Guaranteed Collision Callback

[Geometrian]

Hi,

I am attempting to figure out when two objects collide.

What I want:
Ideally, whenever a collision actually occurs, Bullet would call a callback, giving me the option to ignore the collision or allow Bullet to continue with collision response (restitution, reaction, etc.).

Some things close and my analysis thereof:

I found two pages of interest:

• Page 1: http://www.bulletphysics.org/mediawiki- ... d_Triggers
  Page 2: http://bulletphysics.org/mediawiki-1.5. ... _Filtering

Both are quite outdated as of May 2013, but I eventually was able to cobble together a method that I can use, working nicely in my revision of Bullet 2.81 (mostly from Page 1):

Code: Select all

//dynamics->stepSimulation above

int numof_manifolds = collision_dispatcher->getNumManifolds();
for (int i=0;i<numof_manifolds;++i)
{
	btPersistentManifold* contactManifold =  collision_dispatcher->getManifoldByIndexInternal(i);
	const btCollisionObject* obA = contactManifold->getBody0();
	const btCollisionObject* obB = contactManifold->getBody1();

	int numof_contacts = contactManifold->getNumContacts();
	for (int j=0;j<numof_contacts;++j) {
		btManifoldPoint& pt = contactManifold->getContactPoint(j);
		if (pt.getDistance()<=0.0f) {
			//->setUserPointer can be called when rigid bodies are created with an id that allows for their identification here
			printf("Collided: %p %p\n",obA->getUserPointer(),obB->getUserPointer());
			//getchar();

			//const btVector3& ptA = pt.getPositionWorldOnA();
			//const btVector3& ptB = pt.getPositionWorldOnB();
			//const btVector3& normalOnB = pt.m_normalWorldOnB;
		}
	}
}

The major problem with this is that a collision is not guaranteed to be detected. If two objects collide but are then completely restored all in a single stepSimulation call above, then the if-block might never be called. This can be largely resolved in practice by using some epsilon > 0.0f instead if 0.0f as the test, but I don't think it's really a fix.

The other options on Page 1 would suffer from the same problem, I think. Maybe the contact method with gContactProcessedCallback could do what I want? On Page 2, I tried the last suggestion of deriving btCollisionDispatcher and overriding needsResponse, but this seems to be called at times when the two objects do not collide.

Suggestions, tips, comments, etc.?
Ian

[pccsoares]

I'm having the same problem.
Did you find a solution?

[c6burns]

The major problem with this is that a collision is not guaranteed to be detected. If two objects collide but are then completely restored all in a single stepSimulation call above, then the if-block might never be called.

Isn't that the exact case for which CCD (continuous collision detection) is designed? There's a CCD demo that ships with the library.

[pccsoares]

It's not a problem for CCD. I'm missing collisions even with slow moving objects.
The only way I found to get all the collisions is with the callback gContactProcessedCallback.