Pre-multiplied alpha compositing

Question

I am trying to implement pre-multiplied alpha blending. On this page : What Is Color Blending?, they do explain standard alpha blending but not for pre-multiplied values.

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Answer 1

A wild guess: are you varying the amount of blending (srcA)? If so you must recalculate your pre-multiplied alpha values in the bitmap. If you don't, you will get an add-like effect which could be what you describe.