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251
SECTION 4.5 Color Spaces
TABLE 4.15 Entries in a Lab color space dictionary
KEY TYPE VALUE
WhitePoint array (Required) An array of three numbers [ XW YW ZW ] specifying the tristimulus value,
in the CIE 1931 XYZ space, of the diffuse white point; see “CalRGB Color Spaces” on
page 247 for further discussion. The numbers XW and ZW must be positive, and YW
must be equal to 1.0.
BlackPoint array (Optional) An array of three numbers [ XB YB ZB ] specifying the tristimulus value, in
the CIE 1931 XYZ space, of the diffuse black point; see “CalRGB Color Spaces” on
page 247 for further discussion. All three of these numbers must be non-negative.
Default value: [ 0.0 0.0 0.0 ].
Range array (Optional) An array of four numbers [ amin amax bmin bmax ] 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 ].
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
Figure 4.14 on page 245). Instead, these parameters have constant implicit values.
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 = XW × g ( L )
Y = YW × g ( M )
Z = ZW × g ( N )
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