Graphics Reference
In-Depth Information
Figure 8.3.
The chromaticity values of a color.
CIE color specification
for the color
C
based on chromaticity is written CIE (x,y,Y)
(rather than (X,Y,Z)).
Now the tri-stimulus values of this system really have only
relative
meaning
because we do not know what the units are when a color is measured. The CIE spec-
ification gives them an absolute meaning by interpreting Y appropriately. What one
does is to observe that an important characteristic of imaginary red and blue is that they
have zero luminance. Luminance is essentially the intensity. By assigning the lumi-
nance, which can be obtained by a separate measurement, to Y (the imaginary green
value), the tristimulus values can now be given an
absolute
meaning. For example,
for the tuple (X,Y,Z) = (1000,800,1200), if the measurement for the luminance gave Y =
200, then one would adjust the X and Z in proportion and use the tuple (250,200,300)
instead. By convention, the tri-stimulus values of a color are specified with respect to
Y = 100 and if we had Y = 200, that that value would be reported separately.
The CIE chromaticity diagram shown in Figure 8.4 is based on test data for a
narrow 2 degree angle of vision. The diagram is a two-dimensional projection of three-
dimensional color space which plots the x and y values of the CIE (x,y,Y) specifica-
tion of a color. The horseshoe region corresponds to the visible colors. A slightly
different diagram generated in 1964 applied to a 10 degree angle of vision, but is con-
sidered less useful for computer graphics.
8.5
Color Models
The range of colors produced by an RGB monitor is called its
gamut
. There are a
number of models for the gamut of an RGB monitor.
The Color Cube or RGB Model.
This is the “natural” gamut model that represents
the gamut as the unit cube [0,1] ¥ [0,1] ¥ [0,1]. See Figure 8.5.
The CMY Model.
This model uses the
subtractive primary colors
cyan, magenta, and
yellow, which are the complements of red, green, and blue, that is,
C
M
Y
1
1
1
R
G
B
Ê
ˆ
Ê
ˆ
Ê
ˆ
Á
Á
˜
˜
Á
Á
˜
˜
Á
Á
˜
˜
=
-
.
Ë
¯
Ë
¯
Ë
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