SECTION 7.3
539
Transparency Groups
color, object shape, and object alpha for the group are therefore independent of
the group backdrop. The only interaction with the group backdrop occurs when
the group’s computed color, shape, and alpha are then composited with it.
In particular, the special effects produced by the blend modes of objects within
the group take into account only the intrinsic colors and opacities of those ob-
jects; they are not influenced by the group’s backdrop. For example, applying the
Multiply
blend mode to an object in the group produces a darkening effect on
other objects lower in the group’s stack but not on the group’s backdrop.
Plate 17 illustrates this effect for a group consisting of four overlapping circles in a
light gray color (C =
M
=
Y
= 0.0;
K
= 0.15). The circles are painted within the
group with opacity 1.0 in the
Multiply
blend mode; the group itself is painted
against its backdrop in
Normal
blend mode. In the top row, the group is isolated
and thus does not interact with the rainbow backdrop. In the bottom row, the
group is non-isolated and composites with the backdrop. The plate also illustrates
the difference between knockout and non-knockout groups (see Section 7.3.5,
The effect of an isolated group can be represented by a simple object that directly
specifies a color, shape, and opacity at each point. This
flattening
of an isolated
group is sometimes useful for importing and exporting fully composited artwork
in applications. Furthermore, a group that specifies an explicit blending color
space must be an isolated group.
For an isolated group, the group compositing formulas are altered by simply add-
ing one statement to the initialization:
α
0
=
0.0
if the group is isolated
That is, the initial backdrop on which the elements of the group are composited is
transparent rather than inherited from the group’s backdrop. This substitution
also makes
C
0
undefined, but the normal compositing formulas take care of that.
Also, the result computation for
C
automatically simplifies to
C
=
C
n
, since there
is no backdrop contribution to be factored out.
7.3.5 Knockout Groups
In a knockout group, each individual element is composited with the group’s
initial backdrop rather than with the stack of preceding elements in the group.