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                                                                                  541
SECTION 7.3                                                                                 Transparency Groups



    computation is essentially the same as the unsimplified color compositing for-
    mula given in Section 7.2.5, “Interpretation of Alpha,” but using a source shape
    of 1.0.)
α t = Union ( α g , q s )
                 b     i

Ct = ( 1 – qs ) × αb × Cb + qs × ( ( 1 – αb ) × Cs + αb × Bi ( Cb , Cs ) )
             i                i                   i                   i

2. Compute a weighted average of this result with the object’s immediate back-
   drop, using the source shape as the weighting factor. Then normalize the result
   color by the result alpha:

αg = ( 1 – fs ) × αg                                + fs × αt
    i                      i               i–1          i

 α i = Union ( α 0 , α g )
                                              i
       ( 1 – fs ) × αi – 1 × Ci – 1 + fs × Ct
                      i                                                      i
 C i = ----------------------------------------------------------------------------------
                                              α     i

This averaging computation is performed for both color and alpha. The formulas
above show this averaging directly. The formulas in Section 7.3.7, “Summary of
Group Compositing Computations,” are slightly altered to use source shape and
alpha rather than source shape and opacity, avoiding the need to compute a
source opacity value explicitly. (Note that Ct there is slightly different from Ct
above: it is premultiplied by f s .)
                                                               i

The extreme values of the source shape produce the straightforward knockout
effect. That is, a shape value of 1.0 (inside) yields the color and opacity that result
from compositing the object with the initial backdrop. A shape value of 0.0 (out-
side) leaves the previous group results unchanged. The existence of the knockout
feature is the main reason for maintaining a separate shape value rather than only
a single alpha that combines shape and opacity. The separate shape value must be
computed in any group that is subsequently used as an element of a knockout
group.

A knockout group can be isolated or non-isolated; that is, isolated and knockout
are independent attributes. A non-isolated knockout group composites its top-
most enclosing element with the group’s backdrop. An isolated knockout group
composites the element with a transparent backdrop.

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