DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Introduction to luma
and chroma 10
Video systems convey image data in the form of one
component that represents lightness, and two components
that represent color, disregarding lightness. This
scheme exploits the reduced color acuity of vision
compared to luminance acuity: As long as lightness is
conveyed with full detail, detail in the color components
can be reduced by subsampling (filtering, or averaging).
This chapter introduces the concepts of luma
and chroma encoding; details will be presented in Luma
and color differences, on page 281.
Luma
A certain amount of noise is inevitable in any image
digitizing system. As explained in Nonlinear image
coding, on page 12, we arrange things so that noise has
a perceptually similar effect across the entire tone scale
from black to white. The lightness component is
conveyed in a perceptually uniform manner that minimizes
the amount of noise (or quantization error) introduced
in processing, recording, and transmission.
Ideally, noise would be minimized by forming a signal
proportional to CIE luminance, as a suitably weighted
sum of linear R, G, and B tristimulus signals. Then, this
signal would be subjected to a transfer function that
imposes perceptual uniformity, such as the CIE L* function
of color science that will be detailed on page 208.
As explained in Constant luminance, on page 75, there
are practical reasons in video to perform these operations
in the opposite order. First, a nonlinear transfer
function – gamma correction – is applied to each of the
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