[cairo] Cairo + GdkPixbuf

Jonathan Morton jonathan.morton at movial.com
Wed Jun 30 13:09:47 PDT 2010

>> Can you explain how experience moves one to favour premultiplied
>> alphas?

Very simply, for performance reasons.  The calculations required per
pixel (for anything more demanding than fills and copies) are
simplified if all of the images involved are premultiplied, so it all
goes faster.

Photo manipulation programs usually keep images in un-premultiplied
form because, as you say, it preserves the most information for the
artist's benefit.  But when calculating the filters, they will
probably convert a copy to premultiplied form to simplify and speed
things up.

 - Jonathan Morton

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