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                                             535
SECTION 7.3                                                          Transparency Groups



VARIABLE                MEANING

C0                      Color of the group’s backdrop

C                       Computed color of the group, to be used as the source color when
                        the group is treated as an object

f                       Computed shape of the group, to be used as the object shape when
                        the group is treated as an object

α0                      Alpha of the group’s backdrop

α                       Computed alpha of the group, to be used as the object alpha when
                        the group is treated as an object

Note that the opacity is not given explicitly as an argument or result of this func-
tion. Almost all of the computations use the product of shape and opacity (alpha)
rather than opacity alone; therefore, it is usually convenient to work directly with
shape and alpha rather than shape and opacity. When needed, the opacity can be
computed by dividing the alpha by the associated shape.

The result of applying the group compositing function is then treated as if it were
a single object, which in turn is composited with the group’s backdrop according
to the usual formulas. In those formulas, the color, shape, and alpha (C, f, and α)
calculated by the group compositing function are used, respectively, as the source
color Cs , the object shape fj , and the object alpha αj .

The group compositing formulas for a non-isolated, non-knockout group are de-
fined as follows:

• Initialization:
    f g = α g = 0.0
       0     0


• For each group element Ei ∈ G (i = 1, … , n):
                          ⎧ Composite ( C i – 1, α i – 1, E i )                      if E i is a group
     〈 Cs , f j , α j 〉 = ⎨
         i     i     i    ⎩ intrinsic color, shape, and ( shape × opacity ) of E i   otherwise

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