DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Tristimulus values are correctly reproduced by the
arrangement of Figure 8.9, and it is highly practical.
Figure 8.9 encapsulates the basic signal flow for all
video systems; it will be elaborated in later chapters.
In the rearranged encoder, we no longer use CIE L* to
optimize for perceptual uniformity. Instead, we use the
inverse of the transfer function inherent in the CRT.
A 0.4-power function accomplishes approximately
perceptually uniform coding, and reproduces tristimulus
values proportional to those in the original scene.
You will learn in the following chapter, Rendering intent,
that the 0.4 value must be altered to about 0.5 to
accommodate a perceptual effect. This alteration
depends upon viewing environment; display systems
should have adjustments for rendering intent, but they
don’t! Before discussing the alteration, I will outline the
repercussions of the nonideal block diagram.
“Leakage” of luminance into chroma
Until now, we have neglected the color difference
components. In the rearranged block diagram of
Figure 8.9 at the bottom of the facing page, color
differences components are “matrixed” from nonlinear
(gamma-corrected) R’G’B’:
Figure 8.10 Chroma components
R
G
B
0.5
R’
G’
B’
Y’
[P] [P -1
CB
]
CR
In a true constant luminance system, no matter how the
color difference signals are handled, all of the relative
luminance is carried by the luminance channel. In the
rearranged system, most of the relative luminance is
conveyed through the Y’ channel; however, some relative
luminance can be thought of as “leaking” into the
color difference components. If the color difference
components were not subsampled, this would present
no problem. However, the color difference components
are formed to enable subsampling! So, we now turn our
attention to that.
CHAPTER 8 CONSTANT LUMINANCE 79
2.5