An introduction to vehicle physics

A physics vehicle is a physics object used to simulate a vehicle driving on (or gliding above) a smooth surface.

Jolt JNI implements vehicle physics using a VehicleConstraint applied to a rigid body. Each VehicleConstraint includes a step listener. For a vehicle be effective, both its body and its constraint must be added to a physics system and its step listener must be invoked before each simulation step.

Instead of applying forces or updating the body’s velocities directly, an app using a physics vehicle should specify when and how the vehicle accelerates, steers, and brakes. This information is communicated to the constraint by means of a VehicleController object.

Jolt JNI provides 3 implementations of VehicleController:

  • TrackedVehicleController, for vehicles like bulldozers and tanks, whose motive power drives a pair of continuous tracks, one on each side,

  • WheeledVehicleController, for vehicles like automobiles and busses, whose motive power drives pairs of wheels joined by a differential gear train, and

  • MotorcycleController, for simulating motorcycles, which balance on just 2 wheels.

Instead a rotating rigid body for each wheel, a VehicleCollisionTester object is used to simulate contact with the driving surface.

Jolt JNI provides 3 implementations of VehicleCollisionTester:

  • VehicleCollisionTesterCastCylinder, which uses a cylinder shape,

  • VehicleCollisionTesterCastSphere, which uses a sphere shape, and

  • VehicleCollisionTesterRay, which uses a ray.

Creation

To create a physics vehicle:

  1. Create the rigid body and add it to the physics system.

  2. Create and configure a settings object for the wheels, specifying their locations and functionality.

  3. Create and configure a settings object for the controller, specifying how the vehicle is steered and how motive power is distributed.

  4. Create and configure a settings object for the constraint, referencing the wheel settings and controller settings.

  5. Instantiate the constraint.

  6. Create a collision tester and add it to the constraint.

  7. Add the constraint and its step listener to the physics system.

The physics system doesn’t keep track of vehicles; the app is responsible for that. Collision testers, constraints, constraint settings, controller settings, and wheel settings are all refcounted objects.

HelloVehicle is a Sport-Jolt app that demonstrates the creation of a vehicle with a WheeledVehicleController and a VehicleCollisionTesterRay, followed by automated steering and acceleration. Things to notice while running the app:

  1. The vehicle has a wedge-shaped body.

  2. The vehicle has 4 wheels, arranged in a rectangle.

  3. The vehicle circles to the left, accelerating steadily until it "wipes out" and tips over.

Summary

  • A physics vehicle simulates a vehicle accelerating, steering, and braking on a smooth surface.