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      SECTION 7.3                                                      Transparency Groups



      shapes of its constituent objects and whose color and opacity are the result of the
      compositing operations. This object is then composited with the group’s back-
      drop in the usual way.

      In addition to its computed color, shape, and opacity, the group as a whole can
      have several further attributes:

      • All of the input variables that affect the compositing computation for individu-
        al objects can also be applied when compositing the group with its backdrop.
        These variables include mask and constant shape, mask and constant opacity,
        and blend mode.
      • The group can be isolated or non-isolated, determining the initial backdrop
        against which its stack is composited.
      • The group can be knockout or non-knockout, determining whether the objects
        within its stack are composited with one another or only with the group’s back-
        drop.
      • An isolated group can specify its own blending color space, independent of that
        of the group’s backdrop.
      • Instead of being composited onto the current page, a group’s results can be used
        as a source of shape or opacity values for creating a soft mask (see Section 7.4,
        “Soft Masks”).

      The next section introduces some notation for dealing with group compositing.
      Subsequent sections describe the group compositing formulas for a non-isolated,
      non-knockout group and the special properties of isolated and knockout groups.


7.3.1 Notation for Group Compositing Computations

      Since we are now dealing with multiple objects at a time, it is useful to have some
      notation for distinguishing among them. Accordingly, the variables introduced
      earlier are altered to include a second-level subscript denoting an object’s position
      in the transparency stack. Thus, for example, C s stands for the source color of
                                                           i
      the ith object in the stack. The subscript 0 represents the initial backdrop; sub-
      scripts 1 to n denote the bottommost to topmost objects in an n-element stack. In
      addition, the subscripts b and r are dropped from the variables Cb , fb , qb , αb , Cr,
      fr, qr, and αr ; other variables retain their mnemonic subscripts.

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