CHAPTER 4
248
Graphics
cients are specified by the
Gamma
entry in the color space dictionary (see Table
ABC”
in Figure 4.14 is de-
fined by the dictionary’s
Matrix
entry. Since there is no second transformation
stage, “Decode
LMN”
and “Matrix
LMN”
are implicitly taken to be identity trans-
formations.
TABLE 4.14 Entries in a CalRGB color space dictionary
KEY
TYPE
VALUE
WhitePoint
array
(Required)
An array of three numbers
[
X
W
Y
W
Z
W
]
specifying the tristimulus value,
in the CIE 1931
XYZ
space, of the diffuse white point; see below for further discus-
sion. The numbers
X
W
and
Z
W
must be positive, and
Y
W
must be equal to 1.0.
(Optional)
An array of three numbers
[
X
B
Y
B
Z
B
]
specifying the tristimulus value, in
the CIE 1931
XYZ
space, of the diffuse black point; see below for further discussion.
All three of these numbers must be non-negative. Default value:
[ 0.0 0.0 0.0 ]
.
(Optional)
An array of three numbers
[
G
R
G
G
G
B
]
specifying the gamma for the red,
green, and blue
(A, B,
and
C)
components of the color space. Default value:
[ 1.0 1.0 1.0 ]
.
(Optional)
An array of nine numbers
[
X
A
Y
A
Z
A
X
B
Y
B
Z
B
X
C
Y
C
Z
C
]
specifying
the linear interpretation of the decoded
A, B,
and
C
components of the color space
with respect to the final
XYZ
representation. Default value: the identity matrix
[ 1 0 0 0 1 0 0 0 1 ]
.
BlackPoint
array
Gamma
array
Matrix
array
The
WhitePoint
and
BlackPoint
entries in the color space dictionary control the
overall effect of the CIE-based gamut mapping function described in Section 6.1,
WhitePoint
and
BlackPoint
are mapped to the nearly lightest and nearly darkest achromatic
colors that the output device is capable of rendering in a way that preserves color
appearance and visual contrast.
WhitePoint
is assumed to represent the diffuse achromatic highlight, not a specu-
lar highlight. Specular highlights, achromatic or otherwise, are often reproduced
lighter than the diffuse highlight.
BlackPoint
is assumed to represent the diffuse
achromatic shadow; its value is typically limited by the dynamic range of the in-
put device. In images produced by a photographic system, the values of
WhitePoint
and
BlackPoint
vary with exposure, system response, and artistic in-
tent; hence, their values are image-dependent.