Controlling Physics-Based Characters Using Soft Contacts

晃晃

Controlling Physics-Based Characters Using Soft Contacts

Sumit Jain C. Karen Liu

Georgia Institute of Technology

Abstract

In this paper, we investigate the impact of the deformable bodies on

the control algorithms for physically simulated characters. We hy-

pothesize that ignoring the effect of deformable bodies at the site of

contact negatively affects the control algorithms, leading to less ro-

bust and unnatural character motions. To verify the hypothesis, we

introduce a compact representation for an articulated character with

deformable soft tissue and develop a practical system to simulate

two-way coupling between rigid and deformable bodies in a robust

and efficient manner. We then apply a few simple and widely used

control algorithms, such as pose-space tracking control, Cartesian-

space tracking control, and a biped controller (SIMBICON), to sim-

ulate a variety of behaviors for both full-body locomotion and hand

manipulation. We conduct a series of experiments to compare our

results with the motion generated by these algorithms on a char-

acter comprising only rigid bodies. The evaluation shows that the

character with soft contact can withstand larger perturbations in a

more noisy environment, as well as produce more realistic motion.

CR Categories: I.3.7 [Computer Graphics]: Three-Dimensional

Graphics and Realism—Animation;

Keywords: character animation, articulated rigid body, de-

formable body, linear complementarity problem

1 Introduction

One of the fundamental simplifications that researchers in physics-

based human motion synthesis make, is that motion is the product

of an articulated rigid body system with actuated joints representing

bones and active skeletal muscles. On the surface this abstraction

does capture the most fundamental aspects of the human muscu-

loskeletal system. Utilizing this assumption, researchers have de-

veloped several control algorithms that can synthesize movement

for various tasks like balance and walking. Although these con-

trollers work well in their specific problem domain, they still cannot

achieve the same level of agility the human body displays.

In this paper we revisit the fundamental assumption that an articu-

lated rigid body system, by itself, captures the fundamental prop-

erties that enable human-like motion. We focus on one aspect of

the motion that is not captured by this simplified model: the contact

with the environment primarily occurs through the soft tissue. This

factor comes into play in any situation where there is a collision be-

tween the character and the environment. Collisions between rigid

bodies usually result in sporadic contact points and highly discon-

tinuous pressure distribution. Although a character consist of only

rigid bodies is ideal for efficiently simulating human movement,

we postulate that the simplified rigid contact model inadvertently

increases the difficulty in controller design and results in unrealis-

tic motion.



The primary contribution of this paper is to demonstrate that simple

control strategies coupled with the simulation of soft tissue defor-

mation at the site of contact can achieve very robust and realistic

motion. We develop a practical system that allows us to simulate

two-way coupling between rigid and deformable bodies in a robust

and efficient manner. We then apply a few simple and widely used

control algorithms, such as pose-space tracking control, Cartesian-

space tracking control, and SIMBICON, to simulate a variety of

both full-body locomotion and hand manipulation. The resulting

motions are compared with the motion generated by these algo-

rithms on a character comprising only of rigid bodies. These sim-

ple controllers demonstrate that the character with soft contacts can

withstand larger perturbations in a more noisy environment, with-

out the need of designing more sophisticated control algorithms.

Simulating deformable bodies can be achieved in a few different

ways and the design choice often has to balance the required accu-

racy and performance. We hypothesize that the accuracy offered

by sophisticated but expensive methods, such as Finite Element

Method (FEM) is unnecessary for our application for two reasons.

First, unlikemost previous work that simulates deformation of com-

plex volumetric meshes for aesthetic purpose, the primary goal of

our work is to produce deformation for more physically correct con-

tacts. Second, average human body deforms marginally due to the

support of bones. In particular, the deformation due to contacts is

typically small and localized. We take advantage of these proper-

ties to design a simple and accurate model that only computes the

surface of deformable bodies, rather than the entire volume.



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