OpenGL: How to implement an "eraser" tool?

Question

I'm working on a game for the iPhone that has a drawing/paint mechanic involved and I'm having problems trying to create a tool that would erase things already painted.

The main problem is that the background being painted on is not a solid color but a static image or animation. I've tried using different blending options and logical operations in drawing but nothing seemed to work. I'm new to OpenGL so I must be missing something.

Any tips?

EDIT: To give a little more information, I'm using textures for my brushes and using glVertexPointer() and glDrawArrays() to render them. For example:

glBindTexture(GL_TEXTURE_2D, circleBrush);
glVertexPointer(3, GL_FLOAT, 0, verts);
glTexCoordPointer(2, GL_FLOAT, 0, coords);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

EDIT 2: Unfortunately, stencil buffers are not available on the iPhone. : (

EDIT 3: Framebuffer objects are available on the iPhone and that is the road I took. I haven't fully implemented it yet, but so far it looks like it works the way I wanted it to. Thanks everyone!

Answer 1

Draw a full-screen textured quad over your scene. When the user draws a brush stroke, use glTexSubImage2D to update the your texture.

glReadPixels/glDrawPixels is slow.

Using FrameBufferObjects is even better, but I doubt this extention is available on the iPhone (then again, I don't know for sure, so maybe try it). FBO's allow you to draw directly into a texture as if it were another rendering context.