Travelling matte?

[timf32887620]

I'm new to AE.  I want to create a moving mask which follows a shape (a human body) so that I can mask out the background and replace it. How?

[Rick Gerard]

The right tool depends on the shot. Rotobrush works well for shots with sufficient contrast and edge detail. Mocha AE has excellent surface tracking and can be used to generate animated masks or track mattes for more challenging shots. You can always animate a mask. 

Show us your shot, and we can suggest the most efficient workflow.