I'm making a particular effect where a ball splashes into a lake....

The splash creates waves that propagate outward. I implemented this with an active object that is created and expands. The lake is a background object in the lowest layer of my frame, and the borders around the lake are contained within a higher layer.....

This works for me, but is not entirely ideal. For performance reasons, I need to keep my graphics small for mobile development. The whole play area is filled with a quick backdrop pattern in the lowest layer, but the lake borders need to line up with the pattern. Not too ideal...

Another issue occurs when two lakes are close enough that their waves can transfer from one into the other, effectively tunneling beneath the ground....

Anyone know of a way to mask parts of an active object (specific pixels) that do NOT overlap a background object?