I have created a drawing app for iPhone, iPad, iPod in which the user can color on top of an outline image (coloring book). I have an erasure that removes all that is on the page, but I would like to create an erasure that would lift some but not all the color that is on the page off, much like a kneaded erasure does in real life. I have searched the net for such a thing but haven't found anything. Does anyone have any suggestions? I was wondering if one could use a texture filter to let only part of the erasure effect the underlying drawing but don't know how to do that. I have tried setting the alpha on a clearColor but there isn't one. HELP!