Animating sand, mud, and snow

Computer animations often lack the subtle environmental changes that should occur due to the actions of the characters. Squealing cai tires usually le ave no skid marks, airplanes rarely leave jet trails in the sky, and most r unners leave no footprints. In this paper, we describe a simulation model o f ground surfaces that can be deformed by the impact of rigid body models o f animated characters. To demonstrate the algorithms, we show footprints ma de by a runner in sand, mud, and snow as well as bicycle tire tracks, a bic ycle crash, and a falling runner. The shapes of the footprints in the three surfaces are quite different, but the effects were controlled through only five essentially independent parameters. To assess the realism of the resu lting motion, we compare the simulated footprints to human footprints in sa nd.