Overlay two bitmaps and rub out the top one

I'm having trouble trying to create a screen where I have two bitmaps (one on top of the other so it's totally obscuring the lower one) and "rub out" the top one so it reveals the bottom 开发者_StackOverflow中文版one under the path of the finger. I realise I may need 3 separate layers (bottom image, top image and then a custom view that handles the finger tracking) but I can't find a method of manipulating the middle layer from the custom view. Can anyone offer some advice on how I might do this?

You should do the whole thing in one View, see this SO question and its answer for more info and links.