COMPUTER ANIMATION AND VIRTUAL WORLDS Comp. Anim. Virtual Worlds 2007; 18: 463–472 Published online 13 July 2007 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/cav.185 ........................................................................................... A steering model for on-line locomotion synthesis By Taesoo Kwon and Sung Yong Shin * .......................................................................... For applications such as video games and virtual walk-throughs, on-line locomotion control is an important issue. In general, the user prescribes a sequence of motions one by one while providing an input trajectory. Since the input trajectory lacks in human characteristics, one may not synthesize quality motions by blindly following it. In this paper, we present a novel data-driven scheme for transforming a user-prescribed trajectory to a human trajectory in an on-line manner. As preprocessing, we analyze example motion data to extract human steering behavior. At run-time, the input trajectory is refined to reflect the steering behavior. Together with an existing on-line motion synthesis system, our scheme forms a feedback loop, in which the user effectively specifies an intended human trajectory. Copyright © 2007 John Wiley & Sons, Ltd. Received: 11 May 2007; Accepted: 14 May 2007 KEY WORDS: computer animation; character animation; motion control Introduction On-line, real-time locomotion synthesis is an important issue in applications such as video games and virtual walk-throughs. Although rich researches have been done in locomotion synthesis, the issue of on- line steering of human-like character has not been addressed well. To get a concrete feel, consider Figure 1. Figure 1(a) and (b) show how user-specified trajectories (colored red) are different from actual human trajectories (colored green) during straight walking and running. The oscillations of the actual trajectories are due to the pelvis movements (rotations and translations) caused by supporting feet. Figure 1(c) and (d) show the variations of actual pelvis trajectories during curved walking and running. Such oscillations or curvature variations are the unique characteristics of human steering behavior. Simply placing the pelvis along a user-specified trajectory would not produce a natural motion. This immediately raises an issue: how to incorporate these characteristics into a user-specified trajectory. For on-line applications, the user commonly prescribes a motion by interactively providing a motion type and its *Correspondence to: S. Y. Shin, Korea Advanced Institute of Science and Technology, Taejon,Korea. E-mail: syshin@jupiter.kaist.ac.kr trajectory. In particular, the trajectory is specified either explicitly by a point stream that is sampled with an input device such as a mouse, or implicitly by a force profile that is given with a user interface equipped with slide bars (or a joystick). The former directly produces the trajectory of a human-like figure. Although it is easy to specify, the trajectory itself is neither precise nor smooth. Moreover, it is far from a natural human trajectory. On the other hand, integrating an input force profile that is sampled at each frame, the latter yields a smooth trajectory in an equally easy manner. However, the resulting trajectory is not natural, either. In either case, little effort has been made to produce a natural human trajectory. No matter what method we employ, it would be difficult to generate high-quality locomotion with such a poor trajectory. In this paper, we present a data-driven method for refining a input trajectory for on-line, real- time locomotion synthesis, given the type of a locomotive motion. Choosing the center of the pelvis as the root of an articulated character, we describe how to yield a natural pelvis trajectory from the input trajectory. Without loss of generality, we assume that the input trajectory is given in an explicit form, that is, in the form of a point stream sampled at each frame. The refined trajectory gives the global pelvis position and orientation at each frame. The refinement is performed frame by frame in an on-line manner. Our method performs a two-step ............................................................................................ Copyright © 2007 John Wiley & Sons, Ltd.