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      SECTION 7.3                                                        Transparency Groups



      VARIABLE               MEANING

      fk                     Constant shape for object i
           i


      fi                     Result shape after compositing object i

      qs                     Source opacity for object i
           i

      qj                     Object opacity for object i
           i


      qm                     Mask opacity for object i
                   i

      qk                     Constant opacity for object i
               i

      qi                     Result opacity after compositing object i

      αs                     Source alpha for object i
               i


      αi                     Result alpha after compositing object i

      Cs                     Source color for object i
               i


      Ci                     Result color after compositing object i

      B i ( C i – 1, C s )   Blend function for object i
                       i



7.3.2 Group Structure and Nomenclature

      As stated earlier, the elements of a group are treated as a separate transparency
      stack, the group stack. These objects are composited against a selected initial
      backdrop (to be described) and the resulting color, shape, and opacity are then
      treated as if they belonged to a single object. The resulting object is in turn com-
      posited with the group’s backdrop in the usual way.

      This computation entails interpreting the stack as a tree. For an n-element group
      that begins at position i in the stack, it treats the next n objects as an n-element
      substack, whose elements are given an independent numbering of 1 to n. These
      objects are then removed from the object numbering in the parent (containing)

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