CHAPTER 4
246
Graphics
Decode
A
Decode
LMN
L
A
Matrix
A
M
N
Matrix
LMN
X
Y
Z
FIGURE 4.15
Component transformations in a CIE-based
A
color space
A
CalGray
color space is a CIE-based
A
color space with only one transformation
stage instead of two. In this type of space,
A
represents the gray component of a
calibrated gray space. This component must be in the range 0.0 to 1.0. The decod-
ing function (denoted by “Decode
A”
in Figure 4.15) is a gamma function whose
coefficient is specified by the
Gamma
entry in the color space dictionary (see Ta-
A”
in the figure is de-
rived from the dictionary’s
WhitePoint
entry, as described below. Since there is no
second transformation stage, “Decode
LMN”
and “Matrix
LMN”
are implicitly
taken to be identity transformations.
TABLE 4.13 Entries in a CalGray color space dictionary
KEY
TYPE
VALUE
WhitePoint
array
(Required)
An array of three numbers [
X
W
Y
W
Z
W
] specifying the tri-
stimulus value, in the CIE 1931
XYZ
space, of the diffuse white point; see
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 “CalRGB
be non-negative. Default value:
[ 0.0 0.0 0.0 ]
.
(Optional)
A number
G
defining the gamma for the gray
(A)
component.
G
must be positive and is generally greater than or equal to 1. Default value: 1.
BlackPoint
array
Gamma
number