situation of colliding but no contact points

[li9s]

I have searched for the situation but it seems there are no relative topics.
In my scene I have a btCollisionObject with btCompoundShape, and a btRigidBody with a btBoxShape, they are both running in the only btDiscreteDynamicsWorld. And when they collide, I got numManifolds(which is not 0), but I cannot get contact points since numContacts is always 0.
my code for collision detection:

Code: Select all

	int numManifolds = sDynamicsWorld->getDispatcher()->getNumManifolds();
	for(int i=0;i<numManifolds;i++)
	{
		btPersistentManifold * contactManifold = sDynamicsWorld->getDispatcher()->getManifoldByIndexInternal(i);
		int numContacts= contactManifold->getNumContacts();
		if(numContacts>0)
		{
			btCollisionObject * obA = static_cast<btCollisionObject*>(contactManifold->getBody0());
			btCollisionObject * obB = static_cast<btCollisionObject*>(contactManifold->getBody1());
			type_obj1 = obA->getUserPointer();
			type_obj2 = obB->getUserPointer();
			...
		}
	}

I'm still not clear about the relation of contactManifold and contactPoints.
Non-zero contactManifold means collision, am I right?

thanks.

[Dirk Gregorius]

Well, the first thing you should do is to render your scene. The nice thing about collision detection and physics is that you can easily visualize everything. Secondly, I would guess that Bullet creates a manifold as soon as the associated AABBs of the collision shapes overlap. This doesn't necessarily mean that the actual shapes are in contact which explains that the number of contact points is zero.

HTH,
-Dirk

[Erwin Coumans]

Indeed, as Dirk mentioned Bullet pre-allocates contact manifolds ahead of time, when the AABB overlaps.

This means, a btPersistentManifold might be empty/have zero contact points. A non-zero contactManifold means collision indeed.
Thanks,
Erwin

[li9s]

Well, the first thing you should do is to render your scene. The nice thing about collision detection and physics is that you can easily visualize everything. Secondly, I would guess that Bullet creates a manifold as soon as the associated AABBs of the collision shapes overlap. This doesn't necessarily mean that the actual shapes are in contact which explains that the number of contact points is zero.

HTH,
-Dirk

Hi Dirk,
Yes I did some rendering works, I can see the AABBs but the real shape is very small, I guess there's something wrong with the units.
finally it works. I use meters instead of centimeters,then the collision shape's size seems to be correct.

Thanks!
-James Lee.

[li9s]

Indeed, as Dirk mentioned Bullet pre-allocates contact manifolds ahead of time, when the AABB overlaps.

This means, a btPersistentManifold might be empty/have zero contact points. A non-zero contactManifold means collision indeed.
Thanks,
Erwin

thanks Erwin! It really helps.
-James Lee