Okay, say you have a blank image which is coloured completely with white
and another blank image of the same size coloured completely black. You paint a green dot of with 0% hardness on the same position on both images.

How would you be able to take the information from these images and create a blank TRANSPARENT image of the same size as the previous 2 with a green dot blurring into the transparency (varying alpha channel values) of the 3rd image?

Or to put it another way, how do you filter out things from multiple images leaving things that are unchanged with anything else that differs transparent?

Thanks