Real-Time Physics Simulation Forum

Re: btPersistentManifold::getNumContacts always returns zero

2021-03-25T03:57:49+00:00

Yep, I'm seeing the same behavior as @s_kohan. Even if there is a btPersistentManifold, you still have to check getNumContacts() > 0 to actually know if the collision is for-realz.

Statistics: Posted by acu192 — Thu Mar 25, 2021 3:57 am

Re: btPersistentManifold::getNumContacts always returns zero

2019-07-03T18:10:16+00:00

Thank you, I actually ended up solving the problem myself. It turns out the problem was a false assumption I had about the contact manifolds: I thought they would only be created when there were already points of contact, but it turns out those "collisions" are already registered when the bodies are just close (broad-phase detection I think?) - so I just had to make my collision handling logic a bit smarter and wait until the collision manifolds already had contacts before handling it.

Statistics: Posted by s_kohan — Wed Jul 03, 2019 6:10 pm

Re: btPersistentManifold::getNumContacts always returns zero

2019-06-17T15:06:29+00:00

I would like to help, however I don't have any good hypothesises. I looked at some working code that harvests contacts details from the contact manifolds and AFAICT your code should work. Perhaps you could supply enough source to actually build and reproduce your problem?

Statistics: Posted by drleviathan — Mon Jun 17, 2019 3:06 pm

btPersistentManifold::getNumContacts always returns zero

2019-06-15T16:36:37+00:00

I want to get access to the contact point details for a collision. The problem is, whenever I try to get the number of contacts from a manifold, via btPersistentManifold::getNumContacts, the result is always zero. Here's how I'm getting the contacts:

Code:

          // these methods are part of a C wrapper so I can use bullet using the C FFIint btDynamicsWorldGetCollisionCount(btDynamicsWorldRef* world) {return ((btDynamicsWorld*)world)->getDispatcher()->getNumManifolds();}btContact btDynamicsWorldGetCollision(btDynamicsWorldRef* world, int index) {btPersistentManifold* contactManifold = ((btDynamicsWorld*)world)->getDispatcher()->getManifoldByIndexInternal(index);btContact contact; // this is a struct defined to expose contact info to this C wrappercontact.objectA = contactManifold->getBody0()->getUserPointer();contact.objectB = contactManifold->getBody1()->getUserPointer();contact.manifoldRef = (btContactManifoldRef*)contactManifold;return contact;}     int btContactManifordGetContactCount(btContactManifoldRef* manifold) {        btPersistentManifold* contactManifold = (btPersistentManifold*) manifold;        int count = contactManifold->getNumContacts();        return count;    }

The strange thing is, when I debug, I can see that there are 4 btManifoldPoint records inside contactManifold->m_pointCache. What am I missing here?

Statistics: Posted by s_kohan — Sat Jun 15, 2019 4:36 pm