Collision Response without Rotational Response?

Rhekua · **Joined:** Tue May 15, 2007 11:58 pm **Posts:** 4

I integrated Bullet into my project and everything seems to work fine except one thing: When objects collide with other objects their rotation does not update. Things do not bounce off of other things and rotate how they should in response to the collision. They kind of just sit there in their normal orientation, bouncing off eachother.

It's not a graphical error as I can use setAngularVelocity and the objects (which are box shapes at the moment) will rotate and collide, climbing over eachother, etc. but they do not influence eachother's rotation.

I followed the user manual's integration steps and also looked at the other demos and it seems to me that I am initializing everything how I should.

I am using the newest version, 2.50b.

Erwin Coumans · **Posted:** Wed May 16, 2007 2:12 pm

It sounds like the 'angularFactor' is set to zero in the rigidbody.

Did you use this one?

Code:

btRigidBody::setAngularFactor(btScalar angFac)

Hope this helps,
Erwin

Rhekua · **Joined:** Tue May 15, 2007 11:58 pm **Posts:** 4

I didn't use that function, no. But I tried adding it in and setting it to 1 but there is still no rotation.

Should I be including more than just:

Code:

#include "Contrib\bullet-2.50b\src\btBulletDynamicsCommon.h"
#if defined(DEBUG) || defined(_DEBUG)
   #pragma comment( lib, "libbulletcollision_d.lib" )
   #pragma comment( lib, "libbulletdynamics_d.lib" )
   #pragma comment( lib, "libbulletmath_d.lib" )
#else
   #pragma comment( lib, "libbulletcollision.lib" )
   #pragma comment( lib, "libbulletdynamics.lib" )
   #pragma comment( lib, "libbulletmath.lib" )
#endif

Everything else seems to work fine but I can't figure out why they just won't rotate. I can link to a video if you would like to see exactly what is going on.

Erwin Coumans · **Posted:** Wed May 16, 2007 3:18 pm

It might be some initialization issue.

Did you initialize the local inertia properly? Can you compare it more carefully with Bullet Demo initialization (see Bullet/Demos/OpenGL/DemoApplication.cpp )

Can you provide a snippet of collision shape and rigidbody creation?

Thanks,
Erwin

Rhekua · **Joined:** Tue May 15, 2007 11:58 pm **Posts:** 4

Code:

btMotionState* g_motionState = NULL;
btRigidBody* g_rigidBody = NULL;
btCollisionShape* box = NULL;

   btTransform boxTrans;
   boxTrans.setIdentity();
   boxTrans.setOrigin(btVector3( 0.0f,100.0f,0.0f));
   g_motionState = new btDefaultMotionState(boxTrans);
   box = new btBoxShape( btVector3(1.0f, 1.0f, 1.0f) );
   g_rigidBody = new btRigidBody( 1.0f, g_motionState, box, btVector3(0,0,0), btScalar(0), btScalar(0), btScalar(0.5), btScalar(1.0) );
   g_DynamicsWorld->addRigidBody( g_rigidBody );
   g_rigidBody->setAngularFactor( btScalar(1) );
   g_rigidBody->setLinearVelocity( btVector3( 0.0f, 0.0f, 0.0f ) );
   g_rigidBody->setAngularVelocity( btVector3( 0.0f, 0.0f, 0.0f ) );

Erwin Coumans · **Posted:** Wed May 16, 2007 3:44 pm

g_rigidBody = new btRigidBody( 1.0f, g_motionState, box, btVector3(0,0,0), btScalar(0), btScalar(0), btScalar(0.5), btScalar(1.0) );

You should provide a proper local inertia tensor, instead of btVector3(0,0,0) as 4th argument in btRigidBody constructor.

The sample code in Bullet uses the shape and mass to calculate the localInertia:

Code:

btRigidBody*    DemoApplication::localCreateRigidBody(float mass, const btTransform& startTransform,btCollisionShape* shape)
{
        //rigidbody is dynamic if and only if mass is non zero, otherwise static
        bool isDynamic = (mass != 0.f);

        btVector3 localInertia(0,0,0);
        if (isDynamic)
                shape->calculateLocalInertia(mass,localInertia);

        //using motionstate is recommended, it provides interpolation capabilities, and only synchronizes 'active' objects

#define USE_MOTIONSTATE 1
#ifdef USE_MOTIONSTATE
        btDefaultMotionState* myMotionState = new btDefaultMotionState(startTransform);
        btRigidBody* body = new btRigidBody(mass,myMotionState,shape,localInertia);

#else
        btRigidBody* body = new btRigidBody(mass,startTransform,shape,localInertia);
#endif//
        m_dynamicsWorld->addRigidBody(body);

        return body;
}

Hope this helps,
Erwin

Rhekua · **Joined:** Tue May 15, 2007 11:58 pm **Posts:** 4

Awesome! Thank you very much for your help.

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Collision Response without Rotational Response?

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