Color space
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).
A color space in the most generic sense is identical to the concept of gamut. In general use however, a color space is the gamut which can be described by the combination between a given color model and a certain mapping of its colors onto the Lab color space (e.g. Adobe RGB and sRGB are color spaces based on the RGB model).
In the generic sense of the definition 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 colors which are described by the existence of a corresponding set of physical color swatches by which the colors are identified. In the rest of this article color space will be used exclusively to denote the second sense from the definition above.
Since the color space is a more specific term for the certain combination of a color model + 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. Unfortunately this results in ambiguity regarding the two terms and they generally end up being used interchangeably, even when they shouldn't be. For example, although several specific color spaces are based on the RGB model, there is no such thing as the RGB color space.
A color model describes a certain way of encoding color information. A color model with no associated mapping function 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 reference color space results in a measurable "footprint" within the reference color space. The color space used as reference is the Lab color space, which was specifically designed to encompass all colors the average human can see. The "footprint" of a color space within the Lab color space is the color space's gamut.
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 at the time of this writing is the 24 bit implementation, with 8 bits per channel. That means that each of the three channels in such an RGB implementation can store 256 discrete levels of color. Any color space based on the RGB model is thus limited to a gamut of 256*256*256=16.7 million colors. Therefore a "small", "restrictive" color space based on this RGB implementation will result in exactly the same number of colors as a "large", "generous" space. The only difference between the two is the volume their gamut cover, but that is always inverse proportional to its density on the reference space.
Since any color space inherently defines colors as a function of the absolute reference frame, color spaces along with device profiling lend themselves to reproducible representations of color, particularly in digital representations, such as digital printing or digital electronic display.
Some colorspaces in wide use are:
- RGB for computer graphics
- RBGA for computer graphics with variable transparency
- CMYK for printing
- YUV and YIQ for television
- HSV for graphic design
Commercial color spaces:
Specialised application color spaces:
- CIE color spaces
- RG Chromaticity for Computer vision applications
Obsolete colorspaces:
Commonly used color spaces
Additive color mixing
Subtractive color mixing
RGB is typically used to describe additive color. Light is added together to create form from out of the darkness. RGB stores individual values for red, green and blue.
RGBA is used similarly as RGB, but it has an additional channel, alpha, to indicate transparency.
CMYK is a subtractive color space used in the printing process. 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 (also called YCC or more accurately YCbCr) scheme used in PAL television and JPEG image compression, except for the fact that the YIQ color space is rotated 33° with respect to the YUV color space.
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.
There's an overview of the differences between RGB and CMYK at http://www.pixelphoto.com/Main/rgb_cmyk.html
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.
Specialised application color spaces
The reference standard usually used is the CIE Lab color space. This is the most accurate color space but too complex for every day uses.
The RG Chromaticity space is used in Computer vision applications, and shows the color of light (red, yellow, green etc.), but not its intensity (dark, bright).
Obsolete color spaces
Early color spaces had two components. They largely ignored blue light because the added complexity of a 3-component process was 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