Bullet 3D Physics Objects Not Rotating?

Question

So I've got Bullet 3D controlling the motion of objects rendered with DirectX. In the clipping below I've got 2 identical rectangles & 5 balls. The lowest rectangle has a mass of 0 and doesn't move (as expected). All other objects have a mass of 1 and fall as expected.

There are two issues that I'm seeing: 1. When the balls land on an angled surface (not shown) they don't rotate, but they do move 'down hill'. 2. Pictured below you see that the top rectangle is perched on the bottom rectangle when it should've rotated a bit and kept falling.

What do I need to do in Bullet to enable object rotations? I'm not a physicist so I suspect my google-fu is lacking, also the Bullet docs are painfully inadequate.

This code here is how I initialize my rigid bodies:

RESULT_CODE ubAddRigidBody(HANDLE collisionShape, float mass, ubVector3 position, ubVector4 orientation, HANDLE* rigidBodyHandle)
{
if (rigidBodyHandle == nullptr)
{
    return INVALID_PARAM;
}

if (_world->isValidCollisionShapeHandle(collisionShape) != SUCCESS)
{
    return INVALID_HANDLE;
}

auto transform = btTransform(btQuaternion(orientation.x, orientation.y, orientation.z, orientation.w), btVector3(position.x, position.y, position.z));
auto collisionShapeObject = _world->m_collisionShapes[collisionShape].get();

auto motionStateObject = new btDefaultMotionState();
motionStateObject->setWorldTransform(transform);

auto rigidBodyInfo = btRigidBody::btRigidBodyConstructionInfo(btScalar(mass), motionStateObject, collisionShapeObject);

// Enable rotations for the object

collisionShapeObject->calculateLocalInertia(mass, rigidBodyInfo.m_localInertia);
rigidBodyInfo.m_linearDamping = .2f;
rigidBodyInfo.m_angularDamping = .2f;

auto rigidBody = new btRigidBody(rigidBodyInfo);
auto uniqueHandle = _world->getUniqueHandle();

rigidBody->setFriction(.5f);
rigidBody->setRollingFriction(.5f);

_world->m_world->addRigidBody(rigidBody);
_world->m_rigidBodies[uniqueHandle] = unique_ptr<ubRigidBody>(new ubRigidBody(rigidBody, collisionShapeObject, motionStateObject));

*rigidBodyHandle = uniqueHandle;

return SUCCESS;
}

Answer 1

Your problem is that you're not supplying an inertia vector in the btRigidBodyConstructionInfo constructor. Your rigid body dynamic spheres end up with a default inertia of (0,0,0). This prevents angular motion, but not linear, so your objects slide but don't rotate. Look at some of the Bullet demo code and you'll see examples of the inertia being calculated for specific collision shapes, there are helper methods in the collision object or shape class.

Answer 2

you probably didn't calculateLocalInertia()

If I comment out that line here, the objects don't rotate:

btVector3 myStartingInertia(0, 0, 0);
instanceData[thisLocation].myDefaultMotionState = new 
btDefaultMotionState(btTransform(btQuaternion(0, 0, 0, 1), 
btVector3(spawnLocation.x, spawnLocation.y, spawnLocation.z)));
myCollisionShape->calculateLocalInertia(myMass, myStartingInertia);
btRigidBody::btRigidBodyConstructionInfo myRigidBodyConstructionInfo(myMass, 
instanceData[thisLocation].myDefaultMotionState, myCollisionShape, 
myStartingInertia);
myRigidBodyConstructionInfo.m_restitution = 0.35;
myRigidBodyConstructionInfo.m_friction = 0.0;

Answer 3

I don't know about C++ clearly but when I use it in java the bullet library there is method called setToRotation you can check that