A Logic Problem: One Roto for FG and BG

This feels like a logic problem, but maybe it’s more complex.

I am rotoing out an actor from the background as she walks in and the camera tracks her. The goal is to make the BG rotate behind her as she continues to walk in normally (the BG will settle when she stops walking), to indicate impending difficulty. Pretty normal.

As I work on the shot, I expect to refine the roto (a lot) as I experiment with the effects on the BG.

My issue is I only want to have one instance of the track that gets referenced in the effect. So, here’s the logic problem:

In After Effects, I assume I need one layer with just the actor and rotoed out and a second layer with just the background rotating (one layer is a normal matte, second layer is inverted).

It seems that I should precomp the Mocha layer and reference it twice, but each reference is to the same track, right? So, I’m confused how to only have one track using Mocha but keep them separate so I can have the effects on one layer, the actor rotoed out on the other layer but only have one track from Mocha (that I can keep refining independently of the effects I’m apply on the AE layer).

Ugh, I don’t think I’ve described this very well. I can do screen shots if that helps.

Or maybe someone can go, Oh, yeah, I know what you’re trying to do! (I hope.)


For the roto, you will just need the applied matte on the actor in a foreground layer. How you apply this is up to you (Create AE MAsks, use “Apply Matte” etc).

For the background, do you need to clean the background first? As in are you using the same footage the actor was filmed on? If so, you will need to use the remove module to take the actor out first, or rotate in a clean plate of the background that doesn’t have the actor in it before resuming the original footage.
If it’s a different background, then that will not be necessary.

I am using the same footage and understand that I will need to use the Remove Module. (Because the actor is moving away from the door, into the apartment, the BG is constantly shifting, so I can’t simply rotate in a clean plate, as much as I’d like to.)

My issue, at the moment, is I’m just roughing this in, so I’ve put a fairly bad track on the actor and expect to go back to Mocha to clean it up (over and over again).

So, I’m trying to just have the one track in a precomp, rather than two separate Mocha instances.

Maybe this isn’t possible?

You can duplicate the Mocha instances, but you will probably need two it sounds like. One for the roto and one for the track if you’re using Mocha to render the track. If you just need Mocha to apply the track, you can keep it in the same instance of Mocha. However. You still need another instance for the remove.

OK, well that’s sort of what I thought but figured I’d asked. I guess ideally I should be done with the track in Mocha then I can use it as a finished track for roto, remove, mattes or whatever.