DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Here the term color difference refers
to a signal formed as the difference
of two gamma-corrected color
components. In other contexts, the
term can refer to a numerical
measure of the perceptual distance
between two colors.
I introduced interface offsets
on page 23.
vision, a color image can be coded into a wideband
monochrome component representing lightness, and
two narrowband components carrying color information,
each having substantially less spatial resolution
than lightness. In analog video, each color channel has
bandwidth typically one-third that of the monochrome
channel. In digital video, each color channel has half
the data rate (or data capacity) of the monochrome
channel, or less. There is strong evidence that the
human visual system forms an achromatic channel and
two chromatic color-difference channels at the retina.
Green dominates luminance: Between 60% and 70% of
luminance comprises green information. Signal-to-noise
ratio is maximized if the color signals on the other two
components are chosen to be blue and red. The
simplest way to “remove” lightness from blue and red is
to subtract it, to form a pair of color difference (or
loosely, chroma) components.
The monochrome component in color video could have
been based upon the luminance of color science
(a weighted sum of R, G, and B). Instead, as I explained
in Constant luminance, on page 75, luma is formed as
a weighted sum of R’, G’, and B’, using coefficients
similar or identical to those that would be used to
compute luminance. Expressed in abstract terms, luma
ranges 0 to 1. Color difference components B’-Y’ and
R’-Y’ are bipolar; each ranges nearly ±1.
In component analog video, B’-Y’ and R’-Y’ are scaled
to form P B and P R components. In abstract terms, these
range ±0.5. Figure 24.2 opposite shows the unit R’G’B’
cube transformed into luma [Y’, P B , P R ]. (Various interface
standards are in use; see page 303.) In component
digital video, B’-Y’ and R’-Y’ are scaled to form C B and
C R components. In 8-bit Y’C BC R prior to the application
of the interface offset, the luma axis of Figure 24.2
would be scaled by 219, and the chroma axes by 112.
Once color difference signals have been formed, they
can be subsampled to reduce bandwidth or data
capacity, without the observer’s noticing, as I will
explain in Chroma subsampling, revisited, on page 292.
284 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES