DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Figure 24.5 Luminance and
luma notation is necessary
because different primary chromaticity
sets, different luma
coefficients, and different
component scale factors are in
use. Unity scaling suffices for
components in this chapter; in
succeeding chapters, other
scale factors will be introduced.
Figure 24.6 Typesetting Y’C B C R
is a challenge! Luma coefficient
set, scaling, and chromaticities
are set out as in
Figure 24.5 above. The prime
should always be present, to
distinguish luma from the luminance
of color science. C is
appropriate for digital signals,
P for analog. Subscripts B and
R serve as tags, not variables:
They should be in Roman type,
not italics. B comes before R.
Luminance or luma
coefficients: Rec. 601,
SMPTE 240M,
or Rec. 709
Scaling: 1 (implicit) ,
steps, or millivolts
601
219
Y ’
709
Prime indicates
nonlinear (gammacorrected,
or luma)
component
Chromaticity: Rec. 709,
SMPTE 240M, or EBU
For luminance or luma, a leading superscript indicates
the standard specifying the weights used; historically,
the weights of Rec. 601 were implicit, but recent HDTV
standards such as Rec. 709 and SMPTE 240M call for
different weights. Finally, the leading subscript indicates
the overall scaling of the signal. If omitted, an
overall scaling of unity is implicit, otherwise an integer
such as 219, 255, or 874 specifies the black-to-white
excursion in a digital system, or a number such as 661,
700 or 714 specifies the analog excursion in millivolts.
Typesetting Y’C BC R (or Y’P BP R) is a challenge! I illustrate
the main points in Figure 24.6 below. Y’ is augmented
with shaded leading superscript and subscript and
a trailing subscript, according to the conventions of
Figure 24.5. Without these elements, the intended
color cannot be determined with certainty.
Luma details; see
Figure 24.5 above.
Luma
coefficients
Scaling
601
219
Chromaticity
Y’ CBCR 709
Prime indicates
(nonlinear) luma
C for digital,
P for analog
Roman (for tag),
not italic type
CHAPTER 24 LUMA AND COLOR DIFFERENCES 289