CHAPTER 7
568
Transparency
The value of the blend function
B
(c
b
,
c
s
) in the CompatibleOverprint mode is
either
c
b
or
c
s
, depending on the setting of the overprint mode parameter, the
current and group color spaces, and the source color value
c
s
:
•
If the overprint mode is 1 (nonzero overprint mode) and the current color
space and group color space are both
DeviceCMYK
, then only process color
components with nonzero values replace the corresponding component values
of the backdrop. All other component values leave the existing backdrop value
unchanged. That is, the value of the blend function
B
(c
b
,
c
s
) is the source com-
ponent
c
s
for any process (
DeviceCMYK
) color component whose (subtractive)
color value is nonzero; otherwise it is the backdrop component
c
b
. For spot col-
or components, the value is always
c
b
.
•
In all other cases, the value of
B
(c
b
,
c
s
) is
c
s
for all color components specified
in the current color space, otherwise
c
b
. For instance, if the current color space
is
DeviceCMYK
or
CalRGB
, the value of the blend function is
c
s
for process color
components and
c
b
for spot components. On the other hand, if the current col-
or space is a
Separation
space representing a spot color component, the value is
c
s
for that spot component and
c
b
for all process components and all other spot
components.
Note:
In the descriptions above, the term
current color space
refers to the color
space used for a painting operation. This may be specified by the current color space
parameter in the graphics state (see Section 4.5.1, “Color Values”), implicitly by col-
or operators such as
rg
ColorSpace
en-
try of an image XObject (Section 4.8.4, “Image Dictionaries”). In the case of an
Indexed
space, it refers to the base color space (see “Indexed Color Spaces” on page
Separation
and
DeviceN
spaces that revert to their alternate color
space, as described under “Separation Color Spaces” on page 264 and “DeviceN
If the current blend mode when CompatibleOverprint is invoked is any mode
other than
Normal
, the object being painted is implicitly treated as if it were
defined in a non-isolated, non-knockout transparency group and painted using
the CompatibleOverprint blend mode. The group’s results are then painted using
the current blend mode in the graphics state.
Note:
It is not necessary to create such an implicit transparency group if the current
blend mode is
Normal
; simply substituting the CompatibleOverprint blend mode
while painting the object produces equivalent results. There are some additional
cases in which the implicit transparency group can be optimized out.