A kinematic-variational model for animating skin with wrinkles

Realistic skin modeling and animation must include the wrinkling of skin as it is stretched or compressed. We introduce a combination of a kinematic and a variational model to accomplish this. A curvature-driven kinematics forms the wrinkles: the final shape is obtained by minimizing a functional that includes energies or cost for stretching, bending, and self-intersection. In this paper we confine ourselves to curves in the plane as an approximation for contours of a finger. We model the wrinkle as sharp, that is, with infinite curvature, and so we abandon the standard Hookean elasticity in the energy for bending, modifying it so that it remains finite for singular curvature. We also use an 'energy' to ensure that skin surfaces do not interpenetrate each other under bending, instead of geometric methods for collision avoidance. We model the movement of skin with respect to underlying structures such as bones and muscles to allow a more natural effect of skin motion. We illustrate our results by animating a finger where profile curves develop wrinkles. The final model provides the animator with capability to control the location and formation of wrinkles through specification of user-defined parameters such as stretching, bending, spring constants, and curvature evolution rates. Our approach differs from most existing approaches which tend to be based on geometry, texture, and dynamics-based bump mapping.