SECTION 7.3
537
Transparency Groups
VARIABLE
MEANING
f
k
f
g
i
Constant shape for element
E
i
Group shape: the accumulated source shapes of group elements
E
1
to
E
i
, excluding the initial backdrop
i
i
q
m
q
k
Mask opacity for element
E
i
Constant opacity for element
E
i
Source alpha for element
E
i
Object alpha for element
E
i
: the product of its object shape and ob-
ject opacity
Group alpha: the accumulated source alphas of group elements
E
1
to
E
i
, excluding the initial backdrop
Accumulated alpha after compositing element
E
i
, including the ini-
tial backdrop
Source color for element
E
i
Accumulated color after compositing element
E
i
, including the ini-
tial backdrop
Blend function for element
E
i
i
α
s
α
j
i
i
α
g
α
i
i
C
s
C
i
i
B
i
(
C
i
–
1
,
C
s
)
i
Note that the elements of a group are composited onto a backdrop that includes
the group’s initial backdrop. This is done to achieve the correct effects of the
blend modes, most of which are dependent on both the backdrop and source col-
ors being blended. (This feature is what distinguishes non-isolated groups from
isolated groups, discussed in the next section.)
Special attention should be directed to the formulas at the end that compute the
final results,
C, f,
and
α
, of the group compositing function. Essentially, these for-
mulas remove the contribution of the group backdrop from the computed results.
This ensures that when the group is subsequently composited with that backdrop