SECTION 4.5
249
Color Spaces
The transformation defined by the
Gamma
and
Matrix
entries in the
CalRGB
color
space dictionary is
X
=
L
=
X
A
×
A
+
X
B
×
B
+
X
C
×
C
G
R
G
G
G
B
+
Y
B
×
B
+
Y
C
×
C
Y
=
M
=
Y
A
×
A
G
R
G
G
G
B
+
Z
B
×
B
+
Z
C
×
C
Z
=
N
=
Z
A
×
A
G
R
G
G
G
B
In other words, the
A, B,
and
C
components are first decoded individually by the
gamma functions. The results are treated as a three-element vector and multi-
plied by
Matrix
(a 3-by-3 matrix) to obtain the
L, M,
and
N
components of the in-
termediate representation. Since there is no second stage, these are also the
X, Y,
and
Z
components of the final representation.
CalRGB
color space for the CCIR XA/11–
recommended D65 white point with 1.8 gammas and Sony Trinitron phosphor
chromaticities.
Example 4.7
[ /CalRGB
<< /WhitePoint [ 0.9505 1.0000 1.0890 ]
/Gamma [ 1.8000 1.8000 1.8 000]
/Matrix [ 0.4497 0.2446 0.0252
0.3163 0.6720 0.1412
0.1845 0.0833 0.9227
]
>>
]
In some cases, the parameters of a
CalRGB
color space may be specified in terms
of the CIE 1931 chromaticity coordinates (x
R
,
y
R
), (x
G
,
y
G
), (x
B
,
y
B
) of the red,
green, and blue phosphors, respectively, and the chromaticity (x
W
,
y
W
) of the dif-
fuse white point corresponding to some linear
RGB
value
(R, G, B),
where usually
R
=
G
=
B
= 1.0. Note that standard CIE notation uses lowercase letters to specify