SECTION 7.3
531
Transparency Groups
shapes of its constituent objects and whose color and opacity are the result of the
compositing operations. This object is then composited with the group’s back-
drop in the usual way.
In addition to its computed color, shape, and opacity, the group as a whole can
have several further attributes:
•
All of the input variables that affect the compositing computation for individu-
al objects can also be applied when compositing the group with its backdrop.
These variables include mask and constant shape, mask and constant opacity,
and blend mode.
•
The group can be
isolated
or
non-isolated,
determining the initial backdrop
against which its stack is composited.
•
The group can be
knockout
or
non-knockout,
determining whether the objects
within its stack are composited with one another or only with the group’s back-
drop.
•
An isolated group can specify its own blending color space, independent of that
of the group’s backdrop.
•
Instead of being composited onto the current page, a group’s results can be used
as a source of shape or opacity values for creating a
soft mask
(see Section 7.4,
The next section introduces some notation for dealing with group compositing.
Subsequent sections describe the group compositing formulas for a non-isolated,
non-knockout group and the special properties of isolated and knockout groups.
7.3.1 Notation for Group Compositing Computations
Since we are now dealing with multiple objects at a time, it is useful to have some
notation for distinguishing among them. Accordingly, the variables introduced
earlier are altered to include a second-level subscript denoting an object’s position
in the transparency stack. Thus, for example,
C
s
stands for the source color of
i
the
ith
object in the stack. The subscript 0 represents the initial backdrop; sub-
scripts 1 to
n
denote the bottommost to topmost objects in an
n-element
stack. In
addition, the subscripts
b
and
r
are dropped from the variables
C
b
,
f
b
,
q
b
,
α
b
,
C
r
,
f
r
,
q
r
, and
α
r
; other variables retain their mnemonic subscripts.