derivative.cpp example qpos indexing error

Discussion in 'Bug Reports' started by Taylor Apgar, Jul 21, 2017.

  1. Taylor Apgar

    Taylor Apgar

    In the derivative.cpp example it first computes derivative w.r.t force, then velocity, and finally qpos. Care was taken to make sure quaternions are differentiated correctly but I believe an indexing error remains if you have a model with quaternions (m->nq != m->nv).

    In the worker function where the derivative w.r.t qpos is calculated:

    // finite-difference over position: skip = mjSTAGE_NONE
    for( int i=istart; i<iend; i++ ) <- iterating over m->nv
    {
    /.../
    d->qpos += eps; <- I believe the index should be in terms of qpos indices so I changed this to

    this
    d->qpos[m->jnt_dofadr[jid]] += eps;

    and now I get the correct FD matrix.
     
    Taylor Apgar, Jul 21, 2017
    #1
  2. Emo Todorov

    Emo Todorov Administrator Staff Member

    You are right, there is an indexing bug in that code which I also discovered recently. The code sample in the documentation is now updated.

    The correct correction is:

    d->qpos[m->jnt_qposadr[jid] + i - m->jnt_dofadr[jid]] += eps;

    The extra offset is needed because for free joints we have 3 translational dofs for the same jnt_qposadr.
     
    Emo Todorov, Jul 21, 2017
    #2