Laurie Knapp notes: “In AE, you can’t apply a track to a mask shape. But sometimes, you really want to. In this example, I needed to get rid of a background light that showed up in a chromakey. Because of the nature of the key, using roto and paint were not precise enough because I had to work frame-by-frame. What I needed was to make one mask, and then track it to the movement of the subject.”
Thanks for this excellent tutorial! That is a great method. Your explanation is simple and elegant. It’s worth noting that the basic scale, position and rotation info generated from a Mocha track can be applied to a solid in the manner you’re describing for what could be an even closer first pass. Plus an added benefit with the Mocha plugin is that you can track the shape as a whole, and so perhaps not need to lose the track when a specific point exits the screen. Also, I think a shoutout to Pete O’Connell, whose “Advanced Roto for After Effects” describes this technique in detail and adds a few expressions to help in different situations, is due.
Lastly, you mention that the solid should be larger than the comp size. That is a very good point, and there’s no reason, I think, that it shouldn’t be a LOT larger (4x comp size, why not?) , to avoid the gotcha of running out of space in your solid.
Best,
Chris Hart