newton-physics · adenzler-nvidia · Apr 16, 2026 · Apr 16, 2026
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -29,6 +29,7 @@
 - Fix MPR convergence failure on large and extreme-aspect-ratio mesh triangles by projecting the starting point onto the triangle nearest the convex center
 - Fix O(W²·S²) memory explosion in `CollisionPipeline` shape-pair buffer allocation for NXN and SAP broad phase modes by computing per-world pair counts instead of a global N²
 - Fix `SensorRaycast` ignoring `PLANE` geometry
+- Fix multi-world `qfrc_actuator` conversion using the wrong body center of mass for worlds with `worldid > 0`
 - Fix SDF hydroelastic broadphase scatter kernel using a grid-stride loop with binary search instead of per-pair thread launch
 - Fix box support-map sign flips from quaternion rotation noise (~1e-14) producing invalid GJK/MPR contacts for face-touching boxes with non-trivial base rotations
 - Fix USD import of multi-DOF joints from MuJoCo-converted assets where multiple revolute joints between the same two bodies caused false cycle detection; merge them into D6 joints with correct DOF label mapping for MjcActuator target resolution

diff --git a/newton/_src/solvers/mujoco/kernels.py b/newton/_src/solvers/mujoco/kernels.py
@@ -2458,17 +2458,19 @@ def convert_qfrc_actuator_from_mj_kernel(
     """
     worldid, jntid = wp.tid()
 
+    joint_id = joints_per_world * worldid + jntid
+
     # Skip loop joints — they have no MuJoCo DOF entries
     q_i = mj_q_start[jntid]
     if q_i < 0:
         return
 
     qd_i = mj_qd_start[jntid]
-    wqd_i = joint_qd_start[joints_per_world * worldid + jntid]
+    wqd_i = joint_qd_start[joint_id]
 
-    type = joint_type[jntid]
+    jtype = joint_type[joint_id]
 
-    if type == JointType.FREE:
+    if jtype == JointType.FREE:
         # MuJoCo qfrc_actuator for free joint:
         #   [f_x, f_y, f_z] = linear force at body origin (world frame)
         #   [τ_x, τ_y, τ_z] = torque in body frame
@@ -2496,7 +2498,7 @@ def convert_qfrc_actuator_from_mj_kernel(
         )
 
         # CoM offset in world frame
-        child = joint_child[jntid]
+        child = joint_child[joint_id]
         com_world = wp.quat_rotate(rot, body_com[child])
 
         # Rotate torque body -> world and shift reference origin -> CoM
@@ -2508,11 +2510,11 @@ def convert_qfrc_actuator_from_mj_kernel(
         qfrc_actuator[wqd_i + 3] = tau_world[0]
         qfrc_actuator[wqd_i + 4] = tau_world[1]
         qfrc_actuator[wqd_i + 5] = tau_world[2]
-    elif type == JointType.BALL:
+    elif jtype == JointType.BALL:
         for i in range(3):
             qfrc_actuator[wqd_i + i] = mjw_qfrc_actuator[worldid, qd_i + i]
     else:
-        axis_count = joint_dof_dim[jntid, 0] + joint_dof_dim[jntid, 1]
+        axis_count = joint_dof_dim[joint_id, 0] + joint_dof_dim[joint_id, 1]
         for i in range(axis_count):
             qfrc_actuator[wqd_i + i] = mjw_qfrc_actuator[worldid, qd_i + i]
 

diff --git a/newton/tests/test_mujoco_solver.py b/newton/tests/test_mujoco_solver.py
@@ -8275,6 +8275,66 @@ def test_biasprm2_consistent_across_worlds(self):
             )
 
 
+class TestMultiWorldQfrcActuatorCom(unittest.TestCase):
+    """Multi-world qfrc_actuator must use each world's own body CoM.
+
+    Regression test: convert_qfrc_actuator_from_mj_kernel indexed
+    joint_type, joint_child, and joint_dof_dim with per-world jntid
+    instead of the global index, so world 1 read world 0's CoM for
+    the free-joint force transform.
+    """
+
+    MJCF = """
+    <mujoco>
+      <option gravity="0 0 0"/>
+      <worldbody>
+        <body pos="0 0 1" quat="0.7071 0.7071 0 0">
+          <freejoint name="root"/>
+          <geom type="box" size="0.1 0.1 0.05" mass="1"/>
+        </body>
+      </worldbody>
+      <actuator>
+        <motor joint="root" gear="0 0 1 0 0 0"/>
+      </actuator>
+    </mujoco>
+    """
+
+    @classmethod
+    def setUpClass(cls):
+        def make_template(com):
+            t = newton.ModelBuilder()
+            t.add_mjcf(cls.MJCF, ctrl_direct=True)
+            t.request_state_attributes("mujoco:qfrc_actuator")
+            t.body_com[0] = com
+            return t
+
+        builder = newton.ModelBuilder()
+        builder.request_state_attributes("mujoco:qfrc_actuator")
+        builder.add_world(make_template(wp.vec3(0.0, 0.0, 0.0)))
+        builder.add_world(make_template(wp.vec3(0.1, -0.05, 0.03)))
+        cls.model = builder.finalize()
+        cls.solver = SolverMuJoCo(cls.model)
+
+    def test_world1_uses_own_com(self):
+        """World 1 angular qfrc must reflect its non-zero CoM, not world 0's."""
+        state = self.model.state()
+        ctrl = self.model.control()
+        ctrl.mujoco.ctrl = wp.array([10.0, 10.0], dtype=wp.float32)
+        self.solver.step(state, state, ctrl, None, dt=0.01)
+
+        qfrc = state.mujoco.qfrc_actuator.numpy()
+        dofs_per_world = self.model.joint_dof_count // self.model.world_count
+
+        # World 0 has CoM at origin — cross product is zero
+        np.testing.assert_allclose(qfrc[3:6], 0.0, atol=0.01)
+        # World 1: tau = -cross(R * com, f_lin) where R is 90deg around X,
+        # com = (0.1, -0.05, 0.03), f_lin = (0, 0, 10)
+        # => R*com = (0.1, -0.03, -0.05), tau = (0.3, 1.0, 0.0)
+        # The bug would produce zeros here (reading world 0's zero CoM).
+        angular_1 = qfrc[dofs_per_world + 3 : dofs_per_world + 6]
+        np.testing.assert_allclose(angular_1, [0.3, 1.0, 0.0], atol=0.05)
+
+
 class TestActuatorLengthRangeRuntime(unittest.TestCase):
     """Verify actuator lengthrange updates after runtime gear changes."""