iOS: Merging two images, combining the content where it overlaps

Question

I would like to combine two images that partially contain content and otherwise are transparent (alpha = 0). Where the content of the two images overlaps I would like to use half the color value (alpha=0.5) from the first image combined with half the color value of the other image. All pixels that still does not contain content should be transparent. I can't seem to find a convenient way to do this using Core Graphics or Core Image or maybe I am missing something... Does anyone have any tips on how to do this?

Answer 1

If anyone else encounters this problem:

I was able to solve it by using pixel wise processing inspired by this answer https://stackoverflow.com/a/31661519/3652610

and alpha blending described here https://stackoverflow.com/a/727339