Applying force through constraints [Solved]

[nathn123]

Hey all

I have set up an interpretation of a hill-type muscle in bullet using 3 rigid bodies ,a spring2point and a slider. i have calculated the forces generated by the muscle i just need help to apply these forces through the constraints just wondering if anyone knows of any ways to apply these forces

[Erwin Coumans]

Constraints compute their own constraint forces. If you computed your own forces, you don't need to use a constraint to apply them, just apply the force directly to a rigid body (or multi body).

Say, you have a btHingeConstraint, and want to apply a torque along the hinge axis, you can use the following snippet:

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        btScalar torque = ...
        btHingeConstraint* hinge = ...
        btRigidBody& bodyA = hinge->getRigidBodyA();
        btTransform trA = bodyA.getWorldTransform();
        btVector3 hingeAxisInWorld = trA.getBasis()*hinge->getFrameOffsetA().getBasis().getColumn(2);
        hinge->getRigidBodyA().applyTorque(-hingeAxisInWorld*torque);
        hinge->getRigidBodyB().applyTorque(hingeAxisInWorld*torque);

You may also consider using a btMultiBody with revolute joints, instead of btRigidBody and btHingeConstraint.
Then you can apply a joint torque using

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multiBody->addJointTorque(linkIndex,torque);

[nathn123]

thanks for the reply, had a feeling thats what i needed todo but was hoping that there would be a more realistic approach since in real muscles the force doesn't come from the bone
also with regard to the multibody, the way i have designed the muscles and calculation of force it needs a physical representation to work