Rigid Body Overview

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.

Class	Extends	Functionality
`PxBase`	N/A	Reflection/querying object types.
`PxActor`	`PxBase`	Actor name, actor flags, dominance, clients, aggregates, query world bounds.
`PxRigidActor`	`PxActor`	Shapes, transforms, and querying constraints.
`PxRigidBody`	`PxRigidActor`	Mass, inertia, velocities, damping, forces, body flags.
`PxRigidStatic`	`PxRigidActor`	Interface for static body in the scene. This kind of body has implicit infinite mass/inertia.
`PxRigidDynamic`	`PxRigidBody`	Interface for dynamic rigid body in the scene. Introduces support for kinematic targets and object sleeping.
`PxArticulationLink`	`PxRigidBody`	Interface for a dynamic rigid body link in a PxArticulation. Introduces support for querying the articulation and adjacent links.
`PxArticulationReducedCoordinate`	`PxBase`	Defines interface for a PxArticulation. Effectively a container referencing multiple PxArticulationLink rigid bodies.

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

Creating Rigid Actors

Rigid actors along with their shapes can be created through the PxPhysics instance:

PxShapeFlags shapeFlags = PxShapeFlag::eVISUALIZATION | PxShapeFlag::eSCENE_QUERY_SHAPE | PxShapeFlag::eSIMULATION_SHAPE;
PxMaterial* materialPtr = &material;

//plane rigid static
PxRigidStatic* rigidStatic = mPhysics.createRigidStatic(PxTransformFromPlaneEquation(PxPlane(PxVec3(0.f, 1.f, 0.f), 0.f)));
{
        PxShape* shape = mPhysics.createShape(PxPlaneGeometry(), &materialPtr, 1, true, shapeFlags);
        rigidStatic->attachShape(*shape);
        shape->release(); // this way shape gets automatically released with actor
}

//single shape rigid dynamic
PxRigidDynamic* rigidDynamic = mPhysics.createRigidDynamic(PxTransform(PxVec3(0.f, 2.5f, 0.f)));
{
        PxShape* shape = mPhysics.createShape(PxBoxGeometry(2.f, 2.f, 2.f), &materialPtr, 1, true, shapeFlags);
        rigidDynamic->attachShape(*shape);
        shape->release(); // this way shape gets automatically released with actor
}

mScene.addActor(*rigidStatic);
mScene.addActor(*rigidDynamic);

More details on collision geometry and shapes are provided in Rigid Body Collision. In the case of single shape actors the PxRigidActorExt::createExclusiveShape() function from PxRigidActorExt.h can be used:

//plane rigid static
PxRigidStatic* rigidStatic = mPhysics.createRigidStatic(PxTransformFromPlaneEquation(PxPlane(PxVec3(0.f, 1.f, 0.f), 0.f)));
PxRigidActorExt::createExclusiveShape(*rigidStatic, PxPlaneGeometry(), material);

//single shape rigid dynamic
PxRigidDynamic* rigidDynamic = mPhysics.createRigidDynamic(PxTransform(PxVec3(0.f, 2.5f, 0.f)));
PxRigidActorExt::createExclusiveShape(*rigidDynamic, PxBoxGeometry(2.f, 2.f, 2.f), material);

mScene.addActor(*rigidStatic);
mScene.addActor(*rigidDynamic);

Helper functions in PxSimpleFactory.h can further simplify the setup for common cases:

//plane rigid static
PxRigidStatic* rigidStatic = PxCreatePlane(mPhysics, PxPlane(PxVec3(0.f, 1.f, 0.f), 0.f), material);

//single shape rigid dynamic
PxRigidDynamic* rigidDynamic = PxCreateDynamic(mPhysics, PxTransform(PxVec3(0.f, 2.5f, 0.f)), PxBoxGeometry(2.f, 2.f, 2.f), material, 1.f);

mScene.addActor(*rigidStatic);
mScene.addActor(*rigidDynamic);

The helpers in PxSimpleFactory.h further support creating kinematic actors, cloning shapes, actors and scaling actors.

Simulation

For the actual simulation rigid body objects need to be added to a scene and that scene will be stepped forwards in time. See section The Simulation Loop.