This can be an awkward mechanism, often requiring the user to precisely reposition a great many control points to effect conceptually simple changes to the surface. Our alternative is based on active surface models that continually try to maximize their smoothness subject to the constraints imposed on them. The user may grab or pin the surface at arbitrary points or along arbitrary curves, using the points and curves as customized shape controls. Complex models are formed by cutting and joining multiple surface sheets. We are also addressing more specialized design applications, such as the analysis and adjustment of tolerances.
Computer animation raises special problems of motion control.
Standard keyframe methods depend entirely on the animator's skill and perseverance to produce pleasing animation. Our methods make it possible to control motion much more directly and economically. For instance, a character's hand may be accurately dragged along a specified path as if it were sliding on a wire, while the direction of gaze is constrained to follow a moving target, and while the feet are kept firmly planted on the ground. This sort of coordinated action is surprisingly difficult to achieve using traditional methods. Motion control of the virtual camera is an important special case.