You can read about this here: http://dvcs.w3.org/hg/FXTF/rawfile/tip/compositing/index.html#blending
Read part '5. Blending' until '5.1 Separable blend modes'. It is not that much. To summarize what is said in that part: 1. 'Compositing' is 'blending' followed by some of the Porter-Duff operations. By 'blending' they mean mixing colors and ignoring alpha values. 2. There are different functions that can be applied in the 'blending' step, listed in part 5.1 Separable blend modes. It is those that need non-premultiplied color values. I made this image for the final formula in part 5, to make it easier to see things: http://postimage.org/image/mknubjpdn/
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