Game Engine Gems 2

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23

AJitter‐TolerantRigidBody

SleepCondition

Eric Lengyel

Terathon Software

23.1Introduction

One of the primary optimizations employed by any physics engine is the ability

to put a rigid body to sleep when it has reached a resting state. A sleeping rigid

body is not processed by the physics engine until some kind of event occurs, such

as a collision or broken contact, necessitating that it is woken up and simulated

once again. The fact that most rigid bodies are in the sleeping state at any one

time is what allows a large number of simulated objects to exist in a game level

without serious performance problems.

A problem faced by all physics engines is how to decide that a rigid body has

actually come to rest. If we put an object to sleep without being sure that it has

come to rest, then we risk accidentally freezing it in mid-simulation, which can

look odd to the player. On the other hand, if we are too conservative and wait for

too strict of a condition to be met before we put an object to sleep, then we can

run into a situation where too many objects are being simulated unnecessarily

and performance suffers.

The sleep decision problem is complicated by the fact that all physics en-

gines exhibit some jitter no matter how good the constraint solver is. If the right

sleep condition isn’t chosen in the design of a physics engine, then jitter can pre-

vent an object from ever going to sleep. Or at the very least, jitter can delay an

object from entering the sleep state, and this generally causes a higher number of

objects to be simulated at any given time. This chapter discusses a simple condi-

tion that can be used to determine when it is the proper time to put a rigid body to

sleep, and it is highly tolerant to jitter.