SECTION 7.5
547
Specifying Transparency in PDF
G
can be any kind of group—isolated or not, knockout or not—producing vari-
ous effects on the
C
result in each case. The color
C
is then converted to luminos-
ity in one of the following ways, depending on the group’s color space:
•
For CIE-based spaces, convert to the CIE 1931
XYZ
space and use the
Y
com-
ponent as the luminosity. This produces a colorimetrically correct luminosity.
In the case of a PDF
CalRGB
space, the formula is
Y
=
Y
A
×
A
G
R
+
Y
B
×
B
G
G
+
Y
C
×
C
G
B
using components of the
Gamma
and
Matrix
entries of the color space dictio-
nary (see Table 4.14 on page 248). An analogous computation applies to other
CIE-based color spaces.
•
For device color spaces, convert the color to
DeviceGray
by device-dependent
means and use the resulting gray value as the luminosity, with no compensa-
tion for gamma or other color calibration. This method makes no pretense of
colorimetric correctness; it merely provides a numerically simple means to pro-
duce continuous-tone mask values. Here are some recommended formulas for
converting from
DeviceRGB
and
DeviceCMYK
, respectively:
Y
=
0.30
×
R
+
0.59
×
G
+
0.11
×
B
Y
=
0.30
× (
1
–
C
) × (
1
–
K
)
+
0.59
× (
1
–
M
) × (
1
–
K
)
+
0.11
× (
1
–
Y
) × (
1
–
K
)
Following this conversion, the result is passed through a separately specified
transfer function, allowing the masking effect to be customized.
The backdrop color most likely to be useful is black, which causes any areas out-
side the group’s shape to have zero luminosity values in the resulting mask. If the
contents of the group are viewed as a positive mask, this produces the results that
would be expected with respect to points outside the shape.
7.5 Specifying Transparency in PDF
The preceding sections have presented the transparent imaging model at an
abstract level, with little mention of its representation in PDF. This section
describes the facilities available for specifying transparency in PDF 1.4.