Position-based Skin Deformations for Interactive Character Animation

Abstract

Skeletal animation is a widely used technique for animating articulated characters, such as humans and animals. In skeleton-based animation, skinning is the process of defining how the geometric surface of the character deforms according to a function of the skeletal poses. One of the fundamental aspects when animating articulated character is the production of flesh-like deformations for the soft tissues when the character is moving. Creating believable and compelling skin deformations is the central challenge of animated feature films, computer games, and interactive applications. Traditionally, the skin deformations are driven by an underlying skeleton.

The idea can be formulated with a simple expression that binds the character’s skin mesh with its underlying skeleton, whose bones can be transformed in order to obtain a smooth non-rigid deformation of the surrounding mesh. The deformation of each mesh vertex is computed as a weighted blend of the bones transformations. This technique does not generate realistic deformations and it usually suffers from unsightly artefacts. Moreover, skeleton-based deformation methods are incapable of capturing secondary motion effects, such as volume preservation, skin contact effects and the jiggling behaviors of soft tissues when the character is moving. In contrast, by employing a physically based method into the skinning process, the believability and realism of character motions are highly enhanced. Physics-based simulations manage to bring skeleton-driven deformations beyond the purely kinematic approach by simulating secondary motions. Despite offering such interesting effects, physics-based simulation requires complex and intensive computations, and thus it is usually avoided in interactive applications such as computer games. Furthermore, once the deformation parameters are specified in the simulation, it is difficult to control the actual resulting shape of the character in every animation frame.

In this dissertation, we address the problem of creating believable mesh-based skin deformation for soft articulated characters. We present a novel two-layered deformation framework, which is able to mimic the macro-behaviors of the skin and capture secondary effects, such as volume conservation and jiggling. While minimizing the manual post-processing time, our system provides the artist with some level of control over the secondary effects. Our system is practical, relatively easy to implement and fast enough for real-time applications. We also introduce an efficient method for detecting collisions and self-collisions on articulated models, in which we exploit the skeletal nature of the deformation to achieve a good real-time performance. The output of the collision detection algorithm is used to enhance our layered skin deformation with responsive contact handling, and supports contact skin deformation between skin parts.