DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Pointer, M.R., “The Gamut of
Real Surface Colours,” in Color
Research and Application 5 (3):
143–155 (Fall 1980).
situation can be corrected by adjusting or calibrating
the monitor to a white reference with a lower color
temperature. (Studio video standards in Japan call for
viewing with a 9300 K white reference; this is apparently
due to a cultural preference regarding the reproduction
of skin tones.)
Gamut
Analyzing a scene with the CIE analysis functions
produces distinct component triples for all colors. But
when transformed into components suitable for a set of
physical display primaries, some of those colors – those
colors whose chromaticity coordinates lie outside the
triangle formed by the primaries – will have negative
component values. In addition, colors outside the
triangle of the primaries may have one or two primary
components that exceed unity. These colors cannot be
correctly displayed. Display devices typically clip signals
that have negative values and saturate signals whose
values exceed unity. Visualized on the chromaticity
diagram, a color outside the triangle of the primaries is
reproduced at a point on the boundary of the triangle.
If a scanner is designed to capture all colors, its
complexity is necessarily higher and its performance is
necessarily worse than a camera designed to capture
a smaller range of colors. Thankfully, the range of colors
encountered in the natural and man-made world is
a small fraction of all of the colors. Although it is necessary
for an instrument such as a colorimeter to measure
all colors, in an imaging system we are generally
concerned with colors that occur frequently.
M.R. Pointer characterized the distribution of
frequently occurring real surface colors. The naturally
occurring colors tend to lie in the central portion of the
chromaticity diagram, where they can be encompassed
by a well-chosen set of physical primaries. An imaging
system performs well if it can display all or most of
these colors. Rec. 709 does reasonably well; however,
many of the colors of conventional offset printing –
particularly in the cyan region – are not encompassed
by all-positive Rec. 709 RGB. To accommodate such
colors requires wide-gamut reproduction.
CHAPTER 22 COLOR SCIENCE FOR VIDEO 255