Use a hollow shape (ring) in jMonkeyEngine's physics simulation

Question

I'm trying to make a ring tossing game in JMonkey, but when I tried using a Torus it just bounces over the pins(because of the collision spreads over the hollow centre).
I've been looking for a way to get the Torus to work, and alternatives for the ring, but I can't find anything which works.
Any tips or hints for a fix are really appreciated.

Answer 1

By default, the physics engine (Bullet) uses simple shapes that make calculating collisions easy and fast. The torus probably looks like a box to the physics engine. To make it hollow you can convert the mesh of the torus into a CollisionShape that Bullet accepts.
Available classes are listed here: http://hub.jmonkeyengine.org/wiki/doku.php/jme3:advanced:physics

The GImpactCollisionShape is one that will work for dynamic, concave objects. It comes with a constructor that conveniently converts your mesh.

However, a complex shape means more work for the engine. It's often sufficient to use a rough approximation as CollisionShape. Here's an example that uses a complex torus for display and a simpler one for the physics simulation:

public void simpleInitApp() {
    Material mat = new Material(getAssetManager(), "Common/MatDefs/Misc/ShowNormals.j3md");

    // Create pin
    Cylinder cylinderMesh = new Cylinder(4, 12, 1.0f, 7.0f, true);
    Geometry pin = new Geometry("Pin", cylinderMesh);
    pin.setMaterial(mat);
    pin.rotate(90 * FastMath.DEG_TO_RAD, 0, 0);

    // Create a smooth ring
    Torus torusMesh = new Torus(64, 48, 1.0f, 3.0f);
    Geometry ring = new Geometry("Ring", torusMesh);
    ring.setMaterial(mat);
    ring.rotate(90 * FastMath.DEG_TO_RAD, 0, 0);
    ring.rotate(0, 51 * FastMath.DEG_TO_RAD, 0);
    ring.setLocalTranslation(0, 10, 0);

    rootNode.attachChild(pin);
    rootNode.attachChild(ring);

    // Define the shape that the ring uses for collision checks
    Torus simpleTorusMesh = new Torus(16, 12, 1.0f, 3.0f);
    GImpactCollisionShape collisionShape = new GImpactCollisionShape(simpleTorusMesh);
    ring.addControl(new RigidBodyControl(collisionShape, 0.1f));

    // Default CollisionShape for the pin
    pin.addControl(new RigidBodyControl(0));

    BulletAppState bulletState = new BulletAppState();
    stateManager.attach(bulletState);
    bulletState.getPhysicsSpace().add(pin);
    bulletState.getPhysicsSpace().add(ring);

    cam.setLocation(new Vector3f(20, 5, 35));
    cam.lookAt(new Vector3f(0, 4, 0), Vector3f.UNIT_Y);
    flyCam.setMoveSpeed(30);
}