Disable RK4 integrator in mjd_transitionFD by throwing an error and improve documentation for mjd_transitionFD and mjd_inverseFD.

PiperOrigin-RevId: 659492713
Change-Id: I84ab7ebfba34742f239f60311d8f0a55559995fb

doc/APIreference/functions.rst

@@ -2618,6 +2618,9 @@ These matrices and their dimensions are:
   fixed (small) value and solver :ref:`tolerance<option-tolerance>` is set to 0. This insures that
   all calls to the solver will perform exactly the same number of iterations.
 
+.. attention::
+   - The Runge-Kutta 4th-order integrator (``mjINT_RK4``) is not supported.
+
 .. _mjd_inverseFD:
 
 mjd_inverseFD
@@ -2654,6 +2657,12 @@ using finite-differencing. These matrices and their dimensions are:
 - ``eps`` is the (forward) finite-differencing epsilon.
 - ``flg_actuation`` denotes whether to subtract actuation forces (``qfrc_actuator``) from the output of the inverse
   dynamics. If this flag is positive, actuator forces are not considered as external.
+- The model option flag ``invdiscrete`` should correspond to the representation of ``mjData.qacc`` in order to compute
+  the correct derivative information.
+
+.. attention::
+   - The Runge-Kutta 4th-order integrator (``mjINT_RK4``) is not supported.
+   - The noslip solver is not supported.
 
 .. _mjd_subQuat:
 

doc/APIreference/functions_override.rst

@@ -592,6 +592,9 @@ These matrices and their dimensions are:
   fixed (small) value and solver :ref:`tolerance<option-tolerance>` is set to 0. This insures that
   all calls to the solver will perform exactly the same number of iterations.
 
+.. attention::
+   - The Runge-Kutta 4th-order integrator (``mjINT_RK4``) is not supported.
+
 .. _mjd_inverseFD:
 
 Finite differenced continuous-time inverse-dynamics Jacobians.
@@ -623,6 +626,12 @@ using finite-differencing. These matrices and their dimensions are:
 - ``eps`` is the (forward) finite-differencing epsilon.
 - ``flg_actuation`` denotes whether to subtract actuation forces (``qfrc_actuator``) from the output of the inverse
   dynamics. If this flag is positive, actuator forces are not considered as external.
+- The model option flag ``invdiscrete`` should correspond to the representation of ``mjData.qacc`` in order to compute
+  the correct derivative information.
+
+.. attention::
+   - The Runge-Kutta 4th-order integrator (``mjINT_RK4``) is not supported.
+   - The noslip solver is not supported.
 
 .. _mjd_subQuat:
 

src/engine/engine_derivative_fd.c

@@ -548,6 +548,10 @@ void mjd_stepFD(const mjModel* m, mjData* d, mjtNum eps, mjtByte flg_centered,
 //      D: (nsensordata x nu)
 void mjd_transitionFD(const mjModel* m, mjData* d, mjtNum eps, mjtByte flg_centered,
                       mjtNum* A, mjtNum* B, mjtNum* C, mjtNum* D) {
+  if (m->opt.integrator == mjINT_RK4) {
+    mjERROR("RK4 integrator is not supported");
+  }
+
   int nv = m->nv, na = m->na, nu = m->nu, ns = m->nsensordata;
   int ndx = 2*nv+na;  // row length of state Jacobians