Re-positioning a Rigid Body in Bullet Physics

Question

I am writing a character animation rendering engine that uses Bullet Physics as a physics simulation engine.

A sequence will start out with no model on the screen, then an animation will be assigned to that model, the model will be moved to frame 0 of the animation, and the engine will begin rendering the model with the animation.

What is the correct way to re-position the rigid bodies on the character model when it is initialized at frame 0?

Currently I am using this code, which is called immediately after the animation is assigned to the model and the bones are moved to the frame 0 position:

_world->removeRigidBody(_body);
bool k = (_type == Kinematics);
_body->setCollisionFlags(_body->getCollisionFlags() & ~btCollisionObject::CF_NO_CONTACT_RESPONSE);
btTransform tr = BulletPhysics::ConvertD3DXMatrix(&(_bone->getCombinedTrans()));
tr *= _trans;
_body->setCenterOfMassTransform(tr);
_body->clearForces();
_body->setLinearVelocity(btVector3(0,0,0));
_body->setAngularVelocity(btVector3(0,0,0));
_world->addRigidBody(_body, _groupID, _groupMask);

The issue is that sometimes this works, and other times not. For an example, take a skirt of a model. Sometimes it will show up in the natural position, other times slightly misaligned and it will fall into place, and other times it shows up completely clipped through the body, as if collision was turned off and some force pushed it in that direction. This does make sense most of the time, because in the test animation I am using the model's initial position is in the center of the screen, but the animation starts off the left side of the screen. Does anyone know how to solve this?

I know the bones on the skirt are not the problem, because I turned off physics and forced it to manually update the bone positions each frame, and everything was in the correct positions throughout the entire animation.

EDIT: I also have constraints, might that be what's causing this?

Answer 1

Here is my reposition method that does exactly this.

void LimbBt::reposition(btVector3 position,btVector3 orientation) {
    btTransform initialTransform;

    initialTransform.setOrigin(position);
    initialTransform.setRotation(orientation);

    mBody->setWorldTransform(initialTransform);
    mMotionState->setWorldTransform(initialTransform);
}

The motion state mMotionState is the motion state you created for the btRigidBody in the beginning. Just add your clearForces() and velocities to it to stop the body from moving on from the new position as if it went through a portal. That should do it. It works nicely with me here.

Edit: The constraints will adapt if you reposition all rigidbodies correctly. For that purpose, it is easy to calculate the relative position and reposition the whole constrained rigidbody construct according to that. If you do it incorrectly, you will get severe twitching, as the constraints will try to adjust you construct numerically, causing high forces if the constraint gaps are large.

Edit2: Another issue is that if you need deterministic behavior (every time you reset your bodies, they should fall exactly the same), then you will have to kill your old dynamicsWorld, recreate it and add all the bodies again. The world stores some information about the bodies that just can not be cleared for now. This might change in the future as bullet4 is going to support deterministic resets. But for now, if you do experiments with deterministic resets, you need to drop the world and recreate it.

source: discussion with Erwin Coumans, the developer of Bullet Physics.