How to handle alpha in a manual “Overlay” blend operation?

Question

I\'m playing with some manual (walk-the-pixels) image processing, and I\'m recreating the standard \"overlay\" blend. I\'m looking at the \"Photoshop math\" macros here:

Answer 1

Just a guess, but I would try

resultA = 1 - (1-baseAlpha) * (1-blendAlpha)