Graphics Reference
In-Depth Information
Cyan(0, 1, 1)
Blue(0, 0, 1)
White(1, 1, 1)
Magenta(1, 0, 1)
Green(0, 1, 0)
Black(0, 0, 0)
Yellow(1, 1, 0)
Red(1, 0, 0)
Figure 28.24: The RGB cube. Grays lie along the main diagonal.
In this form, grays lie along the main diagonal; moving away from this diagonal
gives increasingly saturated colors. Viewed this way, we are taking a part of the
space of colors and transforming it so that it looks like a rectilinear cube (which is
a skewed parallelepiped in
XYZ
-coordinates). For this reason, people sometimes
refer to an RGB color space, rather than RGB coordinates on colors.
To return to the general (prestandards) case: The color gamut associated with
the RGB color cube depends on the primary colors producible by the display (the
LCD's color stripes or the CRT's phosphors). So an RGB triple like
(
0.5, 0.7, 0.1
)
may represent rather different greenish-yellows on different devices.
Fortunately, we have a universal description—CIE XYZ values—to which we
can convert. Unfortunately, the conversion requires knowing something about the
primary colors of our device. These can be measured with a colorimeter by making
all pixels red, observing the color in XYZ space, that is,
(
X
r
,
Y
r
,
Z
r
)
; then making
all pixels green, observing the XYZ color
(
X
g
,
Y
g
,
Z
g
)
, and then doing the same
for blue to get
(
X
b
,
Y
b
,
Z
b
)
. If we then display
rR
+
gG
+
bB
,
(28.38)
the resultant XYZ color coefficient triple will be
⎡
⎤
⎡
⎤
X
r
X
g
X
b
r
g
b
⎣
⎦
⎣
⎦
.
r
(
X
r
,
Y
r
,
Z
r
)+
g
(
X
g
,
Y
g
,
Z
g
)+
b
(
X
b
,
Y
b
,
Z
b
)=
Y
r
Y
g
Y
b
(28.39)
Z
r
Z
g
Z
b
In other words, the result will be the coefficients of
X
,
Y
, and
Z
in the CIE XYZ
description of the color. If we have two displays with corresponding matrices
M
1
and
M
2
, we can convert the colors of each display to XYZ space with the respec-
tive matrices. Starting with the color
rR
+
gG
+
bB
(28.40)
on display 1, we get to the XYZ color
⎡
⎤
r
g
b
⎣
⎦
,
M
1
(28.41)
which in turn corresponds to the color triple
