How to erase foreground image to make the background visible using OpenGLES?

I'm learning iPhone development specifically OpenGL ES 2. I'm trying to achieve an effect as in steam draw. I have a background image a开发者_JAVA百科nd a foreground image. When I move the finger over the foreground image the corresponding pixels of the foreground image are erased and the background does get visible. Can someone point me in the right direction on how to implement this? Some guide lines?

You can use GL_STENSIL_TEST when drawing foreground to exclude fragments you choose(or your finger ;))