Fix computation of tangential contact forces in MJX _decode_pyramid.

PiperOrigin-RevId: 685665382
Change-Id: Icdbba4a3ee5fec804340d73d525a36ee570c2aa8

doc/changelog.rst

@@ -25,6 +25,7 @@ MJX
 - Added sphere-cylinder and sphere-ellipsoid collision functions (:github:issue:`2126`).
 - Fixed a bug with frictionloss constraints.
 - Added ``TENDONPOS`` sensor.
+- Fixed a bug with the computation of tangential contact forces in ``_decode_pyramid``.
 
 Bug fixes
 ^^^^^^^^^

mjx/mujoco/mjx/_src/support.py

@@ -295,7 +295,7 @@ def _decode_pyramid(
   force = force.at[0].set(pyramid[0 : 2 * (condim - 1)].sum())
 
   # force_tangent_i = (pyramid0_i - pyramid1_i) * mu_i
-  i = np.arange(0, condim)
+  i = np.arange(0, condim - 1)
   force = force.at[i + 1].set((pyramid[2 * i] - pyramid[2 * i + 1]) * mu[i])
 
   return force

mjx/mujoco/mjx/_src/support_test.py

@@ -203,6 +203,11 @@ class SupportTest(parameterized.TestCase):
       force = jax.jit(support.contact_force, static_argnums=(2,))(mx, dx, j)
       np.testing.assert_allclose(result, force, rtol=1e-5, atol=2)
 
+      # check for zeros after first condim elements
+      condim = dx.contact.dim[j]
+      if condim < 6:
+        np.testing.assert_allclose(force[condim:], 0, rtol=1e-5, atol=1e-5)
+
       # test world conversion
       force = jax.jit(
           support.contact_force,