Interactive Construction and Animation of Layered Elastic Characters

Russell Turner

Doctoral Thesis, 1993
Swiss Federal Institute of Technology, Lausanne

Keywords

Character Animation, Physics-Based Animation, Deformation, Elasticity, 3D Interaction.

Abstract

Traditional character animation is a highly-developed art form, but it has several creative and economic limitations. Computer animation techniques have had great success in other aspects of commercial animation, but have only recently begun to make inroads into the area of character animation. To animate three-dimensional computer models of characters, a variety of issues must be addressed. Movement results not only from physical structure of the character, but also from its higher-level control. Animated characters are modeled after real-life animals or humans who are composed of both rigid interior skeletons and deformable exteriors. Articulated figure animation techniques are now established and can model the skeleton quite well, while deformable surface models for the skin are more difficult and have not been as successful. Layered construction techniques attempt to combine these two approaches. Physically based models use physical simulation as a modeling and animating tool. They can generate a rich variety of realistic or surrealistic behavior which can be controlled by the animator through intuitive physical parameters and constraint techniques. With high-speed workstations, they can be simulated in real time, allowing interactive control. Layered elastic models add physically-based deformable components to layered 3D characters, resulting in more natural-looking deformation and movement. However, proposed models do not allow for independent layers of muscle, fat and skin.

To overcome these limitations, an original 3D character model is presented. In this new type of layered construction technique, called the elastic surface layer model, a simulated elastically deformable skin surface is wrapped around a traditional kinematic articulated figure. Unlike previous layered models, the skin is free to slide along the underlying surface layers constrained by reaction forces which push the surface out and spring forces which pull the surface in to the underlying layers. By tuning the parameters of the physically-based model, a variety of surface shapes and behaviors can be obtained such as more realistic-looking skin deformation at the joints, the effects of fat, skin sliding over muscles, and dynamic skin effects. Since the elastic model derives all of its input forces from the underlying articulated figure, the animator may specify all of the physical properties of the character once, during the initial character design process, after which a complete animation sequence can be created using a traditional skeleton animation technique. The elastic surface layer model is demonstrated using an interactive animation system designed for studying layered character models with elastic components. The system, called LEMAN (Layered Elastic Model ANimation), allows three-dimensional animated characters to be built up from successive layers of skeleton, muscle, fat and skin in a completely interactive, direct-manipulation environment, using multi-dimensional input devices.