SECTION 7.3
533
Transparency Groups
VARIABLE
MEANING
f
k
f
i
i
Constant shape for object
i
Result shape after compositing object
i
Source opacity for object
i
Object opacity for object
i
Mask opacity for object
i
Constant opacity for object
i
Result opacity after compositing object
i
Source alpha for object
i
Result alpha after compositing object
i
Source color for object
i
Result color after compositing object
i
Blend function for object
i
q
s
q
j
i
i
q
m
q
k
q
i
i
i
α
s
α
i
C
s
C
i
i
i
B
i
(
C
i
–
1
,
C
s
)
i
7.3.2 Group Structure and Nomenclature
As stated earlier, the elements of a group are treated as a separate transparency
stack, the group stack. These objects are composited against a selected initial
backdrop (to be described) and the resulting color, shape, and opacity are then
treated as if they belonged to a single object. The resulting object is in turn com-
posited with the group’s backdrop in the usual way.
This computation entails interpreting the stack as a tree. For an
n-element
group
that begins at position
i
in the stack, it treats the next
n
objects as an
n-element
substack, whose elements are given an independent numbering of 1 to
n.
These
objects are then removed from the object numbering in the parent (containing)