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.
Understanding the concept
Most people have heard that all colors can be created by the primary colors red, blue, and yellow, if working with paints. Those colors are then a color space. We can see the amount of red color as the X axis, the amount of blue as the Y axis, and the amount of yellow as the Z axis. We now have a 3D space, wherein every possible color has a given position.
However, this is not the only color space around. For instance, when colors are displayed on a computer monitor, they are usually called RGB, for red, green and blue. This is another way of making the same colors, and red, green and blue can be considered as the X, Y and Z axis. Another way of making the same colors is to use their hue (X axis), their saturation (Y axis) and their brightness (z axes). This is called the HSB color space. And there are many more color spaces. Many can be represented as 3D (X,Y,Z) values in this way, but some have more, or less dimensions, and some cannot be represented in this way at all.
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 interchangeably, 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 is implemented in different ways, depending on the capabilities of the system used. By far the most common general-use incarnation as of 2005 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. Some implementations use 16 bits per component, resulting in the same range with a greater density of distinct colors. This is especially important when working with wide-gamut color spaces (where most of the more common colors are located relatively close together), or when a large number of digital filtering algorithms are used consecutively. The same principle applies for any color spaces based on the same color model, but implemented in different bit depths.
Partial list of color spaces
Generic color spaces
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.
Commercial color spaces
Special-purpose color spaces
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.
- 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
Last updated: 07-30-2005 03:28:34
Last updated: 08-15-2005 22:08:26