Developer Journal 46

The little green block is a piece of food.

3:11 PM

I used yesterday to help my partner move house and this morning to go with mum to the hospital for her checkup. I've got a few hours of work time available today until I need to attend to a personal manner tonight.

I'm working on adding food back into the world. The tricky part is figuring out how to call a function when robots touch food. Bullet creates a btPersistantManifold for each pair of touching objects of class btCollisionObject. My class Robot has a pointer to class btRigidBody that inherits from class btCollisionObject. The problem is that btCollisionObject does not have a pointer to class Robot. From the Bullet code I can only get a pointer to btCollisionObject so I can't for example, call a function to increase the amount of energy in class Robot when the robot is touching food.

//Assume world->stepSimulation or world->performDiscreteCollisionDetection has been called

 int numManifolds = world->getDispatcher()->getNumManifolds();
 for (int i=0;igetDispatcher()->getManifoldByIndexInternal(i);
  btCollisionObject* obA = static_cast(contactManifold->getBody0());
  btCollisionObject* obB = static_cast(contactManifold->getBody1());
 
  int numContacts = contactManifold->getNumContacts();
  for (int j=0;jgetContactPoint(j);
   if (pt.getDistance()<0.f)
   {
    const btVector3& ptA = pt.getPositionWorldOnA();
    const btVector3& ptB = pt.getPositionWorldOnB();
    const btVector3& normalOnB = pt.m_normalWorldOnB;
   }
  }
 }

http://bulletphysics.org/mediawiki-1.5.8/index.php/Collision_Callbacks_and_Triggers
(The code is a bit messed up, will fix soon)

Need coffee.

I've got a few choices. Class Robot can inherit from class btCollisionObject and I can call class Robot functions from inside the physics code.

I can create class MyCollisionObject that inherits from class btCollisionObject that has a pointer to an object of class Robot. Class Robot can also have a pointer to an object of class MyCollisionObject. I don't know.

4:20 PM

Still thinking. I'm going for the second approach. I want to keep inheritance to a minimum. I hope this doesn't come back to bite me.

6:05 PM

Worked out how to check for collisions during a simulation tick, using callbacks.

void MyTickCallback( btDynamicsWorld* dynamicsWorld, btScalar ) {
    // call about code to check for collisions
}

bool PhysicsBullet::Init( void ) {
    // create a dynamics world
    ...
    // set callback
    void* worldUserInfo = 0;
    bool isPreTick = false;
    dynamicsWorld->setInternalTickCallback( 
        MyTickCallback, worldUserInfo, isPreTick );
    ...
}

Added a piece of food. Will re-enable code to eat food soon.

http://farm1.static.flickr.com/72/152956077_ab6382ee70.jpg

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