DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

The term luminance is widely
misused in video. See Relative
luminance, on page 206, and
Appendix A, YUV and luminance
considered harmful, on page 595.
Applebaum, Sidney, “Gamma
Correction in Constant Luminance
Color Television Systems,” in Proc.
IRE, 40 (11): 1185–1195
(Oct. 1952).
Constant luminance 8
Video systems convey color image data using one
component to represent lightness, and two other
components to represent color, absent lightness. In
Color science for video, on page 233, I will detail how
luminance can be formed as a weighted sum of linear
RGB values that are proportional to optical power.
Transmitting relative luminance – preferably after imposition
of a nonlinear transfer function – is called the
Principle of Constant Luminance.
Video systems depart from this principle and implement
an engineering approximation. A weighted sum of
linear RGB is not computed. Instead, a nonlinear
transfer function is applied to each linear RGB component,
then a weighted sum of the nonlinear gammacorrected
R’G’B’ components forms what I call luma.
(Many video engineers carelessly call this luminance.)
As far as a color scientist is concerned,
a video system uses the theoretical matrix coefficients
of color science but uses them in the wrong block
diagram: In video, gamma correction is applied before
the matrix, instead of the color scientist’s preference,
after.
In this chapter, I will explain why and how all video
systems depart from the principle. If you are willing to
accept this departure from theory as a fact, then you
may safely skip this chapter, and proceed to Introduction
to luma and chroma, on page 87, where I will introduce
how the luma and color difference signals are
formed and subsampled.
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