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those textures will be rendered correctly. however this will of course mess up if you set the alpha value of some object manually, e.g. through some effect. so I suppose the alpha value is premultiplied somewhere.
so when drawing the image(CGContextDrawImage) in that context it will be premultiplied.
so that's the cause. now the question is what will be a decent solution
this code will just remove the premultiplied alpha, after drawing the image into the context.
that's also how the SDL and the OSG guys did it:
Thanks for the patch, I'll integrate it tonight!