Creating Realistic Motion
Jessica K. Hodgins
James F. O'Brien
Nancy S. Pollard
Wayne L. Wooten
PEOPLE ARE SKILLED at perceiving the subtle details of human motion. We can, for example, often identify friends by the style of their walk when they are too far away to be recognizable otherwise. As a result of this skill, we have high standards for the motion of virtual human actors. If synthesized human motion is to be compelling, the virtual actors must appear realistic and move in a natural fashion.
Synthetic human motion is needed for such entertainment applications as computer animation, virtual environments, and video games. We would like to be able to create a Toy Story that starred kids as well as their toys, sports training environments where virtual competitors motivate aspiring sports stars to become better athletes, and video games with appealing and interactive characters. The ability to simulate human motion also has significant scientific applications in full body ergonomics, gait analysis of individuals, and physical rehabilitation.
The task of specifying the motion of an animated object to the computer is surprisingly difficult. Even animating a simple object like a bouncing ball can be challenging, in part because people can quickly pick out motion that is unnatural or implausible without necessarily knowing exactly what is wrong. Animation is also time consuming because very subtle details of the motion must be specified in order to convey the personality of a character or the mood of an animation. A number of techniques have been developed for computer animation, but all the available tools involve a trade-off between automation and control. The techniques can be classified into three basic groups: keyframing, motion capture, and simulation. Keyframing allows a fine level of control but does little to automatically ensure the naturalness of the result. Motion capture and simulation generate motion in a fairly automatic fashion but offer little control over the fine details of the motion.
Keyframing. Borrowing its name from the traditional hand-animation technique, keyframing requires that the animator specify key positions for the objects being animated. The computer then smoothly interpolates to determine the positions for the in-between frames. The characters of Toy Story were animated in this fashion with over seven hundred separate controls for the subtle motions of each main character. The specification of keyframes can be made easier with techniques that aid in the placement of articulated models. For example, if the hand of an animated character must be in a particular location, inverse kinematics allows the computer to calculate appropriate elbow and shoulder angles. While these techniques make animation easier, keyframing still requires that the animator have a detailed understanding of how the animated object should behave over time and have the talent to express that behavior though keyframed con