So I am just using the roto-brush in one of my projects. I've done propagation in the forward direction from the base frame, and I mainly did automatic propagation by pressing play and then stopping the playback when a mistake is made. However I want to do automatic propagation in the backwards direction, so is there some form of reverse playback I can use in order to go in reverse? Or will I have to go frame by frame backwards?
As you can see in the image the entire right side has been done but unsure how to do it in reverse from the base from.
(After Effects CC 18.104.22.168)
Nope, the best option you have is to split the layer at the 40s and start from the beginning.
See these links from the Search field at the top right corner of After Effects searching "rotobrush":
Unfortunately most folks that start using Rotobrush don't understand how to use the tool. It's not very intuitive.
Yes there is a way!
The rotobrush will only propagate so many frames in any direction, I think 10 or 20. You can ram preview forward and watch it go automatically, but it will stop after that many frames and you need to do a corrective stroke to continue.
if you look at the timeline in your layer, there will be a grey bar filled with >>>>>>>>>> or <<<<<<<<<<< this shows the propagation range.
All you have to do is set your play head anywhere inside that bar, and it will propagate toward the playhead. Forward or reverse.
The only drawback is you won't have any visual feedback until propagation reaches your playhead.
Hope that makes sense!