[SOLVED] Applying Torque From Point Other Than Center Of Mass

DevilZ · February 2, 2020, 12:14pm

Hello everybody,

I would like to apply torque to a rigidbody from a point other than it’s center of mass. For example, if a body’s position is (1, 1, 1), then by default torque will be applied from the same point. What I would like to do is apply torque from a different point, say (0, 0, 0). Any way to do this?

Leonidas · February 2, 2020, 1:06pm

I don’t think this is doable since the underlying physics engine (Ammo.js) doesn’t seem to support, in the same manner you can do it when applying a force:

github.com

kripken/ammo.js/blob/00506d296df957d80bd192d27ff8a3ef2aa9f176/bullet/src/BulletDynamics/Dynamics/btRigidBody.h#L313


	{
		m_invInertiaLocal = diagInvInertia;
	}


	void	setSleepingThresholds(btScalar linear,btScalar angular)
	{
		m_linearSleepingThreshold = linear;
		m_angularSleepingThreshold = angular;
	}


	void	applyTorque(const btVector3& torque)
	{
		m_totalTorque += torque*m_angularFactor;
	}


	// XXX AMMO local addition to bullet
	void	applyLocalTorque(const btVector3& torque)
	{
		m_totalTorque += m_worldTransform.getBasis() * torque * m_angularFactor;
	}

And I think it makes sense since a rotational force makes sense to be applied to center/origin point of the body.

DevilZ · February 2, 2020, 1:21pm

So @Leonidas is there a workaround for this? My game won’t be able to function without this…

Leonidas · February 2, 2020, 1:31pm

Not sure, what are you trying to achieve?

DevilZ · February 2, 2020, 1:44pm

https://playcanvas.com/editor/scene/870258

Take a look at this👆. Use arrow keys to turn the object. If I make the circles on the end point upwards, the images don’t fall on it, but the bigger circle below it. I suspect this is because the object have moved, but the collision shapes haven’t. Or is it something else?

Leonidas · February 2, 2020, 1:49pm

There are several issues with this project:

All of the SpinningCenter bodies are set to Static, this won’t work if you are moving the objects.
You are moving the objects using entity translation methods (rotateLocal) instead of applying forces/torques to the rigid bodies.

Try updating the project to fix this and you will be in a better shape. If it still doesnt’ work try posting again, here.

DevilZ · February 2, 2020, 2:04pm

I don’t know why this is there, I told my dev to use torque.

I don’t want the objects to respond to gravity. How would I do it then?

Once I know the answer to this, I can fix it.

Leonidas · February 2, 2020, 2:08pm

Then use Kinematic objects, this will do the trick for you, take a look at the manual towards the bottom there is a good explanation on how they work:

https://developer.playcanvas.com/en/user-manual/physics/physics-basics/

DevilZ · February 2, 2020, 2:15pm

Ok, thanks. Will try this out tomorrow, and get back with the result.

DevilZ · February 3, 2020, 2:00pm

I forked the project and did what you asked me to. Works great. Marking as solved.