Dynamic object not moving (Noob alert)

[Hydrael]

Hello everyone,

I'm trying to implement Bullet as the physics engine within my project, and I seem to have problems using it correctly:
I have a static object (ground) and a dynamic object (sphere).
I was following some tutorials/example codes and implemented the following:

Bullet initialization:

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          mCollisionConfiguration = new btDefaultCollisionConfiguration();
          mCollisionDispatcher = new btCollisionDispatcher(mCollisionConfiguration);
          btVector3 worldAabbMin(-150, -150, -150);
          btVector3 worldAabbMax(150, 150, 150);
          mWorldDimension = new btAxisSweep3(worldAabbMin,worldAabbMax);
          mSolver = new btSequentialImpulseConstraintSolver;
          mPhysicsWorld = new btDiscreteDynamicsWorld(mCollisionDispatcher, mWorldDimension, mSolver, mCollisionConfiguration);

          if ( 0 == mPhysicsWorld )
          {
              std::string msg = "CGameSession::CGameSession: Error creating physics world";
              MESSAGELOGGER.Log(msg);
              throw std::exception(msg.c_str());
          }

          mPhysicsWorld->setGravity(btVector3(0, -9.81f, 0));
          DEBUGLOGGER.Log("CGameSession::CGameSession: Successfully initialized physics");

Rigid body creation function:

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  btRigidBody* CGameSession::CreateRigidBody(float mass, const btTransform& startTransform, btCollisionShape* shape)
  {
      btAssert((!shape || shape->getShapeType() != INVALID_SHAPE_PROXYTYPE));

      //rigidbody is dynamic if and only if mass is non zero, otherwise static
      bool isDynamic = (mass != 0.f);

      btVector3 localInertia(0,0,0);
      if (isDynamic)
          shape->calculateLocalInertia(mass,localInertia);

      btDefaultMotionState* myMotionState = new btDefaultMotionState(startTransform);

      btRigidBody::btRigidBodyConstructionInfo cInfo(mass,myMotionState,shape,localInertia);

      btRigidBody* body = new btRigidBody(cInfo);
      mPhysicsWorld->addRigidBody(body);

      return body;
  }

Ground initialization:

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btTransform groundTransform;
      groundTransform.setIdentity();
      groundTransform.setOrigin(btVector3(0, 0, 0));
      mPlayingFieldGroundBodyShape = new btBoxShape(btVector3(groundSizeX, groundSizeY, groundSizeZ));
      mPlayingFieldGroundBody = CreateRigidBody(mPlayingFieldMass, groundTransform, mPlayingFieldGroundBodyShape);

Sphere initialization:

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btTransform transform;
      transform.setIdentity();
      transform.setOrigin(btVector3(mSphereStartPosX, mSphereStartPosY, mSphereStartPosZ));
      mSphereBodyShape = new btSphereShape(mSphereRadius);
      mSphereBody = CreateRigidBody(mSphereMass, transform, mSphereBodyShape);

Update code:

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void UpdatePhysics(float ms) { mPhysicsWorld->stepSimulation(ms / 1000.0f , 60); }

I excpeted the sphere to be falling downwards and bounce off the ground, but it isn't moving at all.
mPlayingFieldMass is 0 and mSphereMass is 1.0f
What am I doing wrong?

Thanks a lot in advance for any help.

Greets,
Chris

[shogun]

Try to give your sphere some gravity.

[Hydrael]

Thanks for your help, but unfortunately that doesn't work either

I altered my rigid body creation function in the following way:

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  btRigidBody* CGameSession::CreateRigidBody(float mass, const btTransform& startTransform, btCollisionShape* shape)
  {
      btAssert((!shape || shape->getShapeType() != INVALID_SHAPE_PROXYTYPE));

      //rigidbody is dynamic if and only if mass is non zero, otherwise static
      bool isDynamic = (mass != 0.f);

      btVector3 localInertia(0,0,0);
      if (isDynamic)
          shape->calculateLocalInertia(mass,localInertia);

      btDefaultMotionState* myMotionState = new btDefaultMotionState(startTransform);

      btRigidBody::btRigidBodyConstructionInfo cInfo(mass,myMotionState,shape,localInertia);

      btRigidBody* body = new btRigidBody(cInfo);
      if (isDynamic)
      {
          body->setGravity(btVector3(0,-9.81f,0));
          body->activate();
          printf("%d\n", body->isActive());
      }
      mPhysicsWorld->addRigidBody(body);

      return body;
  }

That printf will tell, that the body is active, which seems to be correct, but later on in my processing loop, after calling ->stepSimulation, another printf will tell that the body became inactive.

[RBD]

Update code:

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void UpdatePhysics(float ms) { mPhysicsWorld->stepSimulation(ms / 1000.0f , 60); }

May be obvious, but just in case: ms != microseconds?

[Hydrael]

Update code:

Code: Select all

void UpdatePhysics(float ms) { mPhysicsWorld->stepSimulation(ms / 1000.0f , 60); }

May be obvious, but just in case: ms != microseconds?

Nope, it was milliseconds - thanks for the advice!
The printf now says the object is active throughout runtime.
It still isn't moving though, but that could also be a renderer problem.

I'm gonna check that out tommorow.

[Hydrael]

Ok, unfortunately this didn't work either

I'm polling position updates with this code:

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btTransform t;
      mSphereBody->getMotionState()->getWorldTransform(t);

But t.m_origin always stays the same.

Any other ideas anyone?

[rrck]

Hello.

If I fully understand your code, wouldn't you need to multiply ms by 1000 instead of dividing? As I understand if you divide ms by 1000 you get very small number.

[Hydrael]

Hello.

If I fully understand your code, wouldn't you need to multiply ms by 1000 instead of dividing? As I understand if you divide ms by 1000 you get very small number.

I fixed that already. Multiplying the timestep value by 1000 caused the object to stay active - which is correct. But it still wouldn't move

[rrck]

Hm I think, and understood that when you have miliseconds it is correct to divide them by 1000 when passing to bullet, because bullet gets time in seconds.

[Hydrael]

I've been playing around with that value pretty much.
But no matter how high or low I scale the timestep value, I will get no movement

[Hydrael]

No more ideas?