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## Bullet Collision Callback vs Iterating [SOLVED]

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### #1Fredericvo  Members

Posted 08 August 2014 - 09:49 AM

Hello,

I'm trying to get my feet wet with Bullet Physics and watched the following tutorial

and I'm not saying I don't get it but there are a few hurdles I'm expecting to encounter soon. He uses globals and a simple framework with SDL & outdated OGL (glBegin,glFloat3d style) so it is good enough for tutorials but if I want to retrofit this into a proper OOP framework like Rastertek I'm already expecting a lot of scope problems.

gContactAddedCallBack seems like a very easy to use and understand feature, but.. it is global and is not called with my own subsystems as parameters so that reeks of having to declare globals to them also for something as simple as wanting to play a bang sound when two spheres collide. (which is not even remotely simple because of the frequency at which the same collision keeps triggering the callback but I digress)

So I went through the doc and there seem to be several ways to act on collisions but I'm not sure on which one to settle.

"The best way to determine if collisions happened between existing objects in the world, is to iterate over all contact manifolds." they claim initially.

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

int numManifolds = world->getDispatcher()->getNumManifolds();

for (int i=0;i<numManifolds;i++)

{

btPersistentManifold* contactManifold = world->getDispatcher()->getManifoldByIndexInternal(i);

btCollisionObject* obA = static_cast<btCollisionObject*>(contactManifold->getBody0());

btCollisionObject* obB = static_cast<btCollisionObject*>(contactManifold->getBody1());

int numContacts = contactManifold->getNumContacts();

for (int j=0;j<numContacts;j++)

{

btManifoldPoint& pt = contactManifold->getContactPoint(j);

if (pt.getDistance()<0.f)

{

const btVector3& ptA = pt.getPositionWorldOnA();

const btVector3& ptB = pt.getPositionWorldOnB();

const btVector3& normalOnB = pt.m_normalWorldOnB;

}

}

}


Hurray! Got my solution. I can do this from the scope of my PhysicsManager class which could possibly have stored a pointer to some other subsystem.

Yeah but I kept reading and then it goes "A more efficient way is to iterate only over the pairs of objects that you are interested in."

Enters the ghost object and it stops being clear.

btManifoldArray manifoldArray;

int numPairs = pairArray.size();

for (int i=0;i<numPairs;i++)

{

manifoldArray.clear();

//unless we manually perform collision detection on this pair, the contacts are in the dynamics world paircache:

if (!collisionPair)

continue;

if (collisionPair->m_algorithm)

collisionPair->m_algorithm->getAllContactManifolds(manifoldArray);

for (int j=0;j<manifoldArray.size();j++)

{

btPersistentManifold* manifold = manifoldArray[j];

btScalar directionSign = manifold->getBody0() == m_ghostObject ? btScalar(-1.0) : btScalar(1.0);

for (int p=0;p<manifold->getNumContacts();p++)

{

const btManifoldPoint&pt = manifold->getContactPoint(p);

if (pt.getDistance()<0.f)

{

const btVector3& ptA = pt.getPositionWorldOnA();

const btVector3& ptB = pt.getPositionWorldOnB();

const btVector3& normalOnB = pt.m_normalWorldOnB;

/// work here

}

}

}
}

LOL wut?!? The parameters are totally different. Where are my bodies? How to access the UserPointer to my own object?

Also I think it is a callback too so no more access to my own subsystems. Next.

ContactTest

"Bullet 2.76 onwards let you perform an instant query on the world (btCollisionWorld or btDiscreteDynamicsWorld) using the contactTest query. The contactTest query will peform a collision test against all overlapping objects in the world, and produces the results using a callback."

"However, a downside is that collision detection is being duplicated for the target object" Doesn't sound too good although I don't even know what they mean.

It is implemented by your own derived class so that again provides access to my own subsystems as they seem to mention themselves for the first time:

struct ContactSensorCallback : public btCollisionWorld::ContactResultCallback {
//! Constructor, pass whatever context you want to have available when processing contacts
/*! You may also want to set m_collisionFilterGroup and m_collisionFilterMask
* (supplied by the superclass) for needsCollision() */
ContactSensorCallback(btRigidBody& tgtBody , YourContext& context /*, ... */)
: btCollisionWorld::ContactResultCallback(), body(tgtBody), ctxt(context) { }

btRigidBody& body; //!< The body the sensor is monitoring
YourContext& ctxt; //!< External information for contact processing

//! If you don't want to consider collisions where the bodies are joined by a constraint, override needsCollision:
/*! However, if you use a btCollisionObject for #body instead of a btRigidBody,
* then this is unnecessary—checkCollideWithOverride isn't available */
virtual bool needsCollision(btBroadphaseProxy* proxy) const {
// superclass will check m_collisionFilterGroup and m_collisionFilterMask
if(!btCollisionWorld::ContactResultCallback::needsCollision(proxy))
return false;
// if passed filters, may also want to avoid contacts between constraints
return body.checkCollideWithOverride(static_cast<btCollisionObject*>(proxy->m_clientObject));
}

//! Called with each contact for your own processing (e.g. test if contacts fall in within sensor parameters)
const btCollisionObjectWrapper* colObj0,int partId0,int index0,
const btCollisionObjectWrapper* colObj1,int partId1,int index1)
{
btVector3 pt; // will be set to point of collision relative to body
if(colObj0->m_collisionObject==&body) {
pt = cp.m_localPointA;
} else {
assert(colObj1->m_collisionObject==&body && "body does not match either collision object");
pt = cp.m_localPointB;
}
// do stuff with the collision point
return 0; // not actually sure if return value is used for anything...?
}
};

// USAGE:
btRigidBody* tgtBody /* = ... */;
YourContext foo;
ContactSensorCallback callback(*tgtBody, foo);
world->contactTest(tgtBody,callback);

This might seem a better solution except for that "downside" they talked about.

So, which is usually used now for a simple game that has to be able to act upon collisions and triggers, do something to the game objects themselves and access to the sound subsystem among perhaps a few others without using ugly hacks, globals, a complex message passing system (although that's also a subsystem so it's back to square 1) etc?

EDIT

I figured it out thanks to this old post: http://www.bulletphysics.org/Bullet/phpBB3/viewtopic.php?p=15575&f=9&t=3997

Apparently iterating over all manifolds after stepping the world is the answer. Callbacks shouldn't have any game logic and are meant for modifying contacts.

Edited by Fredericvo, 10 August 2014 - 08:54 AM.

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