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

doc/changelog.rst

@@ -63,6 +63,13 @@ Simulate
 General
 ^^^^^^^
 
+- Added :ref:`mjContact.solreffriction<mjContact>`, allowing different :ref:`solref<CSolver>` parameters for the normal
+  and frictional axes of contacts when using :ref:`elliptic friction cones<option-cone>`.  This attribute is required
+  for elastic frictional collisions, see associated
+  `example model <https://github.com/deepmind/mujoco/blob/main/test/engine/testdata/bounce_recoil.xml>`__ mimicking the
+  spin-bounce recoil behaviour of `elastic rubber balls <https://www.youtube.com/watch?v=uFLJcRegIVQ&t=3s>`__.
+  This is an advanced option currently only supported by explicit :ref:`contact pairs<contact-pair>`, using the
+  :ref:`solreffriction<contact-pair-solreffriction>` attribute.
 - Added :ref:`mjd_inverseFD` for finite-differenced inverse-dynamics derivatives.
 - Added functions for operations on banded-then-dense "arrowhead" matrices. Such matrices are common when doing direct
   trajectory optimization. See :ref:`mju_cholFactorBand` documentation for details.