How can I create Kinematic Objects in jBullet?

Question

I am trying to make a game engine and I want a camera controlled by the player and also effected by other jBullet entities in Java. I got suggested to use Kinematic Objects so I looked up about them. I couldn't find any documentation which I could understand.

Can someone explain how to set up and use kinematic objects or at least show me where I can start?

Answer 1

The documentation for KinematicCharacterController, found here isn't too entirely helpful, but the source in the CharacterDemo can be. Two main properties are defined in the demo.

    public KinematicCharacterController character;
    public PairCachingGhostObject ghostObject;

The ghost can be used for dynamic collision detection, as it does not automatically react to those events. The character can be moved by changing the Transform.

    //from the source src\com\bulletphysics\demos\character\CharacterDemo.java
    Transform startTransform = new Transform();
    startTransform.setIdentity();
    startTransform.origin.set(0.0f, 4.0f, 0.0f);

    Vector3f worldMin = new Vector3f(-1000f,-1000f,-1000f);
    Vector3f worldMax = new Vector3f(1000f,1000f,1000f);
    AxisSweep3 sweepBP = new AxisSweep3(worldMin, worldMax);

    ghostObject = new PairCachingGhostObject();
    ghostObject.setWorldTransform(startTransform);
    sweepBP.getOverlappingPairCache().setInternalGhostPairCallback(new GhostPairCallback());
    float characterHeight = 1.75f * characterScale;
    float characterWidth = 1.75f * characterScale;
    ConvexShape capsule = new CapsuleShape(characterWidth, characterHeight);
    ghostObject.setCollisionShape(capsule);
    ghostObject.setCollisionFlags(CollisionFlags.CHARACTER_OBJECT);

    float stepHeight = 0.35f * characterScale;
    character = new KinematicCharacterController(ghostObject, capsule, stepHeight);


    dynamicsWorld.addCollisionObject(ghostObject, CollisionFilterGroups.CHARACTER_FILTER, (short)(CollisionFilterGroups.STATIC_FILTER | CollisionFilterGroups.DEFAULT_FILTER));

    dynamicsWorld.addAction(character);

It would also be wise to extend the MotionState class to hold the transform

   public class MyMotionState extends MotionState {
        private Transform worldTransform;
        public MyMotionState() 
        {
            worldTransform = new Transform();
            worldTransform.setIdentity();
        }

        @Override
        public Transform getWorldTransform(Transform worldTrans) 
        {
            worldTrans.set(worldTransform);
            return worldTrans;
        }

        @Override
        public void setWorldTransform(Transform worldTrans) 
        {
            worldTransform.set(worldTrans);
        }
 }

and linking the Kinematic to a RigidBody for applying the physics to the character, and getting info on rendering.

    rigidBody.setCollisionFlags(rigidBody.getCollisionFlags() | CollisionFlags.KINEMATIC_OBJECT);  
    rigidBody.setActivationState(CollisionObject.DISABLE_DEACTIVATION);

Don't forget to update the physics engine once every iteration of the game loop.

    Transform transform = new Transform();
    transform.setIdentity();
    transform.origin.set(input.getX(), input.getY(), input.getZ());
    myMotionState.setWorldTransform(transform);
    rigidBody.setCenterOfMassTransform(myMotionState.getWorldTransform());

If you prefer, you could put these in your MainCharacter class or whatever you call it (I like it for the object oriented feel and ease to understand)

    public class MainCharacter implements KeyListener, MouseListener
    {
        private DynamicsWorld world;
        private MyMotionState myMotionState;
        private RigidBody rigidBody;
        private KinematicCharacterController character;
        private ConvexShape shape;
        private Texture texture;
        private GhostObject ghost;
        private Vector3f pos;
        public MainCharacter(DynamicsWorld world, Vector3f initialPosition, ConvexShape shape, Texture texture)
        {
            this.world = world;
            RigidBodyConstructionInfo constructInfo = new RigidBodyConstructionInfo(...);
            this.myMotionState = myMotionState;
            rigidBody = new RigidBody(constructInfo);
            ghost = new GhostObject();
            character = new KinematicCharacterController(ghost,shape,1);
        }
        public void render()
        {
            glBegin(GL_QUADS);
                glVertex3f(...
                ...
            glEnd();
        }
        public void mouseMoved(MouseEvent e)
        {
                //pseudocode
                this.yaw = e.getDX();
                this.pitch = e.getDY();
        }
        public void keyPressed(KeyEvent e)
        {
            Vector3f dPos = null;
            if(e.getKeyChar() == 'W')
            {
                dPos.x = 10;
            }
            else if(e.getKeyChar() == 'S')
            {
                dPos.x = -10;
            }
            etc...

            move(dPos.x,dPos.y,dPos.z);
        }
        public void move(float dx, float dy, float dz) {
            pos.z += dx * (float) Math.cos(Math.toRadians(yaw - 90)) + dz *        Math.cos(Math.toRadians(yaw));
            pos.x -= dx * (float) Math.sin(Math.toRadians(yaw - 90)) + dz * Math.sin(Math.toRadians(yaw));
            pos.y += dy * (float) Math.sin(Math.toRadians(pitch - 90)) + dz * Math.sin(Math.toRadians(pitch));
            //pseudocode
            rigidBody.update(pos);
            world.update(pos);
        }
   }

I hope I have helped you.