Fix multi-world joint indexing in warp-to-mj coord conversion

CHANGELOG.md

@@ -47,6 +47,7 @@
 - Fix MJCF importer creating finite planes from MuJoCo visual half-sizes instead of infinite planes
 - Fix ViewerViser mesh popping artifacts caused by viser's automatic LOD simplification creating holes in complex geometry
 - Fix degenerate zero-area triangles in SDF marching-cubes isosurface extraction by clamping edge interpolation away from cube corners and guarding against near-zero cross products
+- Fix multi-world coordinate conversion using the wrong body center of mass for replicated worlds
 
 ## [1.1.0] - 2026-04-13
 

newton/_src/solvers/mujoco/kernels.py

@@ -644,17 +644,19 @@ def convert_warp_coords_to_mj_kernel(
 ):
     worldid, jntid = wp.tid()
 
+    joint_id = joints_per_world * worldid + jntid
+
     # Skip loop joints — they have no MuJoCo qpos/qvel entries
     q_i = mj_q_start[jntid]
     if q_i < 0:
         return
 
     qd_i = mj_qd_start[jntid]
-    type = joint_type[jntid]
-    wq_i = joint_q_start[joints_per_world * worldid + jntid]
-    wqd_i = joint_qd_start[joints_per_world * worldid + jntid]
+    jtype = joint_type[joint_id]
+    wq_i = joint_q_start[joint_id]
+    wqd_i = joint_qd_start[joint_id]
 
-    if type == JointType.FREE:
+    if jtype == JointType.FREE:
         # convert position components
         for i in range(3):
             qpos[worldid, q_i + i] = joint_q[wq_i + i]
@@ -683,7 +685,7 @@ def convert_warp_coords_to_mj_kernel(
         w_world = wp.vec3(joint_qd[wqd_i + 3], joint_qd[wqd_i + 4], joint_qd[wqd_i + 5])
 
         # Get CoM offset in world frame
-        child = joint_child[jntid]
+        child = joint_child[joint_id]
         com_local = body_com[child]
         com_world = wp.quat_rotate(rot, com_local)
 
@@ -702,7 +704,7 @@ def convert_warp_coords_to_mj_kernel(
         qvel[worldid, qd_i + 4] = w_body[1]
         qvel[worldid, qd_i + 5] = w_body[2]
 
-    elif type == JointType.BALL:
+    elif jtype == JointType.BALL:
         # change quaternion order from xyzw to wxyz
         ball_q = wp.quat(joint_q[wq_i], joint_q[wq_i + 1], joint_q[wq_i + 2], joint_q[wq_i + 3])
         ball_q_wxyz = quat_xyzw_to_wxyz(ball_q)
@@ -714,7 +716,7 @@ def convert_warp_coords_to_mj_kernel(
             # convert velocity components
             qvel[worldid, qd_i + i] = joint_qd[wqd_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):
             ref = float(0.0)
             if dof_ref:

newton/tests/test_multiworld_body_properties.py

@@ -0,0 +1,133 @@
+# SPDX-FileCopyrightText: Copyright (c) 2026 The Newton Developers
+# SPDX-License-Identifier: Apache-2.0
+
+"""Test that per-world body property changes produce correct physics.
+
+Regression test for a bug in convert_warp_coords_to_mj_kernel where
+joint_type and joint_child were indexed with the per-world joint index
+instead of the global index, causing incorrect CoM references for
+worlds with worldid > 0.
+"""
+
+import unittest
+
+import numpy as np
+import warp as wp
+
+import newton
+from newton.solvers import SolverMuJoCo
+from newton.tests.unittest_utils import add_function_test, get_test_devices
+
+
+def _build_model_with_per_world_com(device, world_base_coms):
+    """Build a multi-world model where each world has a free-floating
+    body with its own base CoM.
+    """
+    scene = newton.ModelBuilder()
+
+    for base_com in world_base_coms:
+        template = newton.ModelBuilder()
+        base = template.add_link(
+            mass=5.0,
+            com=base_com,
+            inertia=wp.mat33(np.eye(3) * 0.1),
+        )
+        j_free = template.add_joint_free(parent=-1, child=base)
+        template.add_articulation([j_free])
+
+        scene.begin_world()
+        scene.add_builder(template)
+        scene.end_world()
+
+    return scene.finalize(device=device)
+
+
+def _run_sim(model, num_steps: int = 100, sim_dt: float = 1e-3):
+    """Step simulation and return final joint_q as numpy array."""
+    solver = SolverMuJoCo(model)
+
+    state_0 = model.state()
+    state_1 = model.state()
+    control = model.control()
+
+    # Set initial height and angular velocity for all worlds.
+    # Non-zero angular velocity is critical: the bug in
+    # convert_warp_coords_to_mj_kernel affects the velocity
+    # conversion v_origin = v_com - w x com. With w=0 the cross
+    # product vanishes and the wrong com is harmless.
+    nw = model.world_count
+    q_per_world = model.joint_coord_count // nw
+    qd_per_world = model.joint_dof_count // nw
+    joint_q = state_0.joint_q.numpy().reshape(nw, q_per_world)
+    joint_q[:, 2] = 1.0  # z = 1m for all worlds
+    state_0.joint_q.assign(joint_q.flatten())
+    state_1.joint_q.assign(joint_q.flatten())
+    # Set initial angular velocity (world frame) on the free joint
+    joint_qd = state_0.joint_qd.numpy().reshape(nw, qd_per_world)
+    joint_qd[:, 3] = 1.0  # wx = 1 rad/s
+    joint_qd[:, 4] = 0.5  # wy = 0.5 rad/s
+    state_0.joint_qd.assign(joint_qd.flatten())
+    state_1.joint_qd.assign(joint_qd.flatten())
+    newton.eval_fk(model, state_0.joint_q, state_0.joint_qd, state_0)
+    newton.eval_fk(model, state_1.joint_q, state_1.joint_qd, state_1)
+
+    for _ in range(num_steps):
+        state_0.clear_forces()
+        solver.step(state_0, state_1, control, None, sim_dt)
+        state_0, state_1 = state_1, state_0
+
+    return state_0.joint_q.numpy()
+
+
+def test_perworld_com_produces_consistent_physics(test, device):
+    """Multi-world with different per-world base CoM should match
+    independent single-world runs with the same CoM."""
+    com_a = wp.vec3(0.0, 0.0, 0.0)
+    com_b = wp.vec3(0.05, 0.0, -0.02)
+
+    # --- Reference: single-world runs ---
+    ref_q = {}
+    for com_val, label in [(com_a, "A"), (com_b, "B")]:
+        model = _build_model_with_per_world_com(device, [com_val])
+        ref_q[label] = _run_sim(model)
+
+    # --- Multi-world run ---
+    model = _build_model_with_per_world_com(device, [com_a, com_b])
+    multi_q = _run_sim(model)
+
+    q_per_world = model.joint_coord_count // model.world_count
+    multi_q = multi_q.reshape(model.world_count, q_per_world)
+
+    np.testing.assert_allclose(
+        multi_q[0],
+        ref_q["A"],
+        atol=1e-4,
+        err_msg="World 0 (com_A) diverges from single-world reference",
+    )
+    np.testing.assert_allclose(
+        multi_q[1],
+        ref_q["B"],
+        atol=1e-4,
+        err_msg="World 1 (com_B) diverges from single-world reference",
+    )
+
+
+class TestMultiworldBodyProperties(unittest.TestCase):
+    """Verify that multi-world simulations with per-world body mass/CoM
+    produce physically consistent results across all worlds."""
+
+    pass
+
+
+devices = get_test_devices()
+for device in devices:
+    add_function_test(
+        TestMultiworldBodyProperties,
+        f"test_perworld_com_produces_consistent_physics_{device}",
+        test_perworld_com_produces_consistent_physics,
+        devices=[device],
+    )
+
+
+if __name__ == "__main__":
+    unittest.main()