This is an old chestnut but I haven't seen it answered in over 10 years. I have 2 layers - the bottom plate is a mop moving in all directions over a kitchen floor. The layer above is an image of dirt and spatters (that the mop will clean).
NB: This is NOT a track mask or rotoscope the mop question.
I want to know how to erase the dirt frame by frame while still be able to see the mop/floor layer.