SECTION 7.5
559
Specifying Transparency in PDF
•
Objects painted by operators in the transparency group XObject’s content
stream are composited into the group according to the rules described in Sec-
K
) in the group at-
tributes dictionary and the transparency-related parameters of the graphics
state contribute to this computation.
•
If a group color space (
CS
) is specified in the group attributes dictionary, all
painting operators convert source colors to that color space before compositing
objects into the group, and the resulting color at each point is interpreted in
that color space. If no group color space is specified, the prevailing color space
is dynamically inherited from the parent group or page. (If not otherwise spec-
ified, the page group’s color space is inherited from the native color space of the
output device.)
•
After execution of the transparency group XObject’s content stream, the graph-
ics state reverts to its former state before the invocation of the
Do
operator (as it
does for any form XObject). The group’s shape—the union of all objects paint-
ed into the group, clipped by the group XObject’s bounding box—is then paint-
ed into the parent group or page, using the group’s accumulated color and
opacity at each point.
Note:
If the
Do
operator is invoked more than once for a given transparency group
XObject, each invocation is treated as a separate transparency group. That is, the
result is as if the group were independently composited with the backdrop on each
invocation. Applications that perform caching of rendered form XObjects must take
this requirement into account.
The actions described above occur only for a transparency group XObject—a
form XObject having a
Group
entry that designates a group attributes subdic-
tionary whose group subtype (
S
) is
Transparency
. An ordinary form XObject—
one having no
Group
entry—is not subject to any grouping behavior for transpar-
ency purposes. That is, the graphics objects it contains are composited individu-
ally, just as if they were painted directly into the parent group or page.
7.5.6 Patterns and Transparency
In the transparent imaging model, the graphics objects making up the pattern cell
of a tiling pattern (see Section 4.6.2, “Tiling Patterns”) can include transparent
objects and transparency groups. Transparent compositing can occur both within
the pattern cell and between it and the backdrop wherever the pattern is painted.