Compute forces programatically

[jjcasmar]

I want to debug some forces on the initial state of the simulation. What I am doing right now is set the dt to 0 and do a step, that way the correct visitors are launched and the forces are computed, but looks like a hack.

What is the correct way to ask SOFA to compute force? I guess that I will also need to apply mappings before computing forces, and also apply mappings backwards to get the forces back to the DoF nodes.

[jnbrunet]

Hi @jjcasmar,

This is usually done using mechanical operators. In your case, you could create a component that does this:

sofa::core::MechanicalParams mparams (sofa::core::ExecParams::defaultInstance());
sofa::simulation::common::VectorOperations vop( &mparams, this->getContext());
sofa::simulation::common::MechanicalOperations mop( &mparams, this->getContext());
sofa::core::behavior::MultiVecDeriv F( &vop, sofa::core::VecDerivId::force());

// Accumulate the force vectors
// 1. Clear the force vector (F := 0)
// 2. Go down in the current context tree calling addForce on every forcefields
// 3. Go up from the current context tree leaves calling applyJT on every mechanical mappings
mop.computeForce(F);

Jean-Nicolas

[jjcasmar]

the mop.computeForce takes care of updating the mappings in both directions?

[jnbrunet]

It takes care of propagating the mapped force vector into the system (top level mechanical object) force vector.

[jjcasmar]

So I still need to update the mapping to propagate the positions, right?

[jnbrunet]

Exact, you can use

sofa::simulation::MechanicalPropagateOnlyPositionAndVelocityVisitor(&mparams).execute(this->getContext());

to do so.

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