CHAPTER 7
538
Transparency
(possibly with additional shape or opacity inputs or a different blend mode), the
backdrop’s contribution is included only once.
For color, the backdrop removal is accomplished by an explicit calculation, whose
effect is essentially the reverse of compositing with the
Normal
blend mode. The
formula is a simplification of the following formulas, which present this opera-
tion more intuitively:
(
1
–
α
g
) ×
α
0
n
-
φ
b
= --------------------------------------
Union
(
α
0
,
α
g
)
n
C
n
–
φ
b
×
C
0
C
= ------------------------------
-
1
–
φ
b
where
φ
b
is the
backdrop fraction,
the relative contribution of the backdrop color
to the overall color.
For shape and alpha, backdrop removal is accomplished by maintaining two sets
of variables to hold the accumulated values. The group shape and alpha,
f
g
and
α
g
, accumulate only the shape and alpha of the group elements, exclud-
i
i
ing the group backdrop. Their final values become the group results returned by
the group compositing function. The complete alpha,
α
i
, includes the backdrop
contribution as well; its value is used in the color compositing computations.
(There is never any need to compute the corresponding complete shape,
f
i
, that
includes the backdrop contribution.)
As a result of these corrections, the effect of compositing objects as a group is the
same as that of compositing them separately (without grouping) if the following
conditions hold:
•
The group is non-isolated and has the same knockout attribute as its parent
group (see Sections 7.3.4, “Isolated Groups,” and 7.3.5, “Knockout Groups”).
•
When compositing the group’s results with the group backdrop, the
blend mode is used, and the shape and opacity inputs are always 1.0.
Normal
7.3.4 Isolated Groups
An
isolated group
is one whose elements are composited onto a fully transparent
initial backdrop rather than onto the group’s backdrop. The resulting source