Human beings are very graceful creatures at times. The complexity and elegance of their movements are often a joy to behold.
However, people don’t move in straight, even lines; in respect to isolating the human figures for roto work, this can create something of a hassle. There is very little economy of movement where humans are concerned, and basic human gestures rarely conform to the linear in-betweens generated by a computer.
Generally speaking, you can break human movement into easily definable stopping points and direction changes, which would imply that motion-based key framing is the way to go. However, the complexity and subtle movement that drive the human body can be hard to nail down. No one key framing technique ...