			0.2738	0.7174	0.1666	0.0089
CIE XYZ		1931	Unlimited		1	0	0	1	0	0	1	0	1	1	0

See also

• CIELUV color space
• Web colors
• RGB color model
• RGBA color model

External links

• Susstrunk, Buckley and Swen. "Standard RGB Color Spaces" (PDF). http://infoscience.epfl.ch/getfile.py?mode=best&recid=34089. 
• Lindbloom, Bruce. "RGB Working Space Information". http://www.brucelindbloom.com/index.html?WorkingSpaceInfo.html. 
• Colantoni, Philippe. "RGB cube transformation in different color spaces". http://www.couleur.org/index.php?page=rgbcube.