CoG of btConvexHullShapes

[Vylsain]

Hello everyone.

I'm doing an application and I use btConvexHullShapes but I encounter a problem.
The collision is well calculated but not the moves.

Here's a video of my problem with a simple cube :

http://www.youtube.com/watch?v=zMasdqb6z34

As you can see, it looks like the Center of gravity is outside of the shape.
All in all, it has a weird behaviour.

As you'll see in my code, I didn't do anything else than creating the object, and giving it a mass and an inertia.
I didn't modify the CoG, thinking it was automatically initialized in the middle of the cube (is it wrong ? Do I have do initiate it ?)


Here's the constructor of the class that buil the btConvexHullShapes :

Code: Select all

ConvexHullShape::ConvexHullShape(btVector3 * tab, btTransform &TransformCHS)
{
	CHS = new btConvexHullShape();
	
	for(int i=0; i<8; i++)
	{
             CHS->addPoint(tab[i]);
	}

        TransformCHS.setIdentity();
	TransformCHS.setOrigin(btVector3(0.0,25.0,0.0));
	TransformCHS.setRotation(btQuaternion(1,0,0,2));

	localInertia=btVector3(0,0,0);

	mass = 0.5f;

	CHS->calculateLocalInertia( mass, localInertia );

	MotionStateCHS = new btDefaultMotionState(TransformCHS);
	btRigidBody::btRigidBodyConstructionInfo myBoxRigidBodyConstructionInfo(0.5, MotionStateCHS, CHS, localInertia );
	bodyCHS = new btRigidBody(myBoxRigidBodyConstructionInfo);
}

In the main :

Code: Select all

        myWorld->setGravity( btVector3(0,-9.8,0) );
        
        //vertices of the cube
        tab[0] = btVector3 (0.0,0.0,0.0);
	tab[1] = btVector3 (10.0,0.0,0.0);
	tab[2] = btVector3 (10.0,0.0,10.0);
	tab[3] = btVector3 (0.0,0.0,10.0);
	tab[4] = btVector3 (0.0,10.0,0.0); 
	tab[5] = btVector3 (10.0,10.0,0.0);
	tab[6] = btVector3 (10.0,10.0,10.0);
	tab[7] = btVector3 (0.0,10.0,10.0);
	
	CHS_mon_Model=ConvexHullShape(tab,myTransform);

	myWorld->addRigidBody(CHS_mon_Model.bodyCHS);

	

	/// FLOOR ===================================================
	
        btCollisionShape* shape_sol = new btBoxShape( btVector3(100,0,100) );

	myTransform.setIdentity();
	myTransform.setOrigin( btVector3(0,0,0) );

	btVector3 localInertiaSol(0,0,0);

	btScalar mass = 0.0f;

	myMotionState_Sol = new btDefaultMotionState(myTransform);
	btRigidBody::btRigidBodyConstructionInfo rbInfo_sol( mass, myMotionState_Sol, shape_sol, localInertiaSol );
	body_sol = new btRigidBody(rbInfo_sol);

	myWorld->addRigidBody(body_sol);
        //=====================================================================

	InitCallbacks();

	glutDisplayFunc(Display);

	glutMainLoop();	
}

And in my Display function :

Code: Select all

if (myWorld && aff==true) //si myWorld est instancié et si 'a' a été appuyée
	{
		myWorld->stepSimulation(1.f/60, 10);
		myWorld->debugDrawWorld();
	}

	CHS_mon_Model.MotionStateCHS->m_graphicsWorldTrans.getOpenGLMatrix(CHS_mon_Model.MatrixCHS);
	
	glPushMatrix();
		glMultMatrixf(CHS_mon_Model.MatrixCHS);
		if(mod==true){
		        DrawBox();
		}
	glPopMatrix();

	DrawFloor();

I tried a lot of things but still don't know where the problem is coming from...

Is it really a problem of CoG and how do I modify it if that's the case ?

Thank you

[Flix]

As you can see, it looks like the Center of gravity is outside of the shape.
All in all, it has a weird behaviour.

Not weird at all to me. In fact this is what you wanted to do...

//vertices of the cube
tab[0] = btVector3 (0.0,0.0,0.0);
tab[1] = btVector3 (10.0,0.0,0.0);
tab[2] = btVector3 (10.0,0.0,10.0);
tab[3] = btVector3 (0.0,0.0,10.0);
tab[4] = btVector3 (0.0,10.0,0.0);
tab[5] = btVector3 (10.0,10.0,0.0);
tab[6] = btVector3 (10.0,10.0,10.0);
tab[7] = btVector3 (0.0,10.0,10.0);

As you can see, the center of the btConvexHullShape (0,0,0) is a vertex of the cube. Just place (0,0,0) at the center of the cube instead.

PS. The same thing happens in btCompoundShapes (I mean (0,0,0) is considered the center of mass).

[Vylsain]

Oh ok, I thought the CG was calculated as the barycentre of all the points...

Thank you very much Flix !

[Vylsain]

Yeah it works !!!

Thank you again for this good library and it's good community !

[Erwin Coumans]

If you have a btConvexHullShape (or a couple of other collision shapes) and you want to adjust the center of mass/inertia, you can also use the btCompoundShape for that.

Store the shapes in a btCompoundShape and use the calculatePrincipalAxisTransform method. It is not well documented, but the Bullet/Demos/FractureDemo uses it (see btFractureBody::shiftTransform).

Thanks,
Erwin