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SECTION 7.3                                                          Transparency Groups



VARIABLE               MEANING

fk                     Constant shape for element Ei
     i


fg                     Group shape: the accumulated source shapes of group elements E1
     i
                       to Ei , excluding the initial backdrop

qm                     Mask opacity for element Ei
             i

qk                     Constant opacity for element Ei
         i


αs                     Source alpha for element Ei
         i

αj                     Object alpha for element Ei : the product of its object shape and ob-
         i
                       ject opacity

αg                     Group alpha: the accumulated source alphas of group elements E1
         i
                       to Ei, excluding the initial backdrop

αi                     Accumulated alpha after compositing element Ei , including the ini-
                       tial backdrop

Cs                     Source color for element Ei
         i

Ci                     Accumulated color after compositing element Ei , including the ini-
                       tial backdrop

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


Note that the elements of a group are composited onto a backdrop that includes
the group’s initial backdrop. This is done to achieve the correct effects of the
blend modes, most of which are dependent on both the backdrop and source col-
ors being blended. (This feature is what distinguishes non-isolated groups from
isolated groups, discussed in the next section.)

Special attention should be directed to the formulas at the end that compute the
final results, C, f, and α, of the group compositing function. Essentially, these for-
mulas remove the contribution of the group backdrop from the computed results.
This ensures that when the group is subsequently composited with that backdrop

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