feat(unitree_g1): standalone PICO SMPL publisher + dedup/replay fixes

Add a self-contained rt/smpl publisher in the pico_headset teleoperator
(pico_publisher.py + numpy SMPL FK in smpl_fk.py + vendored skeleton table)
so headset whole-body teleop no longer depends on gear_sonic/torch; only
xrobotoolkit_sdk is needed at the headset.

Also: share lowstate_to_obs/get_gravity_orientation via g1_utils (dedup
sonic_pipeline and UnitreeG1.get_observation), and fix dataset-replay joint
ordering (Unitree -> IsaacLab) for sonic.py --replay-dataset.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/lerobot/robots/unitree_g1/controllers/sonic_pipeline.py

@@ -30,7 +30,10 @@ from enum import IntEnum
 import numpy as np
 import onnxruntime as ort
 
-from lerobot.robots.unitree_g1.g1_utils import G1_29_JointIndex
+from lerobot.robots.unitree_g1.g1_utils import (
+    G1_29_JointIndex,
+    get_gravity_orientation,
+)
 
 # ── Constants ────────────────────────────────────────────────────────────────
 
@@ -174,12 +177,6 @@ def _to_mujoco(a):
     return a[MUJOCO_TO_ISAACLAB]
 
 
-def _to_runtime(a):
-    r = np.zeros(29, np.float32)
-    r[MUJOCO_TO_ISAACLAB] = a
-    return r
-
-
 DEFAULT_ANGLES_MUJOCO = _to_mujoco(DEFAULT_ANGLES)
 ENCODER_STANDING_REF = DEFAULT_ANGLES.copy()
 
@@ -214,28 +211,6 @@ def compute_kp_kd():
 _kp_kd = compute_kp_kd  # backward-compatible alias
 
 
-def lowstate_to_obs(lowstate) -> dict:
-    """Build a robot observation dict from Unitree lowstate."""
-    obs: dict = {}
-    for motor in G1_29_JointIndex:
-        idx = motor.value
-        obs[f"{motor.name}.q"] = float(lowstate.motor_state[idx].q)
-        obs[f"{motor.name}.dq"] = float(lowstate.motor_state[idx].dq)
-    quat = lowstate.imu_state.quaternion
-    obs["imu.quat.w"] = float(quat[0])
-    obs["imu.quat.x"] = float(quat[1])
-    obs["imu.quat.y"] = float(quat[2])
-    obs["imu.quat.z"] = float(quat[3])
-    gyro = lowstate.imu_state.gyroscope
-    obs["imu.gyro.x"] = float(gyro[0])
-    obs["imu.gyro.y"] = float(gyro[1])
-    obs["imu.gyro.z"] = float(gyro[2])
-    wr = getattr(lowstate, "wireless_remote", None)
-    if wr is not None:
-        obs["wireless_remote"] = bytes(wr) if not isinstance(wr, (bytes, bytearray)) else wr
-    return obs
-
-
 # ── Quaternion helpers ────────────────────────────────────────────────────────
 
 
@@ -257,12 +232,6 @@ def quat_mul(q1, q2):
     )
 
 
-def gravity_dir(q):
-    q = q / (np.linalg.norm(q) + 1e-8)
-    qv = np.array([0, 0, 0, -1], dtype=np.float32)
-    return quat_mul(quat_mul(quat_conj(q), qv), q)[1:]
-
-
 def quat_to_6d(q):
     w, x, y, z = q
     return np.array(
@@ -631,7 +600,7 @@ class StandingEncoderDecoder:
                 obs[off : off + sz] = h[f]
                 off += sz
         for q in reversed(self.h_quat):
-            obs[off : off + 3] = gravity_dir(q)
+            obs[off : off + 3] = get_gravity_orientation(q)
             off += 3
         assert off == 994, f"Decoder obs mismatch: {off}"
         return obs
@@ -689,7 +658,9 @@ class StandingEncoderDecoder:
         print(
             f"  anchor_6d(identity): {self._anchor_6d(np.array([1, 0, 0, 0], np.float32), np.array([1, 0, 0, 0], np.float32))}"
         )
-        print(f"  gravity(identity): {gravity_dir(np.array([1, 0, 0, 0], np.float32))} (expect [0,0,-1])")
+        print(
+            f"  gravity(identity): {get_gravity_orientation(np.array([1, 0, 0, 0], np.float32))} (expect [0,0,-1])"
+        )
         dec0 = self.build_decoder_obs()
         print(f"  decoder q-delta max: {np.max(np.abs(dec0[94:384])):.6f}")
         print(f"  decoder dq max:      {np.max(np.abs(dec0[384:674])):.6f}")
@@ -1122,7 +1093,6 @@ class PlannerController(StandingEncoderDecoder):
 
 # ── Keyboard ──────────────────────────────────────────────────────────────────
 
-
 class RawKeyboard:
     def __init__(self):
         self.fd = sys.stdin.fileno()

src/lerobot/robots/unitree_g1/controllers/sonic_whole_body.py

@@ -37,10 +37,10 @@ from lerobot.robots.unitree_g1.controllers.sonic_pipeline import (
     _snapshot_ms,
     clamp_mode_params,
     compute_kp_kd,
-    lowstate_to_obs,
     process_joystick,
     should_replan_request,
 )
+from lerobot.robots.unitree_g1.g1_utils import lowstate_to_obs
 
 logger = logging.getLogger(__name__)
 
@@ -185,25 +185,49 @@ class SonicWholeBodyController:
         self._smpl_stream = SmplStream(host=host, port=port)
         logger.info("SONIC subscribed to rt/smpl @ tcp://%s:%d", host, port)
 
+    def _enter_wholebody(self) -> None:
+        """Switch into SMPL whole-body tracking (encode_mode 2)."""
+        self.controller.encode_mode = 2
+        self.controller.reinit_heading = True
+        logger.info("SONIC: SMPL stream active -> whole-body tracking (mode 2)")
+
+    def _exit_wholebody(self) -> None:
+        """Revert to locomotion/standing (encode_mode 0) after SMPL is lost.
+
+        Mirrors the 'M' toggle in sonic.py so the handoff is clean: the robot holds
+        a standing reference and (if a joystick teleop is attached) can be driven.
+        """
+        self.controller.encode_mode = 0
+        self.controller.playing = True
+        self.controller.reinit_heading = True
+        self.ms.needs_replan = True
+        logger.warning("SONIC: SMPL stream lost/stale -> reverting to locomotion (standing)")
+
     def run_step(self, action: dict, lowstate) -> dict:
         if lowstate is None:
             return {}
         obs = lowstate_to_obs(lowstate)
 
         # Prefer SMPL delivered via the teleop action (pico_headset). Fall back to a
-        # direct rt/smpl subscription when SONIC_SMPL_STREAM is enabled.
+        # direct rt/smpl subscription when SONIC_SMPL_STREAM is enabled. A stale
+        # stream (headset silent past its timeout) is treated as "no SMPL" so the
+        # robot doesn't stay frozen tracking the last pose.
         smpl = _extract_smpl_from_action(action)
         if smpl is None and self._smpl_stream is not None:
             window = self._smpl_stream.step()
-            if self._smpl_stream.has_data:
+            if self._smpl_stream.has_data and not self._smpl_stream.is_stale:
                 smpl = window
 
         if smpl is not None:
             # Full-body whole-body tracking: SMPL drives the reference, not joystick.
-            self.controller.encode_mode = 2
+            if self.controller.encode_mode != 2:
+                self._enter_wholebody()
             self.controller.smpl_joints_10frame_step1 = smpl
             return self._runtime.tick(obs, debug=False, use_joystick=False)
 
+        # No (or stale) SMPL: fall back to locomotion so the robot stays balanced.
+        if self.controller.encode_mode == 2:
+            self._exit_wholebody()
         return self._runtime.tick(obs, debug=False)
 
     def reset(self):

src/lerobot/robots/unitree_g1/g1_utils.py

@@ -63,6 +63,47 @@ class G1_29_JointArmIndex(IntEnum):
     kRightWristYaw = 28
 
 
+def lowstate_to_obs(lowstate) -> dict:
+    """Build a robot observation dict from a Unitree lowstate.
+
+    Shared by ``UnitreeG1.get_observation`` and the SONIC pipeline so the
+    lowstate -> obs mapping lives in exactly one place. Keys match the
+    ``<joint>.q``/``imu.*`` schema consumed across the controllers.
+    """
+    obs: dict = {}
+
+    for motor in G1_29_JointIndex:
+        idx = motor.value
+        obs[f"{motor.name}.q"] = lowstate.motor_state[idx].q
+        obs[f"{motor.name}.dq"] = lowstate.motor_state[idx].dq
+        obs[f"{motor.name}.tau"] = lowstate.motor_state[idx].tau_est
+
+    imu = lowstate.imu_state
+    if imu.gyroscope:
+        obs["imu.gyro.x"] = imu.gyroscope[0]
+        obs["imu.gyro.y"] = imu.gyroscope[1]
+        obs["imu.gyro.z"] = imu.gyroscope[2]
+    if imu.accelerometer:
+        obs["imu.accel.x"] = imu.accelerometer[0]
+        obs["imu.accel.y"] = imu.accelerometer[1]
+        obs["imu.accel.z"] = imu.accelerometer[2]
+    if imu.quaternion:
+        obs["imu.quat.w"] = imu.quaternion[0]
+        obs["imu.quat.x"] = imu.quaternion[1]
+        obs["imu.quat.y"] = imu.quaternion[2]
+        obs["imu.quat.z"] = imu.quaternion[3]
+    if imu.rpy:
+        obs["imu.rpy.roll"] = imu.rpy[0]
+        obs["imu.rpy.pitch"] = imu.rpy[1]
+        obs["imu.rpy.yaw"] = imu.rpy[2]
+
+    wr = getattr(lowstate, "wireless_remote", None)
+    if wr:
+        obs["wireless_remote"] = bytes(wr) if not isinstance(wr, (bytes, bytearray)) else wr
+
+    return obs
+
+
 def make_locomotion_controller(name: str | None):
     """Instantiate a locomotion controller by class name. Returns None if name is None."""
     if name is None:

src/lerobot/robots/unitree_g1/smpl_stream.py

@@ -112,6 +112,24 @@ class SmplStream:
         """True once at least one real frame has been received."""
         return self._got_first
 
+    @property
+    def seconds_since_last(self) -> float:
+        """Wall-clock seconds since the last real frame (inf before the first)."""
+        if not self._got_first:
+            return float("inf")
+        return time.time() - self._last_recv_t
+
+    @property
+    def is_stale(self) -> bool:
+        """True when the stream has gone silent past ``stale_after_s``.
+
+        Consumers use this to stop feeding a frozen pose and let the controller
+        fall back to a safe standing/locomotion mode.
+        """
+        if not self._got_first or not self.stale_after_s:
+            return False
+        return self.seconds_since_last > self.stale_after_s
+
     def reset(self):
         self._buf.clear()
         self._got_first = False

src/lerobot/robots/unitree_g1/unitree_g1.py

@@ -38,6 +38,7 @@ from .g1_utils import (
     G1_29_JointArmIndex,
     G1_29_JointIndex,
     default_remote_input,
+    lowstate_to_obs,
     make_locomotion_controller,
 )
 
@@ -428,44 +429,8 @@ class UnitreeG1(Robot):
         if lowstate is None:
             return {}
 
-        obs = {}
-
-        # Motors - q, dq, tau for all joints
-        for motor in G1_29_JointIndex:
-            name = motor.name
-            idx = motor.value
-            obs[f"{name}.q"] = lowstate.motor_state[idx].q
-            obs[f"{name}.dq"] = lowstate.motor_state[idx].dq
-            obs[f"{name}.tau"] = lowstate.motor_state[idx].tau_est
-
-        # IMU - gyroscope
-        if lowstate.imu_state.gyroscope:
-            obs["imu.gyro.x"] = lowstate.imu_state.gyroscope[0]
-            obs["imu.gyro.y"] = lowstate.imu_state.gyroscope[1]
-            obs["imu.gyro.z"] = lowstate.imu_state.gyroscope[2]
-
-        # IMU - accelerometer
-        if lowstate.imu_state.accelerometer:
-            obs["imu.accel.x"] = lowstate.imu_state.accelerometer[0]
-            obs["imu.accel.y"] = lowstate.imu_state.accelerometer[1]
-            obs["imu.accel.z"] = lowstate.imu_state.accelerometer[2]
-
-        # IMU - quaternion
-        if lowstate.imu_state.quaternion:
-            obs["imu.quat.w"] = lowstate.imu_state.quaternion[0]
-            obs["imu.quat.x"] = lowstate.imu_state.quaternion[1]
-            obs["imu.quat.y"] = lowstate.imu_state.quaternion[2]
-            obs["imu.quat.z"] = lowstate.imu_state.quaternion[3]
-
-        # IMU - rpy
-        if lowstate.imu_state.rpy:
-            obs["imu.rpy.roll"] = lowstate.imu_state.rpy[0]
-            obs["imu.rpy.pitch"] = lowstate.imu_state.rpy[1]
-            obs["imu.rpy.yaw"] = lowstate.imu_state.rpy[2]
-
-        # Wireless remote (raw bytes for teleoperator)
-        if lowstate.wireless_remote:
-            obs["wireless_remote"] = lowstate.wireless_remote
+        # Motors + IMU + wireless remote (shared lowstate -> obs mapping)
+        obs = lowstate_to_obs(lowstate)
 
         # Cameras - read images from ZMQ cameras
         for cam_name, cam in self._cameras.items():

src/lerobot/teleoperators/pico_headset/pico_headset.py

@@ -91,9 +91,11 @@ class PicoHeadset(Teleoperator):
         if self._stream is None:
             raise RuntimeError(f"{self} is not connected")
         window = self._stream.step()
-        # Hold back until the headset is actually streaming, so the controller does
-        # not switch to whole-body tracking of an all-zero (collapsed) pose.
-        if not self._stream.has_data:
+        # Emit SMPL only while the headset is actively streaming: hold back before
+        # the first frame (so the controller doesn't track an all-zero collapsed
+        # pose) and once the stream goes stale (so the controller falls back to a
+        # safe standing/locomotion mode instead of freezing on the last pose).
+        if not self._stream.has_data or self._stream.is_stale:
             return {}
         return {f"{SMPL_ACTION_PREFIX}{i}": float(v) for i, v in enumerate(window)}
 

src/lerobot/teleoperators/pico_headset/pico_publisher.py

@@ -0,0 +1,153 @@
+#!/usr/bin/env python
+
+# Copyright 2025 The HuggingFace Inc. team. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Standalone ``rt/smpl`` publisher for the PICO headset (no gear_sonic / torch).
+
+Reads 24 body-joint poses from the XRoboToolkit SDK, runs pure-numpy SMPL forward
+kinematics + canonicalization (``smpl_fk.py``), and publishes one canonical
+``(24, 3)`` SMPL frame per tick over ZMQ on the ``rt/smpl`` topic — the exact
+message ``lerobot.robots.unitree_g1.smpl_stream.SmplStream`` consumes.
+
+This makes the LeRobot side self-contained: the only runtime dependency to drive
+SONIC whole-body teleop from the headset is the ``xrobotoolkit_sdk`` Python package
+(plus numpy/scipy/pyzmq), not the full ``gear_sonic`` stack.
+
+Usage:
+    # Real headset (XRoboToolkit PC Service must be running and connected):
+    python -m lerobot.teleoperators.pico_headset.pico_publisher --fps 50 --port 5560
+
+    # No hardware — emit a synthetic waving motion to test the consumer end-to-end:
+    python -m lerobot.teleoperators.pico_headset.pico_publisher --fake
+"""
+
+from __future__ import annotations
+
+import argparse
+import contextlib
+import json
+import time
+
+import numpy as np
+import zmq
+
+from lerobot.teleoperators.pico_headset.smpl_fk import SmplForwardKinematics
+
+SMPL_TOPIC = "rt/smpl"
+DEFAULT_SMPL_PORT = 5560
+
+
+def pack_message(
+    smpl_joints_local: np.ndarray,
+    frame_index: int,
+    stamp_ns: int,
+    root_quat: np.ndarray | None = None,
+    root_transl: np.ndarray | None = None,
+) -> bytes:
+    """Build the rt/smpl JSON message (single frame, topic embedded in payload)."""
+    data = {
+        "smpl_joints_local": np.asarray(smpl_joints_local, np.float32).reshape(-1).tolist(),
+        "frame_index": int(frame_index),
+        "stamp_ns": int(stamp_ns),
+    }
+    if root_quat is not None:
+        data["root_quat"] = np.asarray(root_quat, np.float32).reshape(-1).tolist()
+    if root_transl is not None:
+        data["root_transl"] = np.asarray(root_transl, np.float32).reshape(-1).tolist()
+    return json.dumps({"topic": SMPL_TOPIC, "data": data}).encode("utf-8")
+
+
+def _fake_body_poses(t: float) -> np.ndarray:
+    """Synthetic (24, 7) body poses: identity rotations + a gently waving right arm."""
+    poses = np.zeros((24, 7), np.float32)
+    poses[:, 6] = 1.0  # unit quaternion (qw = 1), scalar-last
+    poses[:, 1] = 1.0  # ~1 m pelvis height (positions only matter for root_transl)
+    # Wave the right shoulder (SMPL body joint 17) about Z.
+    ang = 0.5 * np.sin(2.0 * np.pi * 0.5 * t)
+    poses[17, 3:7] = [0.0, 0.0, np.sin(ang / 2), np.cos(ang / 2)]
+    return poses
+
+
+def main() -> None:
+    p = argparse.ArgumentParser(description=__doc__)
+    p.add_argument("--port", type=int, default=DEFAULT_SMPL_PORT, help="ZMQ PUB port for rt/smpl")
+    p.add_argument("--fps", type=float, default=50.0, help="Target publish rate (Hz)")
+    p.add_argument("--skeleton", type=str, default=None, help="Path to smpl_skeleton.npz")
+    p.add_argument("--fake", action="store_true", help="Publish synthetic motion (no headset)")
+    args = p.parse_args()
+
+    fk = SmplForwardKinematics(args.skeleton) if args.skeleton else SmplForwardKinematics()
+
+    xrt = None
+    if not args.fake:
+        try:
+            import xrobotoolkit_sdk as xrt  # noqa: PLC0415
+        except ImportError as e:
+            raise SystemExit(
+                "xrobotoolkit_sdk not available. Install it, or run with --fake to test "
+                "the pipeline without a headset."
+            ) from e
+        xrt.init()
+        print("[pico_publisher] XRoboToolkit initialized")
+
+    ctx = zmq.Context.instance()
+    sock = ctx.socket(zmq.PUB)
+    sock.bind(f"tcp://*:{args.port}")
+    print(f"[pico_publisher] '{SMPL_TOPIC}' bound to tcp://*:{args.port} @ {args.fps:.0f} Hz")
+
+    period = 1.0 / max(1.0, args.fps)
+    frame_index = 0
+    t0 = time.time()
+    try:
+        while True:
+            loop_start = time.time()
+            if args.fake:
+                body_poses = _fake_body_poses(loop_start - t0)
+                stamp_ns = time.time_ns()
+            else:
+                body_poses = np.asarray(xrt.get_body_joints_pose(), np.float32)
+                stamp_ns = int(xrt.get_time_stamp_ns())
+                if body_poses.shape != (24, 7):
+                    time.sleep(0.005)
+                    continue
+
+            out = fk.compute(body_poses)
+            sock.send(
+                pack_message(
+                    out["smpl_joints_local"],
+                    frame_index,
+                    stamp_ns,
+                    root_quat=out["root_quat"],
+                    root_transl=out["root_transl"],
+                )
+            )
+            frame_index += 1
+            if frame_index % int(max(1, args.fps)) == 0:
+                print(f"[pico_publisher] sent {frame_index} frames", end="\r")
+
+            dt = time.time() - loop_start
+            if dt < period:
+                time.sleep(period - dt)
+    except KeyboardInterrupt:
+        print("\n[pico_publisher] stopping")
+    finally:
+        sock.close(0)
+        if xrt is not None:
+            with contextlib.suppress(Exception):
+                xrt.close()
+
+
+if __name__ == "__main__":
+    main()

src/lerobot/teleoperators/pico_headset/smpl_fk.py

@@ -0,0 +1,194 @@
+#!/usr/bin/env python
+
+# Copyright 2025 The HuggingFace Inc. team. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Standalone SMPL forward kinematics + canonicalization in pure numpy/scipy.
+
+This mirrors the ``rt/smpl`` producer path in ``gear_sonic`` (``compute_from_body_poses``
+-> ``process_smpl_joints`` -> ``compute_human_joints``) without depending on torch,
+the SMPL mesh model, or ``gear_sonic``. It only needs a small fixed skeleton table
+(rest-pose joints + kinematic tree), vendored in ``assets/smpl_skeleton.npz``.
+
+Given the 24 body-joint poses reported by the XRoboToolkit headset SDK
+(``xrt.get_body_joints_pose()`` -> (24, 7) of ``[x, y, z, qx, qy, qz, qw]``), it
+produces the root-orientation-removed 24x3 SMPL joints the SONIC encoder expects,
+plus the root orientation quaternion and pelvis translation.
+
+Quaternions are scalar-first (w, x, y, z) unless noted.
+"""
+
+from pathlib import Path
+
+import numpy as np
+from scipy.spatial.transform import Rotation as R
+
+# 24-joint parent tree used by the headset body-pose stream (SMPL-X body subset).
+# Matches PoseStreamer.parent_indices in gear_sonic's pico_manager_thread_server.py.
+BODY24_PARENTS = np.array(
+    [-1, 0, 0, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 9, 9, 12, 13, 14, 16, 17, 18, 19, 20, 22],
+    dtype=np.int64,
+)
+
+# FK output joints: first 22 SMPL body joints + two thumb tips (SMPL-X indices 39, 54).
+OUTPUT_JOINT_INDEX = np.concatenate([np.arange(22), np.array([39, 54])])
+
+_SKELETON_PATH = Path(__file__).parent / "assets" / "smpl_skeleton.npz"
+
+
+# ── quaternion helpers (scalar-first w, x, y, z) ─────────────────────────────
+
+
+def aa_to_quat(aa: np.ndarray) -> np.ndarray:
+    aa = np.asarray(aa, np.float64)
+    angle = np.linalg.norm(aa, axis=-1, keepdims=True)
+    small = angle < 1e-8
+    safe = np.where(small, 1.0, angle)
+    axis = np.where(small, 0.0, aa / safe)
+    half = angle * 0.5
+    return np.concatenate([np.cos(half), axis * np.sin(half)], axis=-1)
+
+
+def quat_to_aa(q: np.ndarray) -> np.ndarray:
+    q = np.asarray(q, np.float64)
+    w = q[..., 0:1]
+    xyz = q[..., 1:]
+    n = np.linalg.norm(xyz, axis=-1, keepdims=True)
+    angle = 2.0 * np.arctan2(n, w)
+    small = n < 1e-8
+    safe = np.where(small, 1.0, n)
+    axis = np.where(small, 0.0, xyz / safe)
+    return axis * angle
+
+
+def quat_mul(a: np.ndarray, b: np.ndarray) -> np.ndarray:
+    aw, ax, ay, az = a[..., 0], a[..., 1], a[..., 2], a[..., 3]
+    bw, bx, by, bz = b[..., 0], b[..., 1], b[..., 2], b[..., 3]
+    return np.stack(
+        [
+            aw * bw - ax * bx - ay * by - az * bz,
+            aw * bx + ax * bw + ay * bz - az * by,
+            aw * by - ax * bz + ay * bw + az * bx,
+            aw * bz + ax * by - ay * bx + az * bw,
+        ],
+        axis=-1,
+    )
+
+
+def quat_conj(q: np.ndarray) -> np.ndarray:
+    return np.concatenate([q[..., 0:1], -q[..., 1:]], axis=-1)
+
+
+def quat_inv(q: np.ndarray) -> np.ndarray:
+    c = quat_conj(q)
+    return c / (np.linalg.norm(c, axis=-1, keepdims=True) + 1e-12)
+
+
+def quat_apply(q: np.ndarray, v: np.ndarray) -> np.ndarray:
+    xyz = q[..., 1:]
+    w = q[..., 0:1]
+    t = 2.0 * np.cross(xyz, v)
+    return v + w * t + np.cross(xyz, t)
+
+
+def smpl_root_ytoz_up(root_quat: np.ndarray) -> np.ndarray:
+    """Rotate the root quaternion 90 deg about X to map SMPL Y-up to robot Z-up."""
+    base = aa_to_quat(np.array([np.pi / 2.0, 0.0, 0.0]))
+    return quat_mul(base, root_quat)
+
+
+def remove_smpl_base_rot(root_quat: np.ndarray) -> np.ndarray:
+    """Conjugate out SMPL's default rest orientation ([0.5, 0.5, 0.5, 0.5])."""
+    base_conj = quat_conj(np.array([0.5, 0.5, 0.5, 0.5]))
+    return quat_mul(root_quat, base_conj)
+
+
+# ── forward kinematics ───────────────────────────────────────────────────────
+
+
+class SmplForwardKinematics:
+    """Rest-skeleton SMPL forward kinematics (no mesh, no torch)."""
+
+    def __init__(self, skeleton_path: str | Path = _SKELETON_PATH):
+        data = np.load(skeleton_path)
+        self.J = data["J"].astype(np.float64)  # (55, 3) rest joint positions
+        self.parents = data["parents"].astype(np.int64)  # (55,) kinematic tree
+        self.n_joints = self.J.shape[0]
+
+    def _fk(self, full_pose_aa: np.ndarray) -> np.ndarray:
+        """full_pose_aa: (n_joints, 3) axis-angle (joint 0 = global). Returns (24, 3)."""
+        rot = R.from_rotvec(full_pose_aa).as_matrix()  # (n, 3, 3)
+        rel = self.J.copy()
+        rel[1:] -= self.J[self.parents[1:]]
+
+        transforms = np.zeros((self.n_joints, 4, 4), np.float64)
+        transforms[:, :3, :3] = rot
+        transforms[:, :3, 3] = rel
+        transforms[:, 3, 3] = 1.0
+
+        chain = [transforms[0]]
+        for i in range(1, self.n_joints):
+            chain.append(chain[self.parents[i]] @ transforms[i])
+        joints = np.stack(chain)[:, :3, 3]
+        return joints[OUTPUT_JOINT_INDEX]
+
+    def compute(self, body_poses_np: np.ndarray) -> dict:
+        """Convert (24, 7) headset body poses to canonical SMPL joints.
+
+        Args:
+            body_poses_np: (24, 7) rows of [x, y, z, qx, qy, qz, qw] (scalar-last).
+
+        Returns:
+            dict with:
+              - smpl_joints_local: (24, 3) root-orientation-removed joints
+              - root_quat: (4,) root/torso orientation (w, x, y, z)
+              - root_transl: (3,) pelvis translation
+        """
+        body_poses_np = np.asarray(body_poses_np, np.float64)
+        positions = body_poses_np[:, :3]
+
+        # Global joint rotations from the headset (scalar-last), with the SMPL
+        # +180 deg-about-Y frame fix, converted to per-joint local axis-angle.
+        global_rots = R.from_quat(body_poses_np[:, 3:7]) * R.from_euler("y", 180, degrees=True)
+        gm = global_rots.as_matrix()  # (24, 3, 3)
+
+        local_aa = np.zeros((24, 3), np.float64)
+        for i in range(24):
+            p = BODY24_PARENTS[i]
+            m = gm[i] if p == -1 else gm[p].T @ gm[i]
+            local_aa[i] = R.from_matrix(m).as_rotvec()
+
+        global_orient = local_aa[0]
+        body_pose = local_aa[1:].reshape(-1)[:63]  # 21 body joints
+
+        # Root: Y-up -> Z-up, then run FK with the transformed root.
+        root_quat = aa_to_quat(global_orient)
+        root_quat = smpl_root_ytoz_up(root_quat)
+        global_orient_new = quat_to_aa(root_quat)
+
+        full_pose = np.concatenate(
+            [global_orient_new, body_pose, np.zeros(3 * self.n_joints - 66)]
+        ).reshape(self.n_joints, 3)
+        joints = self._fk(full_pose)  # (24, 3)
+
+        # Canonicalize: strip SMPL base rot and the root orientation.
+        root_quat = remove_smpl_base_rot(root_quat)
+        inv = np.broadcast_to(quat_inv(root_quat), (joints.shape[0], 4))
+        smpl_joints_local = quat_apply(inv, joints)
+
+        return {
+            "smpl_joints_local": smpl_joints_local.astype(np.float32),
+            "root_quat": root_quat.astype(np.float32),
+            "root_transl": positions[0].astype(np.float32),
+        }