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     SECTION 7.3                                                       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,
     “Knockout Groups”).

     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 C0 undefined, but the normal compositing formulas take care of that.
     Also, the result computation for C automatically simplifies to C = Cn , 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.

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