Getting body velocity in local space

[Murphy]

I know this is probably very simple but I cannot figure it out. Is there a way to get the velocity of a body in it's local space. For example, if the body is moving forward I would expect the z component of the velocity to be +10 perhaps. If it is moving backwards I would expect -10 for the z velocity.

Currently I am trying this:

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getRigidBody()->getVelocityInLocalPoint(getRigidBody()->getCenterOfMassPosition()).z()

or

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getRigidBody()->getVelocityInLocalPoint(btVector3(0, 0, 0)).z()

and I am not getting correct results. It seems like it is perhaps returning the velocity in world coordinates...

Any ideas?

[chunky]

I'm not entirely sure I understand the question, but...

I think what you want to do is get the rotation of the body, and the velocity of the body.
Normalise the velocity [so that it's just a direction], subtract one from the other.
Multiply the new vector by the speed [length of the original velocity vector]

Does that make sense?

Gary (-;

[Nathanael]

The following should work,

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btVector3 getVelocityAtWorldPosition(btRigidBody* body,const btVector3& worldposition,bool local)
{
/* That's the 'localpoint' */ 
const btVector3 relpos=worldposition-body->getCenterOfMassPosition();
/* That's velocity in world space */ 
const btVector3 wvel=body->getVelocityInLocalPoint(relpos);
/* Now you can transform it in body local frame */ 
if(local) return(body->getWorldTransform().getBasis().transpose()*wvel);
/* Or keep it in world frame */ 
return(wvel);
}

Hope it help.

[Murphy]

Thanks guys. I tried both of your methods.

It does seem like Nathanael's method is closer to what I need. It does work the way I expect however something funny is happening...

I am using this with a raycast vehicle and it works great if I am going forward or backward only. However, whenever I turn right or left in the vehicle the forward velocity changes in unexpected ways. If I turn left it might make the z component of the local velocity very high or very low. Same if I turn right.

It seems like the x component is somehow affecting to the z component.

[Nathanael]

The function i gave you include angular velocity, call it with body->getCenterOfMassPosition() as worldposition if you only want linear velocity, or:

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static btVector3 getLinearVelocityInBodyFrame(btRigidBody* body)
{
return(	body->getWorldTransform().getBasis().transpose() *
		body->getLinearVelocity());
}

Is faster, but less general.

[Murphy]

Exactly what I needed!

Thanks Nathanael!