diff --git a/data_only_viz/scripts/extract_j3d_offline.py b/data_only_viz/scripts/extract_j3d_offline.py
new file mode 100644
index 0000000..1ad9254
--- /dev/null
+++ b/data_only_viz/scripts/extract_j3d_offline.py
@@ -0,0 +1,124 @@
+"""Extract j3d (22 SMPL-X joint anchors) from a recorded MP4 using the
+Multi-HMR CoreML backend, write per-frame per-person jsonl rows.
+
+Usage:
+    uv run python -m data_only_viz.scripts.extract_j3d_offline \
+        --session sess03 \
+        --video ~/.cache/av-live-action/raw/sess03.mp4 \
+        --out   ~/.cache/av-live-action/raw/sess03.jsonl
+"""
+from __future__ import annotations
+
+import argparse
+import json
+import logging
+from pathlib import Path
+
+import cv2
+import numpy as np
+
+from data_only_viz.action_head_pub import SMPLX_JOINT_ANCHOR_VERTS
+from data_only_viz.multihmr_coreml import MultiHMRCoreMLBackend
+
+LOG = logging.getLogger("extract_j3d_offline")
+IMG_SIZE = 672
+DEFAULT_OUT_DIR = Path("~/.cache/av-live-action/raw").expanduser()
+
+
+def _default_K(size: int = IMG_SIZE) -> np.ndarray:
+    """Synthetic camera intrinsics, focal ~ image size, principal point centred."""
+    f = float(size)
+    cx = cy = f * 0.5
+    return np.array(
+        [[f, 0.0, cx], [0.0, f, cy], [0.0, 0.0, 1.0]],
+        dtype=np.float32,
+    )
+
+
+def _frame_to_chw(frame_bgr: np.ndarray, size: int = IMG_SIZE) -> np.ndarray:
+    """BGR uint8 (H, W, 3) -> float32 CHW (3, size, size) in [0, 1]."""
+    h, w = frame_bgr.shape[:2]
+    side = min(h, w)
+    y0 = (h - side) // 2
+    x0 = (w - side) // 2
+    crop = frame_bgr[y0:y0 + side, x0:x0 + side]
+    resized = cv2.resize(crop, (size, size))
+    rgb = cv2.cvtColor(resized, cv2.COLOR_BGR2RGB).astype(np.float32) / 255.0
+    return rgb.transpose(2, 0, 1)  # CHW
+
+
+def _person_to_j3d22(person: dict, anchors: tuple[int, ...]) -> np.ndarray | None:
+    v3d = person.get("v3d")
+    if v3d is None:
+        return None
+    v3d_np = np.asarray(v3d, dtype=np.float32)
+    if v3d_np.shape[0] < max(anchors) + 1:
+        return None
+    return v3d_np[list(anchors)].astype(np.float32)
+
+
+def extract(session: str, video: Path, out: Path,
+            det_thresh: float = 0.3,
+            mlpackage_path: Path | None = None,
+            anchors: tuple[int, ...] = SMPLX_JOINT_ANCHOR_VERTS) -> int:
+    """Returns the number of (frame, person) rows written."""
+    out.parent.mkdir(parents=True, exist_ok=True)
+    cap = cv2.VideoCapture(str(video))
+    if not cap.isOpened():
+        raise RuntimeError(f"cannot open {video}")
+    fps = cap.get(cv2.CAP_PROP_FPS) or 30.0
+    backend = MultiHMRCoreMLBackend(mlpackage_path) if mlpackage_path \
+        else MultiHMRCoreMLBackend()
+    K = _default_K(IMG_SIZE)
+    n_frames = 0
+    n_rows = 0
+    with out.open("w") as f:
+        while True:
+            ok, frame = cap.read()
+            if not ok:
+                break
+            chw = _frame_to_chw(frame)
+            try:
+                persons = backend.infer(chw, K, det_thresh=det_thresh)
+            except Exception:
+                LOG.exception("infer failed at frame=%d", n_frames)
+                n_frames += 1
+                continue
+            ts = n_frames / fps
+            for i, person in enumerate(persons):
+                j3d = _person_to_j3d22(person, anchors)
+                if j3d is None:
+                    continue
+                f.write(json.dumps({
+                    "ts": ts,
+                    "session": session,
+                    "pid": int(person.get("pid", i)),
+                    "j3d": j3d.tolist(),
+                }) + "\n")
+                n_rows += 1
+            n_frames += 1
+            if n_frames % 100 == 0:
+                LOG.info("frame=%d rows=%d", n_frames, n_rows)
+    cap.release()
+    LOG.info("done: %d frames, %d rows -> %s", n_frames, n_rows, out)
+    return n_rows
+
+
+def _cli() -> None:
+    p = argparse.ArgumentParser()
+    p.add_argument("--session", required=True)
+    p.add_argument("--video", required=True, type=Path)
+    p.add_argument("--out", type=Path)
+    p.add_argument("--det-thresh", type=float, default=0.3)
+    p.add_argument("--mlpackage", type=Path, default=None)
+    args = p.parse_args()
+    logging.basicConfig(level=logging.INFO,
+                        format="%(asctime)s [%(name)s] %(message)s")
+    DEFAULT_OUT_DIR.mkdir(parents=True, exist_ok=True)
+    out = args.out or (DEFAULT_OUT_DIR / f"{args.session}.jsonl")
+    extract(args.session, args.video, out,
+            det_thresh=args.det_thresh, mlpackage_path=args.mlpackage)
+
+
+if __name__ == "__main__":
+    _cli()