DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Y’CBCR
e.g., 4:2:2
CHROMA
INTERPOLATION
Y’CBCR
NONLINEAR
3×3
-1
[P ]
R’G’B’
TRANSFER
FUNCTION
γ D =2.5
matrix (P), to obtain luma and color difference components
Y’C B C R or Y’P B P R . (This matrix depends upon the
luma coefficients in use, and upon color difference scale
factors.) Then, if necessary, a chroma subsampling filter
is applied to obtain subsampled color difference
components; luma is subject to a compensating delay.
A decoder uses the inverse operations of the encoder,
in the opposite order, as sketched in Figure 24.8. In
a digital decoder, the chroma interpolation filter reconstructs
missing chroma samples; in an analog decoder,
no explicit operation is needed. The 3×3 color difference
matrix (P -1 ) reconstructs nonlinear red, green, and
blue primary components. The transfer functions restore
the primary components to their linear-light tristimulus
values. Finally, the tristimulus 3×3 matrix (T 2) transforms
from the primaries of the interchange standard to
the primaries implemented in the display device.
When a decoder is intimately associated with a CRT
monitor, the decoder’s transfer function is performed by
the nonlinear voltage-to-luminance relationship
intrinsic to the CRT: No explicit operations are required
for this step. However, to exploit this transfer function,
the display primaries must be the same as – or at least
very similar to – the interchange primaries.
CHAPTER 24 LUMA AND COLOR DIFFERENCES 295
RGB
TRISTIMULUS
3×3
[ T 2 ]
XYZ
or R 2G 2B 2
Figure 24.8 Luma/color difference decoder involves the inverse of the four stages of Figure 24.7
in opposite order. First, subsampled color difference (chroma) signals are interpolated; luma
undergoes a compensating delay. The matrix P -1 then recovers R’G’B’ from luma and two color
differences. A transfer function having an exponent of about 2.5 is then applied, which produces
linear-light (tristimulus) signals RGB. If the display’s primaries differ from the interchange
primaries, RGB are transformed through the matrix T 2 to produce appropriate R 2 G 2 B 2 .
Figures 24.7 and 24.8 show
3×3 matrix transforms being
used for two distinctly different
tasks. When someone hands
you a 3×3, you have to ascertain
whether it is intended for
a linear or nonlinear task.