Collision Filtering & Materials

Collision Filtering

A key setup step to get vehicles running properly is to define which components should collide with each other and which should not. The following collision groups are common for a simple setup:

• GroundQuery

  • (PhysxSchema.PhysxVehicleWheelAttachmentAPI)

• GroundSurface

  • (UsdPhysics.CollisionAPI)

• Chassis

  • (UsdPhysics.CollisionAPI)

• Wheel

  • (UsdPhysics.CollisionAPI)

Note

The PhysX Vehicle SDK does not simulate the wheel as a separate rigid body, instead a geometrical query along the suspension direction detects impact with the ground. In the given example, the listed GroundQuery collision group represents this geometrical query. The GroundQuery collison group would be set on the wheel attachment prim (PhysxSchema.PhysxVehicleWheelAttachmentAPI, relationship “collisionGroup”).

The following table marks the collisions that may usually be discarded with an “x”.

	GroundQuery	GroundSurface	Chassis	Wheel
GroundQuery			x	x
GroundSurface		x		x
Chassis	x			x
Wheel	x	x	x

Note

Collisions between Wheel and Chassis will be ignored automatically if the wheel and chassis collision geometries are placed under the same PhysxSchema.PhysxVehicleAPI root.

Materials

Another important setup step involves setting physics materials for the ground surfaces a vehicle will drive on. Those materials are used to define the friction values for tire vs. ground. To be recognized as a vehicle compatible material, UsdPhysics.MaterialAPI has to be applied. Physics materials can be set on collision geometry through UsdShade.MaterialBindingAPI. Once the materials are set up, tire friction tables (PhysxSchema.PhysxVehicleTireFrictionTable) can be created to define the friction values of materials for a given tire.

Example:

Material USD Path	Friction Value
/TarmacMaterial	0.7
/GravelMaterial	0.6
…	…

The tire friction table can be linked to in the corresponding relationship in PhysxSchema.PhysxVehicleTire. Please have a look at the basic setup samples for details.

Note

If the vehicle tires touch an unknown material, the default friction value will be used which can be specified per tire friction table (1 is the default value).