DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Figure 23.5 Rec. 709,
sRGB, and CIE L*
transfer functions
are compared. They
are all approximately
perceptually uniform;
however, they are not
close enough to be
interchangeable.
Encoded value, V’ (normalized)
1.0
0.8
0.6
0.4
0.2
0
0
CIE L*
sRGB
Rec. 709
0.2 0.4 0.6 0.8 1.0
Tristimulus value, L (relative)
Use this relation to recover scene tristimulus values –
but not display tristimulus values! – from sRGB:
⎧ 1
⎪ V'sRGB; 0 ≤V'sRGB ≤ 0. 03928
⎪12.
92
L = ⎨
24 .
⎪⎛
V'sRGB
+ 0. 055⎞
⎜
⎟ ; 0. 03928 < V'
≤1
⎪
sRGB
⎩⎝
1. 055 ⎠
Figure 23.5 sketches the sRGB transfer function, alongside
the Rec. 709 and CIE L* functions.
Transfer functions in SDTV
Historically, transfer functions for 480i SDTV have been
very poorly specified. The FCC NTSC standard has, since
1953, specified R’G’B’ encoding for a display with
a “transfer gradient (gamma exponent) of 2.2.”
However, modern CRTs have power function laws very
close to 2.5! The FCC statement is widely interpreted to
suggest that encoding should approximate a power of
1 ⁄2.2; the reciprocal of 1 ⁄2.2, 0.45, appears in modern
standards such as Rec. 709. However, as I mentioned
on page 263, Rec. 709’s overall curve is very close to
a square root. The FCC specification should not be
taken too seriously: Use Rec. 709 for encoding.
Standards for 576i STDV also have poorly specified
transfer functions. An “assumed display power func-
268 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES
Eq 23.9