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# Color space

(Redirected from Color model)

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 (e.g. RGB and CMYK are color models). However, a color model with no associated mapping function to a reference color space is a more or less arbitrary color system with little connection to the requirements of any given application.

Adding a certain mapping function between the color model and a certain reference color space results in a definite "footprint" within the reference color space. This "footprint" is known as a gamut, and, in combination with the color model, defines a new color space. For example, Adobe RGB and sRGB are two different different color spaces, both based on the RGB model.

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## Notes

When formally defining a color space, the usual reference standard is the CIE Lab color space, which was specifically designed to encompass all colors the average human can see. This is the most accurate color space but is too complex for everyday uses.

Since "color space" is a more specific term for a certain combination of a color model plus a color mapping function, the term "color space" tends to be used to also identify color models, since identifying a color space automatically identifies the associated color model. Informally, the two terms are often used interchangably, though this is strictly incorrect. For example, although several specific color spaces are based on the RGB model, there is no such thing as the RGB color space.

In the generic sense of the definitions above, color spaces can be defined without the use of a color model. These spaces, such as Pantone, are in effect a given set of names or numbers which are defined by the existence of a corresponding set of physical color swatches.

Since any color space defines colors as a function of the absolute reference frame, color spaces, along with device profiling, allow reproducible representations of color, in both analogue and digital representations.

### Color space density

The RGB color model can be implemented in various specific incarnations, depending on the capabilities of the system used. By far the most common general-use incarnation as of 2004 is the 24-bit implementation, with 8 bits, or 256 discrete levels of color per channel . Any color space based on such a 24-bit RGB model is thus limited to a gamut of 256×256×256 ≈ 16.7 million colors. An implementation which uses 16 bits per component will result in exactly the same range of colors as the 8 bit version, but to twice the accuracy. The gamuts of the two spaces cover the same volume, but individual colors within that volume can be identified more accurately in the "denser" 16-bit implementation. The same principle applies for any color spaces based on the same color model, but implemented in different bit depths.

## List of color spaces

### Generic color spaces

Subtractive color mixing

RGB uses additive color mixing, because it describes what kind of light needs to be emitted to produce a given color. Light is added together to create form from out of the darkness. RGB stores individual values for red, green and blue. RGBA is RGB with an additional channel, alpha, to indicate transparency.

Color spaces based on the RGB model include sRGB, Adobe RGB and Adobe Wide Gamut RGB.

CMYK uses subtractive color mixing used in the printing process, because it describes what kind of inks need to be applied so the light reflected from them produces a given color. One starts with a white canvas, and uses ink to subtract color from white to create an image. CMYK stores ink values for cyan, magenta, yellow and black.

YIQ is used in NTSC (North American) television broadcasts for historical reasons. YIQ stores a luminance value with two chrominance values, corresponding approximately to the amounts of blue and red in the color. It corresponds closely to the YUV scheme used in PAL television except that the YIQ color space is rotated 33° with respect to the YUV color space. The YDbDr scheme used by SECAM television is rotated in another way. (work needed)

YPbPr is a scaled version of YUV. It is most commonly seen in its digital form, YCbCr, used widely in video and image compression schemes such as MPEG and JPEG.

HSV is often used by artists because it is often more natural to think about a color in terms of hue and saturation than in terms of additive or subtractive color components. HSV stores a hue value, a saturation value and an intensity value.

HLS is quite similar to HSV, with lightness replacing intensity value.

Once you've decided which color space you want to work in, if you are working on a computer, you must then address the problem of color space encoding.

### Obsolete color spaces

Early color spaces had two components. They largely ignored blue light because the added complexity of a 3-component process provided much less of a marginal increase in fidelity than the jump from monochrome to 2-component color.

## References

• R. W. G. Hunt , The Reproduction of Colour in Photography, Printing & Television, 5th Ed. Fountain Press, England, 1995. ISBN 0863433812
• Mark D. Fairchild, Color Appearance Models, Addison-Wesley, Reading, MA (1998). ISBN 0-201-63464-3
• Charles A. Poynton, Introduction to Video Colour Spaces http://groups.google.co.uk/groups?selm=kvpnumINN4rr%40exodus.Eng.Sun.COM&rnum=1

• Charles Poynton's Color FAQ http://www.poynton.com/ColorFAQ.html
• FAQ about color physics http://www.colourware.co.uk/cpfaq.htm
• Dan Bruton's Color Science http://www.physics.sfasu.edu/astro/color.html
• Color-Scheme - open source color space management package written in Scheme http://www-swiss.ai.mit.edu/~jaffer/Color/index.html
• RGB-Color Mixer Java Applet http://www.wackerart.de/mixer.html Java-Plugin required
• Color Space Conversion Formulas http://www.easyrgb.com/math.php

Last updated: 02-07-2005 10:19:37