DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

McCamy argues that under
normal conditions 1,875,000
colors can be distinguished. See
McCamy, C.S., “On the Number
of Discernable Colors,” in Color
Research and Application, 23 (5):
337 (Oct. 1998).
ITU-T Rec. T.42, Continuous-tone
colour representation for facsimile.
distance in L*u*v* or L*a*b* corresponds roughly to
a just-noticeable difference (JND) of color.
Consider that L* ranges 0 to 100, and each of u* and v*
range approximately ±100. A threshold of unity ∆E* uv
defines four million colors. About one million colors can
be distinguished by vision, so CIE L*u*v* is somewhat
conservative. A million colors – or even the four million
colors identified using a ∆E* uv or ∆E* ab threshold of
unity – are well within the capacity of the 16.7 million
colors available in a 24-bit truecolor system that uses
perceptually appropriate transfer functions, such as the
function of Rec. 709. (However, 24 bits per pixel are far
short of the number required for adequate performance
with linear-light coding.)
The L*u*v* or L*a*b* systems are most useful in color
specification. Both systems demand too much computation
for economical realtime video processing,
although both have been successfully applied to still
image coding, particularly for printing. The complexity
of the CIE L*u*v* and CIE L*a*b* calculations makes
these systems generally unsuitable for image coding.
The nonlinear R’G’B’ coding used in video is quite
perceptually uniform, and has the advantage of being
suitable for realtime processing. Keep in mind that
R’G’B’ typically incorporates significant gamut limitation,
whereas L*u*v* and CIE L*a*b* represent all colors.
L*a*b* is sometimes used in desktop graphics with
[a*, b*] coordinates ranging from -128 to +127 (e.g.,
Photoshop). The ITU-T Rec. T.42 standard for color fax
accommodates L*a*b* coding with a* ranging -85 to 85,
and b* ranging -75 to 125. Even with these restrictions,
CIE L*a*b* covers nearly all of the colors.
Color specification
A color specification system needs to be able to represent
any color with high precision. Since few colors are
handled at a time, a specification system can be computationally
complex. A system for color specification
must be intimately related to the CIE system.
The systems useful for color specification are CIE XYZ
and its derivatives xyY, L*u*v*, and L*a*b*.
CHAPTER 21 THE CIE SYSTEM OF COLORIMETRY 229