BulletSharp object is bouncing too much

[peterkort]

I'm having a problem when using the bulletsharp port/wrapper.
I have three cylinders, I want the top one(dynamic) to rest on the other two(static). My setup looks like this:


For some reason, after jittering a bit, the top cylinder bounces off of the other two and falls out of view.

Here's my code to set up the scene.

• //now figure out bulletsharp stuff...
  CollisionConfiguration collConfig = new DefaultCollisionConfiguration();
  Dispatcher collDispatch = new CollisionDispatcher(collConfig);
  
  BroadphaseInterface broadphase = new DbvtBroadphase();
  ConstraintSolver sol = new SequentialImpulseConstraintSolver();
  world = new DiscreteDynamicsWorld(collDispatch, broadphase, sol, collConfig);
  
  world.Gravity = new Vector3(0.0f, -10.0f, 0.0f);
  
  //log (moving object)
  MotionState still = new DefaultMotionState();
  CylinderShape shape = new CylinderShapeZ(0.5f, 1.0f, 1.0f);
  still.WorldTransform = Matrix.Translation(0.0f, 0.4f, 0.0f);
  RigidBodyConstructionInfo constructInfo = new RigidBodyConstructionInfo(1.0f, still, shape);
  logBody = new RigidBody(constructInfo);
  logBody.SetDamping(0.04f, 0.1f);
  world.AddRigidBody(logBody);
  
  //rollers (static objects)
  CylinderShape r1s = new CylinderShapeZ(0.1f, 1.0f, 1.0f);
  MotionState r1m = new DefaultMotionState();
  r1m.WorldTransform = Matrix.Translation(-0.2f, -0.4f, 0.0f);
  RigidBodyConstructionInfo r1ci = new RigidBodyConstructionInfo(0.0f, r1m, r1s);
  r1 = new RigidBody(r1ci);
  world.AddRigidBody(r1);
  
  CylinderShape r2s = new CylinderShapeZ(0.1f, 1.0f, 1.0f);
  MotionState r2m = new DefaultMotionState();
  r2m.WorldTransform = Matrix.Translation(0.2f, -0.4f, 0.0f);
  RigidBodyConstructionInfo r2ci = new RigidBodyConstructionInfo(0.0f, r2m, r2s);
  r2 = new RigidBody(r2ci);
  world.AddRigidBody(r2);

And every frame is updated using:

• world.StepSimulation(0.05f, 100, 0.0005f);

While debugging, the Restitution for all three objects is listed as 0.0f.
Is there anything I might be missing?

I can also mention that I have put together a similar setup in Blender using the Bullet physics there, and there is no jitter.

[anthrax11]

You need to calculate the inertia as well:

Code: Select all

            float mass = 1.0f;
            Vector3 localInertia;
            shape.CalculateLocalInertia(mass, out localInertia);
            RigidBodyConstructionInfo constructInfo = new RigidBodyConstructionInfo(mass, still, shape, localInertia);

And for the static objects:

Code: Select all

            RigidBodyConstructionInfo r1ci = new RigidBodyConstructionInfo(0.0f, r1m, r1s, Vector3.Zero);

Vector3.Zero is actually the default inertia parameter value, so it's not really needed, but there's currently a bug in BulletSharp where the default value isn't passed correctly. I'll fix that soon, but for now you need to pass the zero vector.