CHAPTER 7
534
Transparency
stack and replaced by the group object, numbered
i,
followed by the remaining
objects to be painted on top of the group, renumbered starting at
i
+
1. This oper-
ation applies recursively to any nested subgroups. Henceforth, the term
element
(denoted
E
i
) refers to a member of some group; it can be either an individual ob-
ject or a contained subgroup.
From the perspective of a particular element in a nested group, there are three
different backdrops of interest:
•
The group backdrop
is the result of compositing all elements up to but not in-
cluding the first element in the group. (This definition is altered if the parent
group is a knockout group; see Section 7.3.5, “Knockout Groups.”)
•
The initial backdrop
is a backdrop that is selected for compositing the group’s
first element. This is either the same as the group backdrop (for a non-isolated
group) or a fully transparent backdrop (for an isolated group).
•
The immediate backdrop
is the result of compositing all elements in the group
up to but not including the current element.
When all elements in a group have been composited, the result is treated as if the
group were a single object, which is then composited with the group backdrop.
(This operation occurs whether the initial backdrop chosen for compositing the
elements of the group was the group backdrop or a transparent backdrop. There
is a special correction to ensure that the backdrop’s contribution to the overall re-
sult is applied only once.)
7.3.3 Group Compositing Computations
The color and opacity of a group are defined by the
group compositing function:
〈
C
,
f
,
α
〉
=
Composite
(
C
0
,
α
0
,
G
)
where the variables have the meanings shown in Table 7.7.
TABLE 7.7 Arguments and results of the group compositing function
VARIABLE
MEANING
G
The transparency group: a compound object consisting of all ele-
ments
E
1
, … ,
E
n
of the group—the
n
constituent objects’ colors,
shapes, opacities, and blend modes