18

Thedynamics code, together with code implementing the ground model, basic pose

motion.

control and the balance control formulation of Chapter 3 is then compiled into the simulator

executable.

The

control

script

provides

the

particular

control

parameters

for

the

desired

simulation. Sample control scripts can be found in Appendices B and C.

The primary outputs of

each simulation are the final balanced motion of the creature and the aperiodic PCG which was

ultimately responsible for generating it.

Note that the resulting aperiodic PCG output provides

open loop control.

It is therefore only reusable given an identical initial state.

In essence it is a

record of the applied control actions for the motion, already complete with feedback actions.

The dynamics compiler used is a commercially available software package [SDFAST].

The

animation environment currently supports the simulation

and

control

of

a

single

articulated

creature consisting of rigid links in a tree structure with rotary joints of up to 3 DOF each and no

joint limits. Each DOF has individual PD constants which remain fixed for the entire simulation.

Collision forces due to interpenetration of the links of the articulated figure are not simulated.

The equations of motion are integrated using

a

fixed

time

step,

fourth

order

Runge-Kutta

integrator which is part of the dynamics compiler software.

Performance of the simulator varies

with model complexity with the most complex human model (described in Section 2.5) requiring

approximately 1 minute of wall clock time to compute 1 second of simulated motion on a Sun

Sparkstation 10. The use of a fixed integration time step has a significant impact on performance

since the worst-case (i.e. smallest) time step for the complete simulation must be used.

It is

estimated that the use of a variable integration time step could improve performance by a factor of

5-10. Recorded simulation results can be played back in real-time on a Silicon Graphics Indigo²

Workstation with GR3-XZ graphics hardware.

using the SGI-GL graphics library.

Display functions are implemented using the