Footskate Cleanup for Motion Capture Editing

Lucas Kovar, John Schreiner, Michael Gleicher

Summary

While motion capture is commonplace in character animation, often the raw motion data itself is not used. Rather, it is first fit onto a skeleton and then edited to satisfy the particular demands of the animation. This process can introduce artifacts into the motion. One particularly distracting artifact is when the character’s feet move when they ought to remain planted, a condition known as footskate. In this paper we present a simple and efficient algorithm for removing footskate. Our method exactly satisfies footplant constraints while introducing only smooth changes, and it requires no nonlinear optimizations. We avoid certain common artifacts by violating the standard rigidity assumptions and allowing small changes in the lengths of the character's legs.

Videos

(All videos are compressed with Microsoft MPEG4.))

A typical example of footskate in a motion.
Simply performing IK on every frame independently can create severe discontinuities when constraints turn on and off. Our algorithm combats this by blending changes made on constrained frames into neighboring unconstrained frames.
When a foot has just the heel or the ball planted on one frame and then both planted on the next frame, there may be a discontinuity in the orientation of the ankle. Straightforward blending can avoid this problem.
An example of knee popping. Knee popping is an artifact created by attempting to change the distance between the hip and ankle by when the leg is near full extension. A large change in the knee angle is required to move the ankle by a small amount, causing the knee to "pop". The IK solver used by our agorithm directly addresses this problem.
In order to be able to reach the constraint locations, it may be necessary to move the root. Typically the root is projected to the closest point that allows the ankle to reach the target position. However, when constraints turn on and off this projection can change discontinuously. We avoid this problem by filtering the projections and stretching the leg to make up any residual distance.
A comparison between our algorithm and the "reach-feet" mode of Kaydara's FILMBOX 3, a popular software package for processing motion capture.
Our algorithm can be used to salvage motions that contain footskate even before any editing is done.
Results of our algorithm applied to linear blend transitions between two motions.
Results of our algorithm applied to a motion that has been modified using path editing.
Results of our algorithm applied to a motion that has been retargeted to a new character.
Cleaning an interpolated motion where both the hands and the feet slide across the ground.

	A typical example of footskate in a motion.
	Simply performing IK on every frame independently can create severe discontinuities when constraints turn on and off. Our algorithm combats this by blending changes made on constrained frames into neighboring unconstrained frames.
	When a foot has just the heel or the ball planted on one frame and then both planted on the next frame, there may be a discontinuity in the orientation of the ankle. Straightforward blending can avoid this problem.
	An example of knee popping. Knee popping is an artifact created by attempting to change the distance between the hip and ankle by when the leg is near full extension. A large change in the knee angle is required to move the ankle by a small amount, causing the knee to "pop". The IK solver used by our agorithm directly addresses this problem.
	In order to be able to reach the constraint locations, it may be necessary to move the root. Typically the root is projected to the closest point that allows the ankle to reach the target position. However, when constraints turn on and off this projection can change discontinuously. We avoid this problem by filtering the projections and stretching the leg to make up any residual distance.
	A comparison between our algorithm and the "reach-feet" mode of Kaydara's FILMBOX 3, a popular software package for processing motion capture.
	Our algorithm can be used to salvage motions that contain footskate even before any editing is done.
	Results of our algorithm applied to linear blend transitions between two motions.
	Results of our algorithm applied to a motion that has been modified using path editing.
	Results of our algorithm applied to a motion that has been retargeted to a new character.
	Cleaning an interpolated motion where both the hands and the feet slide across the ground.