i think if i were to try this i would look for a gradient or mask that gives u even blend from 100% opacity to 50% 25% or 0% opacity.then when u find one u like make 4 copies . make 4 flags the same size as ur masks. then try useing the image arithmetic tool. image a (flag) to image B(mask) and u may have some results.then map out in ur mind how to layer these together.just guessing mind u.