DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

In Color science for video, on
page 233, I will describe how
spectral power distributions
(SPDs) in the range 400 nm to
700 nm are related to colors.
The term luminance is often carelessly
and incorrectly used to refer
to luma. See Relative luminance,
on page 206, and Appendix A,
YUV and luminance considered
harmful, on page 595.
Luminance and lightness 20
Nonlinear coding of luminance is essential to maximize
the perceptual performance of an image coding system.
This chapter introduces luminance and lightness, or
what is loosely called brightness.
Luminance, denoted Y, is what I call a linear-light quantity;
it is directly proportional to physical intensity
weighted by the spectral sensitivity of human vision.
Luminance involves light having wavelengths in the
range of about 400 nm to 700 nm; luminance can be
computed as a properly weighted sum of linear-light
red, green, and blue tristimulus components, according
to the principles and standards of the CIE.
Lightness, denoted L*, is defined by the CIE as a
nonlinear transfer function of luminance that approximates
the perception of brightness.
In video, we do not compute the linear-light luminance
of color science; nor do we compute lightness. Instead,
we compute an approximation of lightness, luma
(denoted Y’) as a weighted sum of nonlinear (gammacorrected)
R’, G’, and B’ components. Luma is only
loosely related to true (CIE) luminance. In Constant
luminance, on page 75, I explained why video systems
approximate lightness instead of computing it directly.
I will detail the nonlinear coding used in video in
Gamma, on page 257. In Luma and color differences, on
page 281, I will outline how luma is augmented with
color information.
203