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I have got problem implementing collision callback into my project. I would like to have detection between two specific objects. I have got normall collision but I want one object to stop or change color or whatever when colides with another. I wrote code from bullet wiki:

int numManifolds = dynamicsWorld->getDispatcher()->getNumManifolds();
    for (int i=0;i<numManifolds;i++)
    {
        btPersistentManifold* contactManifold =  dynamicsWorld->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;
                bool x = (ContactProcessedCallback)(pt,fallRigidBody,earthRigidBody);
                if(x)
                    printf("collision\n");
            }
        }
    }

where fallRigidBody is a dynamic body - a sphere and earthRigiBody is static body - StaticPlaneShape and sphere isn't touching earthRigidBody all the time. I have got also other objects that are colliding with sphere and it works fine. But the program detects collision all the time. It doesn't matter if the objects are or aren't colliding.

I have also added after declarations of rigid body:

    earthRigidBody->setCollisionFlags(earthRigidBody->getCollisionFlags() | btCollisionObject::CF_CUSTOM_MATERIAL_CALLBACK);
fallRigidBody->setCollisionFlags(fallRigidBody->getCollisionFlags() | btCollisionObject::CF_CUSTOM_MATERIAL_CALLBACK);

So can someone tell me what I am doing wrong? Maybe it is something simple?

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1 Answer 1

I'm familiar with JBullet but as its the java port of the original the concept should be the same.

It seemed to me that there's always a collision with your static plane and other objects. But you need to do something only on collision of specific objects. What you need to use is the user pointer of the collision shape. Here's a piece of code from my own engine in java. Here GameEntity is the class I wrap my objects and CollisionListener is another class of mine which I use to implement what collision does. I also pass positions and collision normal to the listener to achive more fancy effects.

            if (pt.getDistance() < 0.f) {
                Vector3f ptA = pt.getPositionWorldOnA(new Vector3f());
                Vector3f ptB = pt.getPositionWorldOnB(new Vector3f());
                Vector3f normalOnB = pt.normalWorldOnB;
                GameEntity thisObject = (GameEntity) (obA.getUserPointer());
                GameEntity thatObject = (GameEntity) (obB.getUserPointer());
                if(thisObject == null){
                    return;
                }
                if (thisObject.getCollisionListener() != null) {
                    thisObject.getCollisionListener().collided(
                            ptA, ptB, normalOnB, thatObject);
                }
                if (thatObject.getCollisionListener() != null) {
                    thatObject.getCollisionListener().collided(
                            ptB, ptA, normalOnB, thisObject);
                }
            }
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