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

The problem you've got is depends on whether or not you pre-multiplied your source alpha value by itself as part of your pre-multiplication. If you did, then the srcA you're using in the target multiplications is the square of the real source Alpha, so you need to take the square root for that calculation:

originalSrcA = Math.Sqrt(srcA);
a = ((srcA)) + ((tgtA * (255 - originalSrcA)) >> 8);
r = ((srcR)) + ((tgtR * (255 - originalSrcA)) >> 8);
g = ((srcG)) + ((tgtG * (255 - originalSrcA)) >> 8);
b = ((srcB)) + ((tgtB * (255 - originalSrcA)) >> 8);

If you haven't pre-multiplied by itself (which I think is more likely), you will need to multiply by itself to get the same result as the working one:

a = ((srcA * srcA) >> 8) + ((tgtA * (255 - srcA)) >> 8);
r = ((srcR)) + ((tgtR * (255 - srcA)) >> 8);
g = ((srcG)) + ((tgtG * (255 - srcA)) >> 8);
b = ((srcB)) + ((tgtB * (255 - srcA)) >> 8);