A new sharp-crease bending element for folding and wrinkling surfaces and volumes

Abstract

We present a novel sharp-crease bending element for the folding and wrinkling of surfaces and volumes. Based on a control curve specified by an artist or derived from internal stresses of a simulation, we create a piecewise linear curve at the resolution of the computational mesh. Then, the key idea is to cut the object along the curve using the virtual node algorithm creating new degrees of freedom, while subsequently reattaching the resulting pieces eliminating the translational degrees of freedom so that adjacent pieces may only rotate or bend about the cut. Motivated by an articulated rigid body framework, we utilize the concepts of pre-stabilization and post-stabilization in order to enforce these reattachment constraints. Our cuts can be made either razor sharp or relatively smooth via the use of bending springs. Notably, our sharp-crease bending elements can not only be used to create pleats in cloth or folds in paper but also to create similar buckling in volumetric objects. We illustrate this with examples of forehead wrinkles and nasolabial folds for facial animation. Moreover, our sharp-crease bending elements re- quire minimal extra simulation time as compared to the underlying mesh, and tend to reduce simulation times by an order of magnitude when compared to the alternative of mesh refinement.