Absolute color space
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An absolute color space is a color space in which colors are unambiguous, where they do not depend on any external factors.
An example of an absolute color space is L*a*b*. A L*a*b* color defines an exact color; it is abstract, but if reproduced using an accurate device and viewed in the right conditions, it should look exactly as intended.
A counter-example of a color space that is not absolute is RGB. RGB is made by mixing red, green, and blue, but these are not standardised, precise definitions. Two computer monitors or other RGB devices may show the same RGB image looking very different.
One way to think of this is that L*a*b* is a color, while RGB is a recipe; the results of mixing RGB depend on the ingredients.
A non-absolute color space can be made absolute by defining its ingredients more precisely. For instance, if the red, green, and blue colors in a monitor are measured exactly, together with other properties of the monitor, then RGB values on that monitor can be considered as absolute.
A popular way to make a color space like RGB into an absolute color is to define an ICC profile, which contains the attributes of the RGB. This is not the only way to express an absolute color, but it is the standard in many industries. RGB colors defined by widely accepted profiles include sRGB and Adobe RGB. The process of adding an ICC profile to a graphic or document is sometimes called tagging; tagging therefore marks the absolute meaning of colors in that graphic or document.
[edit] Conversion
One absolute color can be converted to another absolute color, and back again, in general; however, each color space has its own gamut, and converting colors that lie outside that gamut will not produce correct results. There are also likely to be rounding errors, especially if the popular range of only 256 distinct values per component (8 bits) is used.
Also note that one part of the definition of an absolute color is the viewing conditions. The same color, viewed under different natural or artificial lighting conditions, will look different. Those involved professionally with color matching may use viewing rooms, lit by standardised lighting.
Occasionally, there are precise rules for converting between non-absolute color spaces. For example HLS is defined as a mapping of RGB. Both are non-absolute, but the conversion between them should maintain the same color. However, in general, converting between two non-absolute color spaces (for example, RGB to CMYK) or between absolute and non-absolute color spaces (for example, RGB to L*a*b*) is almost a meaningless concept, though there will be formulae that give roughly equivalent results.
[edit] Other absolute color spaces
A different method of defining absolute color spaces is familiar to many consumers as the swatch card, used to select paint, fabrics, and the like. This is a way of agreeing a color between two parties. A more standardised, but still proprietary, method of defining absolute colors is Pantone, which includes swatch cards and recipes that commercial printers can use to make inks that are a particular color.
