Constrain movement to zero on a axis

[cobolt_dink]

As my ball moves and bounces around the level I would like its Y position to never change from where it started (ball radius). I thought this code would do it (modified from something I found here to make a object only fall straight down and not to the side):

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   btTransform identity = btTransform::getIdentity();
	
	btTransform transform;
	transform.setIdentity();
	transform.setOrigin(body->getWorldTransform().getOrigin());

	static btRigidBody s_fixed(0, 0, 0);
	btGeneric6DofConstraint *slider = new btGeneric6DofConstraint(*body, s_fixed, identity,
		                                                          transform, true);

	slider->setAngularLowerLimit(btVector3(-1, -1, -1));
	slider->setAngularUpperLimit(btVector3(1, 1, 1));

	slider->setLinearLowerLimit(btVector3(-1e30f, 0, -1e30f));
	slider->setLinearUpperLimit(btVector3(1e30f, 0, 1e30f));		

	m_dynamicsWorld->addConstraint(slider);

	body->addConstraintRef(slider);

But when logging collisions I can see the Y position is all over the place. I set linear velocity every update and the Y part is zero. So I'm not sure what I'm supposed to do to get that behavior.

[Erwin Coumans]

I would like its Y position to never change from where it started (ball radius).

It is recommended to restrict the motion using btRigidBody::setLinearFactor. For example:

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body->setLinearFactor(btVector3(1,0,1));

This can be combined with or without setAngularFactor to allow rotation around certain axis:

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body->setAngularFactor(btVector3(0,1,0));

The btGeneric6DofConstraint doesn't allow full motion on the Y-axis in combination with X or Z limits.

Hope this helps,
Erwin

[cobolt_dink]

setLinearFactor() doesn't seem to exist in 2.74. Is this a new feature for 2.75?

*edit*
I see its in 2.75 RC7, I will have to build that and see if it fixes the problem.

[cobolt_dink]

So I built 2.75 RC7 and used that and it seems to be working now. I do have a question though

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body->setLinearFactor(btVector3(1, 0, 1));
body->setLinearVelocity(btVector3(0, 10, 0));

That will still make a body go up. Does the linear factor only work on impulses given to a body and not setting the velocity directly?

[Erwin Coumans]

Ok, glad it works fine now.

Using 'setPosition' or 'setVelocity' bypasses the constraint solver indeed. Why would you set the velocity in a direction that you don't want the object to move ?

Thanks,
Erwin

[dphil]

Are the linear and angular factors handled in such a way that they provide more efficient motion updates? I ask because I have a simulation with many spheres, and their rotation is unimportant. So if I "turn off" rotation by setting the angular factor to 0, will that mean *less* processing and greater efficiency? Or would disabling rotation essentially be like a constraint; ie yet another thing that needs to be processed, and should only be used if necessary?

[almatea]

rigid_Body->setLinearFactor(btVector3(0,0,0)) - it is a way to animate activity of a body (static and active mode) - it is correct way?