Get actuator jacobian

Pretty straightforward question: is there a way to get the actuator jacobian in MuJoCo?
It is for instance mentioned here: http://www.mujoco.org/forum/index.php?threads/qfrc_inverse.3463/#post-3986

The actuator jacobian maps the system inputs to generalized forces:

qfrc_actuator = B*u

During forward simulation it must be used in some form.

 

I don't think there is a built-in feature for this. Instead, I now determine it numerically. Which is okay because I only need to do it once, since all my actuators apply directly to a generalized coordinates.

Code:

    MatrixXd jac(m->nv, m->nu);

    const int var_size = m->nu; // Size of ctrl

    mjtNum qfrc_center[m->nv];
    mju_copy(qfrc_center, d->qfrc_actuator, m->nv);

    // Loop over variables to be varied
    for (int i_var = 0; i_var < var_size; i_var++)
    {
        double original = d->ctrl[i_var];

        d->ctrl[i_var] += eps; // Perturb current variable

        // Perform torque forward operation
        mj_fwdActuation(m, d);

        // Loop through columns
        for (int c = 0; c < m->nv; c++)
        {
            mjtNum qfrc_perturbed_c = d->qfrc_actuator[c];

            // Take finite difference
            double derivative = (qfrc_perturbed_c - qfrc_center[c]) / eps;

            // Place inside derivative vector / matrix
            jac(c, i_var) = derivative;
        }

        d->ctrl[i_var] = original;
    }

    mj_fwdActuation(m, d); // Reset actuation

    return jac;