DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

CIE standards established in
1964 were based upon monochromatic
primaries at 444.4,
526.3, and 645.2 nm.
Table 22.1 CIE primaries
were established for the CIE’s
color-matching experiments;
they are unsuitable for image
coding or reproduction.
Characterization of RGB primaries
An additive RGB system is specified by the chromaticities
of its primaries and its white point. The extent – or
gamut – of the colors that can be mixed from a given
set of RGB primaries is given in the [x, y] chromaticity
diagram by a triangle whose vertices are the chromaticities
of the primaries. Figure 22.2 opposite plots the
primaries of several contemporary video standards that
I will describe.
In computing there are no standard primaries or white
point chromaticities, though the sRGB standard is
becoming increasingly widely used. (I will describe
sRGB below, along with Rec. 709.) If you have RGB
image but have no information about its primary chromaticities,
you cannot accurately reproduce the image.
CIE RGB primaries
Color science researchers in the 1920s used monochromatic
primaries – that is, primaries whose chromaticity
coordinates lie on the spectral locus. The particular
primaries that led to the CIE standard in 1931 became
known as the CIE primaries; their wavelengths are
435.8 nm, 546.1 nm, and 700.0 nm, as documented in
the CIE publication Colorimetry (cited on page 216).
These primaries, ennumerated in Table 22.1, are historically
important; however, they are not useful for image
coding or image reproduction.
Red,
700.0 nm
Green,
546.1 nm
Blue,
435.8 nm
White
CIE Ill. B
x 0.73469 0.27368 0.16654 0.34842
y 0.26531 0.71743 0.00888 0.35161
z 0 0.00890 0.82458 0.29997
NTSC primaries (obsolete)
In 1953, the NTSC standardized a set of primaries used
in experimental color CRTs at that time. Those primaries
and white reference are still documented in ITU-R
Report 624. But phosphors changed over the years,
primarily in response to market pressures for brighter
receivers, and by the time of the first videotape
recorder the primaries actually in use were quite
236 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES