SECTION 4.5
247
Color Spaces
The transformation defined by the
Gamma
and
WhitePoint
entries is
X
=
L
=
X
W
×
A
G
Y
=
M
=
Y
W
×
A
G
Z
=
N
=
Z
W
×
A
G
In other words, the
A
component is first decoded by the gamma function, and the
result is multiplied by the components of the white point to obtain the
L, M,
and
N
components of the intermediate representation. Since there is no second stage,
the
L, M,
and
N
components are also the
X, Y,
and
Z
components of the final rep-
resentation.
The following examples illustrate interesting and useful special cases of
CalGray
spaces. Example 4.5 establishes a space consisting of the
Y
dimension of the CIE
1931
XYZ
space with the CCIR XA/11–recommended D65 white point.
Example 4.5
[ /CalGray
<< /WhitePoint [ 0.9505 1.0000 1.0890 ] >>
]
recommended D65 white point and opto-electronic transfer function.
Example 4.6
[ /CalGray
<< /WhitePoint [ 0.9505 1.0000 1.0890 ]
/Gamma 2.222
>>
]
CalRGB Color Spaces
A
CalRGB
color space is a CIE-based
ABC
color space with only one transforma-
tion stage instead of two. In this type of space,
A, B,
and
C
represent calibrated
red, green, and blue color values. These three color components must be in the
range 0.0 to 1.0; component values falling outside that range are adjusted to the
nearest valid value without error indication. The decoding functions (denoted by
“Decode
ABC”
in Figure 4.14 on page 245) are gamma functions whose coeffi-