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.

Watch to find out how, or...
1. Precomp FG and BG
2. Track the movement of the subject in the precomp
3. Apply to a null
4. Create a black solid
5. Decrease opacity of solid so you can see through
6. Draw your initial mask on the black solid
7. Raise the opacity back to 100, set transfer mode to Silhouette Alpha, parent to null - now there's a hole
8. Precomp all of this
9. Reinsert your background behind the precomp to fill the hole
10. Adjust the mask shape as needed with keyframes

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