I know how to do this with QT not sure how to do this with.... CG
Last time I looked there was no way to get Quartz to draw with
straight alpha , only premultiplied. You can use vImage to
unpremultiply the pixels.
I don't believe this is the case. I should write some code to
check it, though.
Part of the information you supply to a bitmap context when you
create it is the alpha channel behavior you desire. At that point
you can specify whether or not you want the context to contain
premultiplied or "straight alpha" pixels. Do you really get
premultiplied even if you ask the context for straight alpha?
It's been some time since I tried this, but I believe that
CGBitmapContextCreate just returns NULL if you try to create one
with, say, kCGImageAlphaFirst.
Jerry
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