CHAPTER 7
558
Transparency
KEY
TYPE
VALUE
I
boolean
(Optional)
A flag specifying whether the transparency group is isolated (see Sec-
true
, objects within the group are
composited against a fully transparent initial backdrop; if
false
, they are com-
posited against the group’s backdrop. Default value:
false
.
In the group attributes dictionary for a page, the interpretation of this entry is
slightly altered. In the normal case in which the page is imposed directly on the
output medium, the page group is effectively isolated and the specified
I
value is
ignored. But if the page is in turn used as an element of some other page, it is
treated as if it were a transparency group XObject; the
I
value is interpreted in
the normal way to determine whether the page group is isolated.
K
boolean
(Optional)
A flag specifying whether the transparency group is a knockout
group (see Section 7.3.5, “Knockout Groups”). If this flag is
false
, later objects
within the group are composited with earlier ones with which they overlap; if
true
, they are composited with the group’s initial backdrop and overwrite
(“knock out”) any earlier overlapping objects. Default value:
false
.
The transparency group XObject’s content stream defines the graphics objects be-
longing to the group. Invoking the
Do
operator on the XObject executes its con-
tent stream and composites the resulting group color, shape, and opacity into the
group’s parent group or page as if they had come from an elementary graphics ob-
ject. When applied to a transparency group XObject,
Do
performs the following
actions in addition to the normal ones for a form XObject (as described in Sec-
•
If the transparency group is non-isolated (the value of the
I
entry in its group
attributes dictionary is
false
), its initial backdrop, within the bounding box
specified by the XObject’s
BBox
entry, is defined to be the accumulated color
and alpha of the parent group or page—that is, the result of everything that has
been painted in the parent up to that point. (However, if the parent is a knock-
out group, the initial backdrop is the same as that of the parent.) If the group is
isolated (
I
is
true
), its initial backdrop is defined to be transparent.
•
Before execution of the transparency group XObject’s content stream, the cur-
rent blend mode in the graphics state is initialized to
Normal
, the current strok-
ing and nonstroking alpha constants to 1.0, and the current soft mask to
None
.
Note:
The purpose of initializing these graphics state parameters at the beginning
of execution is to ensure that they are not applied twice: once when member ob-
jects are painted into the group and again when the group is painted into the par-
ent group or page.