DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Figure 21.1 Example
coordinate system
For an approachable, nonmathematical
introduction to color
physics and perception, see
Rossotti, Hazel, Colour: Why the
World Isn’t Grey (Princeton, N.J.:
Princeton Univ. Press, 1983).
The CIE system
of colorimetry 21
The Commission Internationale de L’Éclairage (CIE) has
defined a system that maps a spectral power distribution
(SPD) of physics into a triple of numerical values –
CIE XYZ tristimulus values – that form the mathematical
coordinates of color space. In this chapter,
I describe the CIE system. In the following chapter,
Color science for video, I will explain how these XYZ tristimulus
values are related to linear-light RGB values.
Color coordinates are analogous to coordinates on
a map (see Figure 21.1). Cartographers have different
map projections for different functions: Some projections
preserve areas, others show latitudes and longitudes
as straight lines. No single map projection fills all
the needs of all map users. There are many “color
spaces.” As in maps, no single coordinate system fills all
of the needs of users.
In Chapter 20, Luminance and lightness, I introduced
the linear-light quantity luminance. To reiterate, I use
the term luminance and the symbol Y to refer to CIE
luminance. I use the term luma and the symbol Y’ to
refer to the video component that conveys an approximation
to lightness. Most of the quantities in this
chapter, and in the following chapter Color science for
video, involve values that are proportional to intensity.
In Chapter 8, Constant luminance, I related the theory of
color science to the practice of video. To approximate
perceptual uniformity, video uses quantities such as R’,
G’, B’, and Y’ that are not proportional to intensity.
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