DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

My notation is outlined in
Figure 24.5, on page 289. The
coefficients are derived in Color
science for video, on page 233.
To compute luminance using
(R+G+B)/3 is at odds with the
spectral response of vision.
Luminance from red, green, and blue
The luminous efficiency of vision peaks in the mediumwave
(green) region of the spectrum: If three monochromatic
sources appear red, green, and blue, and
have the same radiant power in the visible spectrum,
then the green will appear the brightest of the three;
the red will appear less bright, and the blue will be the
darkest of the three. As a consequence of the luminous
efficiency function, the most saturated blue colors are
quite dark, and the most saturated yellows are quite
light.
If the luminance of a scene element is to be sensed by
a scanner or camera having a single spectral filter, then
the spectral response of the scanner’s filter must – in
theory, at least – correspond to the luminous efficiency
function of Figure 20.1. However, luminance can also be
computed as a weighted sum of suitably chosen red,
green, and blue tristimulus components. The coefficients
are functions of vision, of the white reference,
and of the particular red, green, and blue spectral
weighting functions employed. For realistic choices of
white point and primaries, the green coefficient is quite
large, the blue coefficient is the smallest of the three,
and the red coefficient has an intermediate value.
The primaries of contemporary CRT displays are standardized
in Rec. ITU-R BT.709. Weights computed from
these primaries are appropriate to compute relative
luminance from red, green, and blue tristimulus values
for computer graphics, and for modern video cameras
and modern CRT displays in both STDV and HDTV:
709
Y = 0. 2126 R+ 0. 7152 G+ 0. 0722 B Eq 20.1
Luminance comprises roughly 21% power from the red
(longwave) region of the spectrum, 72% from green
(mediumwave), and 7% from blue (shortwave).
Blue has a small contribution to luminance. However,
vision has excellent color discrimination among blue
hues. If you give blue fewer bits than red or green, then
blue areas of your images are liable to exhibit
contouring artifacts.
CHAPTER 20 LUMINANCE AND LIGHTNESS 207