pigmented colorants: dependence on media and time - Cornell ...

color into the appropriate RGB color or vise versa, post multiply by the conversion
matrix M or its inverse:
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[RGB] =[XY Z][M] (3.6)
[XY Z]=[RGB][M] −1
(3.7)
However, the resulting RGB color from the transformation may be invalid if
the color is not within the gamut of colors supported by the RGB primaries. There
are two types of out-of-gamut colors: those that have a chromaticity that cannot
be matched by the primaries and those that experience luminance overflow. The
first set of colors, when mapped to a set of primary phosphors, gives RGB values
of less than zero. The second set, yields RGB values greater than one, as the
magnitude of the color in one or more channels may greater than the maximum
luminance of the display.
The problem of displaying such out-of-gamut colors is an important issue for
painting, since many commonly used real-world pigments fall outside the gamut
of existing color monitors.
Most gamut-matching methods seem to fall into two general categories: global
and local approaches. A local approach examines each pixel, or color, in an image
individually and adjusts only those that are out of gamut. A global approach
applies information gathered from the entire image when considering how to modify
every pixel in the image, even those within the gamut.
Figure 3.22 describes an example of a visible color Q which is unable to be
realized by the RGB primaries. The chromaticity diagram makes it immediately
clear why mixtures of three visible light sources can never reproduce all visible
colors. It is geometrically impossible for a triangle inscribed within the spectral
locus to simultaneously circumscribe it. In this case, we have: Q − Rˆr = Gˆg + B ˆ b.