DVD Demystified

DVD Technology Primer
Figure 3.2
Color and luminance
sensitivity of the eye
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tral sensitivity of the eye, which is about 70 percent green, 20 percent red,
and 10 percent blue (Figure 3.2).
The remaining color information is called chrominance (denoted as C),
which is made up of hue (the proportion of color: the redness, orangeness,
greenness, etc.), and saturation (the purity of the color, from pastel to vivid).
For the purposes of compression and converting from RGB, however, it is
easier to use color difference information rather than hue and saturation. In
other words, the color information is what is left after the luminance is
removed. By subtracting the luminance value from each RGB value, three
color difference signals are created—R-Y, G-Y, and B-Y. Only three stimulus
values are needed, so only two color difference signals need be included
with the luminance signal. Since green is the largest component of luminance,
it has the smallest difference signal (G makes up the largest part of
Y, so G-Y results in the smallest values). The smaller the signal, the more it
is subject to errors from noise, so B-Y and R-Y are the best choice. The green
color information can be recreated by subtracting the two difference signals
from the Y signal (roughly speaking). Different weightings are used to
derive Y and color differences from RGB, such as YUV, YIQ, and YC bC r.