#30 Missing Screen composite operation

[Brendan Lane]

I've attached a patch implementing the missing Screen composite operation.

If I knew where all the hooks for the composite operations were, I would implement the rest of the missing PDF blend modes (Overlay, ColorDodge, ColorBurn, SoftLight, Exclusion).

[Bob Friesenhahn]

The primary mechanism (besides the actual composition code) for adding a new composition operator is to add to the end of the CompositeOperator enumeration in image.h (if the enumeration name does not already exist), add another case to the switch statement in GetCompositionPixelIteratorCallback() in composite.c (as you did), and add more cases to CompositeOperatorToString() and StringToCompositeOperator() in enum_strings.c. Magick++ and PerlMagick need a few more edits to make new composition operators available. The PerlMagick test suite does test many of the composition operators so it could be extended to verify that the new composition operator produces consistent results.

[Bob Friesenhahn]

Your contribution is now available via Mercurial changeset 14085:dfce90af33f6. Thanks for the excellent work!