Efficient elasticity for character skinning with contact and collisions

Abstract

Figure 1: Our method takes a geometric internal skeleton (left) and a source surface mesh (not pictured) as input. Based on a hexahedral lattice (center) it then simulates a deformed surface (right) obeying self-collision and volumetric elasticity. The example shown here has 106,567 cells and simulates at 5.5 seconds per frame. AbstractWe present a new algorithm for near-interactive simulation of skeleton driven, high resolution elasticity models. Our methodology is used for soft tissue deformation in character animation. The algorithm is based on a novel discretization of corotational elasticity over a hexahedral lattice. Within this framework we enforce positive definiteness of the stiffness matrix to allow efficient quasistatics and dynamics. In addition, we present a multigrid method that converges with very high efficiency. Our design targets performance through parallelism using a fully vectorized and branch-free SVD algorithm as well as a stable one-point quadrature scheme. Since body collisions, self collisions and soft-constraints are necessary for real-world examples, we present a simple framework for enforcing them. The whole approach is demonstrated in an end-toend production-level character skinning system.

Simulating articulated subspace self-contact

“…In our current work, we have found that it can be applied to the material model of interest [McAdams et al 2011], and more importantly, to self-collisions.…”

“…In addition to their use in feature animation [Clutterbuck and Jacobs 2010], realistic virtual humans also have numerous medical applications [Hirota et al 2001]. Thus, many efficient methods have been developed, including co-rotational elasticity and multigrid solvers [Zhu et al 2010;McAdams et al 2011;Liu et al 2013]. These full-rank methods are complementary to our current approach.…”

“…These full-rank methods are complementary to our current approach. For example, we use the material and contact model from McAdams et al [2011] to generate data for our subspace solver. As these solvers become more efficient, our preprocessing stage will proportionally accelerate.…”

“…However, a variety of other materials cannot be written in this form, particularly the co-rotational material model we use in this paper [McAdams et al 2011]. The main issue is that per-element polar decompositions must be performed, which are non-trivial to formulate as a tensor.…”

“…Penalty Force Formulation: Like many recent works [Tang et al 2012;McAdams et al 2011;Harmon et al 2009], we use a penalty force formulation to resolve contacts. For each penetrating surface triangle and vertex, (tp, vp), we find the surface point vs on the triangle tp that is closest to vp and apply a force to vp,…”