So before I say anything, I'm kinda new to AE, and thus I may not understand everything you say to me. Recently I've been experimenting with some old footage. I'd like to make a ball dissapear behind a building. The ball is a png image and the building is on a video, on the layer below. I figured it would be done with masks, but I have no idea where to put one, and I animated the ball so when I put the mask on its layer the mask spins along with it. I have no idea how to fix it
I would use track matte to solve this masking.
I attached a picture to show an example
1. draw a shape where you want the ball to be masked out, over your ball layer
2. set alpha inverted matte for the ball layer