@@ -0,0 +1,182 @@
+#!/usr/bin/env python3
+"""Measure how far each committed gallery hero is from what its script renders.
+
+For every entry in ``examples/gallery.json`` and ``showcase/gallery.json``,
+render the entry's README ``--output`` command to a lossless PNG with the
+given Blender binary, then compare it with the committed
+``docs/gallery/assets/<name>-hero.webp``:
+
+* ``mean_abs`` - mean absolute per-channel difference, 0..1
+* ``gt2pct``   - share of pixels whose mean channel difference exceeds 2%
+* ``vs_q90``   - ``gt2pct`` against the fresh render re-encoded as webp q90
+* ``luma``     - mean luminance (Rec. 709) of the committed and fresh image
+
+This is the method from issue #200, with one correction. Renders are
+deterministic (a re-render is pixel-identical), but the webp encode floor is
+not a constant: re-encoding a frame at q90 alone moves 0.1-4% of pixels past
+2%, depending on the image. ``gt2pct`` against the PNG therefore mixes drift
+with encode noise. The verdict uses ``vs_q90``, which cancels the encode: on
+the 2026-09-23 sweep every matching hero scored <= 1.8% and every drifted one
+>= 3.8%, hence DRIFT_THRESHOLD.
+
+Authoring tool, not a CI step: one full render per entry. Requires Pillow
+and numpy on the host Python (not Blender's).
+
+Usage:
+    python scripts/measure_hero_drift.py --blender PATH [--only NAME ...]
+        [--out DIR] [--json FILE]
+
+Exit codes: 0 measured (drift is reported, not judged), 2 usage, 3 a render
+failed for at least one entry.
+"""
+from __future__ import annotations
+
+import argparse
+import io
+import json
+import re
+import shlex
+import subprocess
+import sys
+import time
+from pathlib import Path
+
+import numpy as np
+from PIL import Image
+
+REPO = Path(__file__).resolve().parent.parent
+SOURCES = ("examples/gallery.json", "showcase/gallery.json")
+DRIFT_THRESHOLD = 0.025  # vs_q90 share; the gap on the full sweep is 1.8% .. 3.8%
+
+_RUN_LINE = re.compile(r"blender\s+--background\s+--python\s+\S+\.py\s+--\s+(.*--output\s.*)$")
+
+
+def entries() -> list[dict]:
+    out = []
+    for rel in SOURCES:
+        data = json.loads((REPO / rel).read_text(encoding="utf-8"))
+        items = next(v for v in data.values() if isinstance(v, list))
+        out.extend(items)
+    return out
+
+
+def render_args(entry: dict, png: Path) -> list[str] | None:
+    """The README's documented ``--output`` invocation, retargeted at *png*.
+
+    Returns None when the README documents no render command. Only the
+    output path is replaced; every other documented flag (``--engine``,
+    ``--samples`` ...) is kept, because the hero was rendered with them.
+    """
+    readme = REPO / entry["dir"] / "README.md"
+    if not readme.is_file():
+        return None
+    for line in readme.read_text(encoding="utf-8").splitlines():
+        m = _RUN_LINE.search(line.strip())
+        if not m:
+            continue
+        args = shlex.split(m.group(1))
+        i = args.index("--output")
+        args[i + 1] = str(png)
+        return args
+    return None
+
+
+def load(path: Path) -> np.ndarray:
+    return np.asarray(Image.open(path).convert("RGB"), dtype=np.float64) / 255.0
+
+
+def luma(a: np.ndarray) -> float:
+    return float((a @ np.array([0.2126, 0.7152, 0.0722])).mean())
+
+
+def gt2pct(a: np.ndarray, b: np.ndarray) -> float:
+    # Mean over the channels, not any channel: the definition #200 used,
+    # which reproduces its 22.45% (shipping-crate) and 0.50% (crate-stack).
+    return round(float((np.abs(a - b).mean(axis=2) > 0.02).mean()), 5)
+
+
+def webp_q90(path: Path) -> np.ndarray:
+    buf = io.BytesIO()
+    Image.open(path).convert("RGB").save(buf, "WEBP", quality=90)
+    return np.asarray(Image.open(buf).convert("RGB"), dtype=np.float64) / 255.0
+
+
+def compare(committed: Path, fresh: Path) -> dict:
+    a, b = load(committed), load(fresh)
+    if a.shape != b.shape:
+        return {"shape_committed": a.shape[:2], "shape_fresh": b.shape[:2]}
+    return {
+        "mean_abs": round(float(np.abs(a - b).mean()), 5),
+        "gt2pct": gt2pct(a, b),
+        "vs_q90": gt2pct(a, webp_q90(fresh)),
+        "luma_committed": round(luma(a), 5),
+        "luma_fresh": round(luma(b), 5),
+        "bytes_committed": committed.stat().st_size,
+    }
+
+
+def main(argv=None) -> int:
+    p = argparse.ArgumentParser(description=__doc__.split("\n")[0])
+    p.add_argument("--blender", required=True, help="Blender binary to render with")
+    p.add_argument("--only", action="append", help="measure only this entry (repeatable)")
+    p.add_argument("--out", default=str(REPO / ".scratch" / "hero-drift"),
+                   help="directory for fresh PNG renders (default: .scratch/hero-drift)")
+    p.add_argument("--json", default=None, help="also write the results as JSON here")
+    args = p.parse_args(argv)
+
+    blender = str(Path(args.blender).resolve())
+    out_dir = Path(args.out)
+    out_dir.mkdir(parents=True, exist_ok=True)
+    version = subprocess.run([blender, "--version"], capture_output=True, text=True,
+                             encoding="utf-8", errors="replace").stdout.splitlines()[0]
+    print(f"# renderer: {blender}\n# version: {version}", flush=True)
+    print(f"{'name':32} {'mean_abs':>9} {'gt2pct':>8} {'vs_q90':>8} {'luma c/f':>17} {'secs':>6}  verdict",
+          flush=True)
+
+    results, failed = [], []
+    for e in entries():
+        name = e["name"]
+        if args.only and name not in args.only:
+            continue
+        png = out_dir / f"{name}.png"
+        cmd = render_args(e, png)
+        if cmd is None:
+            print(f"{name:32} no --output command in README", flush=True)
+            failed.append(name)
+            continue
+        script = next((REPO / e["dir"]).glob("*.py"))
+        if png.exists():
+            png.unlink()
+        t0 = time.time()
+        proc = subprocess.run(
+            [blender, "--background", "--factory-startup", "--python", str(script), "--", *cmd],
+            capture_output=True, text=True, encoding="utf-8", errors="replace", cwd=REPO,
+        )
+        secs = time.time() - t0
+        if proc.returncode != 0 or not png.is_file():
+            print(f"{name:32} render failed: exit {proc.returncode}", flush=True)
+            failed.append(name)
+            continue
+        row = {"name": name, "args": cmd, "secs": round(secs, 1),
+               **compare(REPO / e["hero"], png)}
+        results.append(row)
+        if "mean_abs" not in row:
+            print(f"{name:32} size mismatch {row}", flush=True)
+            continue
+        verdict = "matches" if row["vs_q90"] <= DRIFT_THRESHOLD else "DRIFTED"
+        row["verdict"] = verdict
+        print(f"{name:32} {row['mean_abs']:9.5f} {row['gt2pct']:8.2%} {row['vs_q90']:8.2%} "
+              f"{row['luma_committed']:.4f}/{row['luma_fresh']:.4f} {secs:6.1f}  {verdict}",
+              flush=True)
+
+    if args.json:
+        Path(args.json).write_text(json.dumps(
+            {"renderer": version, "results": results, "failed": failed}, indent=2),
+            encoding="utf-8")
+    drifted = [r["name"] for r in results if r.get("verdict") == "DRIFTED"]
+    print(f"\n# {len(results)} measured, {len(drifted)} drifted, {len(failed)} failed")
+    return 3 if failed else 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())