DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

If you are unfamiliar with the term
luminance, or the symbols Y or Y’,
refer to Luminance and lightness,
on page 203.
arrange for the reproduction system to reproduce those
XYZ values. That approach works in many applications
of color reproduction, and it’s the basis for color in
video. However, in image reproduction, direct recreation
of the XYZ values is unsuitable for perceptual
reasons. Some modifications are necessary to achieve
subjectively acceptable results. Those modifications
were described in Constant luminance, on page 75.
Should you wish to skip this chapter, remember that
accurate description of colors expressed in terms of RGB
coordinates depends on the characterization of the RGB
primaries and their power ratios (white reference). If
your system is standardized to use a fixed set of primaries
throughout, you need not be concerned about this;
however, if your images use different primary sets, it is
a vital issue.
Additive reproduction (RGB)
In the previous chapter, I explained how a physical SPD
can be analyzed into three components that represent
color. This section explains how those components can
be mixed to reproduce color.
The simplest way to reproduce a range of colors is to
mix the beams from three lights of different colors, as
sketched in Figure 22.1 opposite. In physical terms, the
spectra from each of the lights add together wavelength
by wavelength to form the spectrum of the
mixture. Physically and mathematically, the spectra add:
The process is called additive reproduction.
I described Grassmann’s Third Law on page 219: Color
vision obeys a principle of superposition, whereby the
color produced by any additive mixture of three primary
SPDs can be predicted by adding the corresponding
fractions of the XYZ tristimulus components of the
primaries. The colors that can be formed from
a particular set of RGB primaries are completely determined
by the colors – tristimulus values, or luminance
values and chromaticity coordinates – of the individual
primaries. Subtractive reproduction, used in photography,
cinema film, and commercial printing, is much
more complicated: Colors in subtractive mixtures are
234 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES