So here is my workspace. My character is covered in black and then they roll across the floor so that the black transfers from their body onto the floor and spreads throughout the whole space. So by the end they are naked and the space has gone from white to black. I've edited the paint disappearing from their body as they roll and now just need to create the ink/paint stuff on the floor. How can I create this? It doesn't ened to look liquidy, although that would be good, plain black matte would be fine.
I would probably start by adding a colored solid to the top of the timeline, change the blend mode to something like Multiply so you can see through it, and then start animating a mask or masks to rotoscope the body as it moves. You might even be able to use Rotobrush. The idea is to get to the point where you have a track matte that follows the body as it moves.
Then you would expand or animate the roto so that the matte grows as the person rolls. You then could add roughen edges, more hand masking or other effects to get the matte to grow the way you want. The actor would then be separated from the background, maybe using the original roto and the animated matte you created would be used as a track matte to reveal your ink texture layer.
Sorry, there's nothing that I know of that would do this in any kind of automated process. It's going to take some masking and painting skills..
Fun concept! Post a link to the finished version.