How to clone a vehicle's state?

zqh0253 · October 27, 2020, 9:01am

I want to create a bullet vehicle node which has the same states of a given bullet vehicle. The states include position, speed and so on. They are nearly the same, except mask bit (I don’t want them to collide with each other of course).
I found that I can only set position but not speed given bullet vehicle’s API. I also tried ‘copy.deepcopy()’ but the object can not be pickled. Is there any possible solution?

Yes, it seems a weird requirement. Actually, I am using Model Predictive Control to implement modular auto-driving in bullet and panda3d. I have to have a ‘virtual’ car that share the same state at time t as the ego vehicle. So that I can search for the optimal control based on the virtual car.

rdb · October 27, 2020, 1:00pm

It looks like Bullet sets the current speed value to the linear velocity of the chassis:

github.com

bulletphysics/bullet3/blob/cdd56e46411527772711da5357c856a90ad9ea67/src/BulletDynamics/Vehicle/btRaycastVehicle.cpp#L265



void btRaycastVehicle::updateVehicle(btScalar step)
{
	{
		for (int i = 0; i < getNumWheels(); i++)
		{
			updateWheelTransform(i, false);
		}
	}

	m_currentVehicleSpeedKmHour = btScalar(3.6) * getRigidBody()->getLinearVelocity().length();

	const btTransform& chassisTrans = getChassisWorldTransform();

	btVector3 forwardW(
		chassisTrans.getBasis()[0][m_indexForwardAxis],
		chassisTrans.getBasis()[1][m_indexForwardAxis],
		chassisTrans.getBasis()[2][m_indexForwardAxis]);

	if (forwardW.dot(getRigidBody()->getLinearVelocity()) < btScalar(0.))
	{

This is the only place where it is set. Bullet offers no way to override this value directly.

zqh0253 · October 27, 2020, 1:26pm

Thx for the reply. It is easy to get the status (e.g. speed) of a vehicle, but I want to set the speed of a vehicle.
For example, I use the API you have shown and get vehicle A’s speed. Then I want to set vehicle B’s speed the same as A. Do you know any API that can do this thing?

rdb · October 27, 2020, 1:38pm

As shown above you need to copy the linear velocity of the rigid body directly since that is where the speed is derived from.