SECTION 4.5
251
Color Spaces
TABLE 4.15 Entries in a Lab 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 “CalRGB Color Spaces” on
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 Color Spaces” on
Default value:
[ 0.0 0.0 0.0 ]
.
(Optional)
An array of four numbers
[
a
min
a
max
b
min
b
max
]
specifying the range of
valid values for the
a*
and
b* (B
and
C)
components of the color space—that is,
a
min
≤
a*
≤
a
max
and
b
min
≤
b*
≤
b
max
Component values falling outside the specified range are adjusted to the nearest valid
value without error indication. Default value:
[
−
100 100
−
100 100 ]
.
BlackPoint
array
Range
array
A
Lab
color space does not specify explicit decoding functions or matrix coef-
ficients for either stage of the transformation from
L*a*b*
space to
XYZ
space
(denoted by “Decode
ABC,
” “Matrix
ABC,
” “Decode
LMN,
” and “Matrix
LMN”
in
The first transformation stage is defined by the equations
L*
+
16
a*
-
-
L
= ----------------- + -------
116
500
L*
+
16
-
M
= -----------------
116
L*
+
16
b*
-
-
N
= ----------------- – -------
116
200
The second transformation stage is given by
X
=
X
W
×
g
(
L
)
Y
=
Y
W
×
g
(
M
)
Z
=
Z
W
×
g
(
N
)