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CHAPTER 7 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 Ei ) 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 E1, … , En of the group—the n constituent objects’ colors,
shapes, opacities, and blend modes
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