Obtain hinge constraint angle

[ghost]

A function to obtain the "current" angle of a hinge constraint would be nice. This is already in Bullet, so it should already be exposed by ammo.js.

[jaredmoore]

This would be a welcome addition, I have been trying to give it a shot, but don't quite understand the communication happening between the two currently.

[chandlerprall]

@jaredmoore do you have a working example of getting the angle out of Ammo?

[jaredmoore]

No I do not have an example getting the Hinge angle yet. There is a call in Bullet, that I'm pretty sure is also in Ammo though I have yet to test it. Should know more later this week.

[jaredmoore]

Chandler, I was recently at the ALIFE 2014 conference and spoke with a few other researchers in the evolutionary robotics community. We would really like to use Physijs for our online work, but not having the ability to define the angle to move a hinge towards limits the usefulness of the simulation.

I'm currently trying to track down what code would be needed. The relevant call in Bullet would be the following lines:

btScalar btHingeConstraint::getHingeAngle() { return getHingeAngle(m_rbA.getCenterOfMassTransform(),m_rbB.getCenterOfMassTransform()); }

btScalar btHingeConstraint::getHingeAngle(const btTransform& transA,const btTransform& transB) { const btVector3 refAxis0 = transA.getBasis() * m_rbAFrame.getBasis().getColumn(0); const btVector3 refAxis1 = transA.getBasis() * m_rbAFrame.getBasis().getColumn(1); const btVector3 swingAxis = transB.getBasis() * m_rbBFrame.getBasis().getColumn(1); // btScalar angle = btAtan2Fast(swingAxis.dot(refAxis0), swingAxis.dot(refAxis1)); btScalar angle = btAtan2(swingAxis.dot(refAxis0), swingAxis.dot(refAxis1)); return m_referenceSign * angle; }

in the ammo.js/blob/master/bullet/src/BulletDynamics/ConstraintSolver/btHingeConstraint.cpp file. This is from the ammo.js distribution.

To make the call in ammo.js is simply the following:

hinge_constraint.getHingeAngle()

I attempted to solve this myself a few months ago, but got sidetracked and cannot locate the code. Do you have any idea on how best to implement this? If I remember correctly, I was getting hung up on the worker communication aspect.

For displacement command of robotic joints, I think the call would best look something like:

hinge_constraint.get_angle() which would return the angle between the two bodies. It's really a simple call, but I'm not sure on the implementation in this system.

If you have any other questions, feel free to ask me.

[chandlerprall]

Sorry about the delay in replying. Most of my github notifications started going to gmail's promotions tab for some reason so I wasn't seeing them. I'll try to find some time to look at making the constraint info accessible. If you have or can put together an example of all you need/want from Ammo itself I can quickly transfer it from the worker.

[jaredmoore]

Chandler, no problem. Really the only thing that needs to be called is something like the following:

hinge_constraint.getHingeAngle()

getHingeAngle() is already in Ammo and returns the angle for a given hinge_constraint. The only thing I haven't been able to figure out is how to pass that back from the worker to the main program. I have an example that's a little convoluted, it's a full hillclimbing evolutionary environment, but all you really need is that one call. I can make it available if you want through email.

Thanks again!