Introduction

This chapter will introduce the fundamentals of simulating rigid body dynamics using the NVIDIA PhysX engine.

Rigid Body Object Model

PhysX uses a hierarchical rigid body object/actor model, which looks like this:

The following diagram shows the relationship between the main types involved in the rigid body pipeline:

The Simulation Loop

Now use the method PxScene::simulate() to advance the world forward in time. Here is simplified code from the samples' fixed stepper class:

mAccumulator = 0.0f;
mStepSize = 1.0f / 60.0f;

virtual bool advance(PxReal dt)
{
    mAccumulator  += dt;
    if(mAccumulator < mStepSize)
        return false;

    mAccumulator -= mStepSize;

    mScene->simulate(mStepSize);
    return true;
}

This is called from the sample framework whenever the app is done with processing events and is starting to idle. It accumulates elapsed real time until it is greater than a sixtieth of a second, and then calls simulate(), which moves all objects in the scene forward by that interval. This is probably the simplest of very many different ways to deal with time when stepping the simulation forward.

To allow the simulation to finish and return the results, simply call:

mScene->fetchResults(true);

True indicates that the simulation should block until it is finished, so that on return the results are guaranteed to be available. When fetchResults completes, any simulation event callback functions that you defined will also be called. See the chapter Callback Sequence.

It is possible to read and write from the scene during simulation. The samples take advantage of this to perform rendering work in parallel with physics. Until fetchResults() returns, the results of the current simulation step are not available. So running rendering in parallel with simulation renders the actors as they were when simulate() was called. After fetchResults() returns, all these functions will return the new, post-simulate state. See the chapter Threading for more details about reading and writing while the simulation is running.

For the human eye to perceive animated motion as smooth, use at least twenty discrete frames per second, with each frame corresponding to a physics time step. To have smooth, realistic simulation of more complex physical scenes, use at least fifty frames per second.

See Simulation memory for details of how memory is used in simulation.