Add mjContact.solreffriction, mjModel.pair_solreffriction and related implementation, allowing contact reference acceleration to be different for normal and frictional contact directions. This is required for e.g. elastic frictional collisions.

PiperOrigin-RevId: 538513357
Change-Id: Id9eb910fe791cf898c712030cb2c0fc481389c91

python/mujoco/structs.cc

@@ -486,6 +486,7 @@ MjContactWrapper::MjWrapper()
       X(frame),
       X(friction),
       X(solref),
+      X(solreffriction),
       X(solimp),
       X(H) {}
 
@@ -495,6 +496,7 @@ MjContactWrapper::MjWrapper(raw::MjContact* ptr, py::handle owner)
       X(frame),
       X(friction),
       X(solref),
+      X(solreffriction),
       X(solimp),
       X(H) {}
 #undef X
@@ -1735,6 +1737,7 @@ This is useful for example when the MJB is not available as a file on disk.)"));
   X(frame);
   X(friction);
   X(solref);
+  X(solreffriction);
   X(solimp);
   X(H);
 #undef X
@@ -1766,6 +1769,7 @@ This is useful for example when the MJB is not available as a file on disk.)"));
   X(mjtNum, includemargin);
   XN(mjtNum, friction);
   XN(mjtNum, solref);
+  XN(mjtNum, solreffriction);
   XN(mjtNum, solimp);
   X(mjtNum, mu);
   XN(mjtNum, H);

python/mujoco/structs.h

@@ -503,6 +503,7 @@ class MjWrapper<raw::MjContact> : public WrapperBase<raw::MjContact> {
   X(frame);
   X(friction);
   X(solref);
+  X(solreffriction);
   X(solimp);
   X(H);
   #undef X