Physics-Based Animation

D. K. Pai, A. Rothwell, P. Wyder-Hodge, A. Wick, Y. Fan, E. Larionov, D. Harrison, D. R. Neog, and C. Shing

Simulating how the human body deforms in contact with clothing, wearables, and other objects is of central importance to many fields. However, the tissue material properties needed to accurately simulate real human bodies had been sorely lacking. We showed that these mechanical properties can be directly measured using a novel hand-held device. We have developed a complete pipeline for measurement, modelling, parameter estimation, and simulation using the finite element method. Our unique data may be used to create personalized models of an individual human or of a population. Consequently, our methods may have many potential applications in apparel design, e-commerce, computer animation, and medicine.

The Human Touch: Measuring Contact with Real Human Soft Tissues

Tassilo Kugelstadt, Dan Koschier, Jan Bender

In this paper we present a novel operator splitting approach for corotated FEM simulations. The deformation energy of the corotated linear material model consists of two additive terms. The first term models stretching in the individual spatial directions and the second term describes resistance to volume changes. By formulating the backward Euler time integration scheme as an optimization problem, we show that the first term is invariant to rotations. This allows us to use an operator splitting approach and to solve both terms individually with different numerical methods. The stretching part is solved accurately with an optimization integrator, which can be done very efficiently because the system matrix is constant over time such that its Cholesky factorization can be precomputed. The volume term is solved approximately by using the compliant constraints method and Gauss-Seidel iterations. Further, we introduce the analytic polar decomposition which allows us to speed up the extraction of the rotational part of the deformation gradient and to recover inverted elements. Finally, this results in an extremely fast and robust simulation method with high visual quality that outperforms standard corotated FEMs by more than two orders of magnitude and even the fast but inaccurate PBD and shape matching methods by more than one order of magnitude without having their typical drawbacks. This enables a very efficient simulation of complex scenes containing more than a million elements.

Fast Corotated FEM using Operator Splitting

Xiaowei He, Huamin Wang, Enhua Wu

Unified simulation of versatile elastoplastic materials and different dimensions offers many advantages in animation production, contact handling, and hardware acceleration. The unstructured particle representation is particularly suitable for this task, thanks to its simplicity. However, previous meshless techniques either need too much computational cost for addressing stability issues, or lack physical meanings and fail to generate interesting deformation behaviors, such as the Poisson effect. In this paper, we study the development of an elastoplastic model under the state-based peridynamics framework, which uses integrals rather than partial derivatives in its formulation. To model elasticity, we propose a unique constitutive model and an efficient iterative simulator solved in a projective dynamics way. To handle plastic behaviors, we incorporate our simulator with the Drucker-Prager yield criterion and a reference position update scheme, both of which are implemented under peridynamics. Finally, we show how to strengthen the simulator by position-based constraints and spatially varying stiffness models, to achieve incompressibility, particle redistribution, cohesion, and friction effects in viscoelastic and granular flows. Our experiments demonstrate that our unified, meshless simulator is flexible, efficient, robust, and friendly with parallel computing.

Projective peridynamics for modeling versatile elastoplastic materials