Additive Curve Fixing

To this end, I developed a process I call Additive Curve Fitting. Once motion capture is mapped to a skeleton, it is fed into a "data holding tank". This data tank can be anything that can hold keyframe data, what that holder is can vary from software to software. For instance, in Alias|Wavefront's Maya, the software which served as the development platform for this technique, adding dynamic attributes to each joint in the skeleton containing animation data.

From here, the animator has complete control over where and when keys are added. A simple press of a button queries the data held in the data tank, and sets a corresponding key in the "real" channel. This data includes not only the obvious time and value of the key, but the tangency as well. Of course, your in game animation system will have to support these features to take full advantage of this.

To use this technique, an animator can slide back and forth in time, adding keys from the data tank, or by hand posing the skeleton. This allows for infinite control and editing of any aspect of the motion, while being in precise control of the data density of the final game asset. In the case of our in house export and in-game animation system, Bridge and Olema, we can even retain all the motion capture data in Maya binary file, without increasing the size of our export. Because of this, we can easily and quickly add keys to our "real" animation curves, balancing export size and motion quality.

	Figure 2: Dynamic Attributes in Maya

If this methodology sounds familiar, it is effectively what any animator does. Breaking down motion into key poses, then layering on the details. The major difference here is that the pose is pulled from the motion capture, not the animator. Of course, at any time the animator can alter the pose to refine or edit the motion. And because we are working with motion capture, we can be assured the timing of the animation is true to life. This dramatically speeds up the asset creation. A first pass at an average asset takes a skilled animator 30 minutes to an hour, usually with an additional 2 hours of refinement to a final in game asset. Other difficulties associated with using motion capture for in game assets, such as looping, are no more challenging than with a traditional animation methodology.

	Figure 3: A comparison of animation curves before and after additive curve fitting.

As you can see, motion capture is nothing to fear. It allows for a speedy and cost effective solution to realistic human motion. It improves our skills as we deal with the accurate timing of recorded motion. But most importantly it frees us as animators to concentrate on animating the things which can't be captured. True, it presents some technical challenges, but they are not insurmountable. Motion capture is not the tool of evil many animators make it out to be. It is a tool of the digital age, and like any tool, once you learn how to use it effectively, you won't remember how you lived without it.

References

Understanding Motion Capture for Computer Animation and Video Games
Alberto Menache
Morgan Kaufmann Publishers
ISBN: 0124906303

The Illusion of Life: Disney Animation
Frank Thomas and Ollie Johnston
Hyperion
ISBN: 0786860707

Special Thanks to Tom Harper Alias|Wavefront, Bryan Ewert H20, Julian Mann Double Negative, Andre Bustanoby Digital Domain, the whole of House of Moves, and most especially Factor 5.