After Effects prior to release 8 used to let you keyframe the first mask and then draw a new completely different mask every frame. You could actually to from a three point mask to a 100 point mask from frame to frame and it didn't care. It was a better way of handling it than the interpolation stunt they have going now. As a creator I liked having as much control as possible and in my opinion this limitation broke the rotoscoping tool.
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