diff --git a/docker/frontend/probe-black.html b/docker/frontend/probe-black.html
new file mode 100644
index 0000000..28b2d9d
--- /dev/null
+++ b/docker/frontend/probe-black.html
@@ -0,0 +1,72 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>probe: develop a real RAW</title>
+<body>
+<pre id="out" style="font:12px monospace">running…</pre>
+<img id="shot" style="max-width:100%">
+<script type="module">
+const log = (...a) => {
+  const s = a.map((x) => (typeof x === 'string' ? x : JSON.stringify(x))).join(' ');
+  document.getElementById('out').textContent += s + '\n';
+  console.log(s);
+};
+const b64 = (bytes) => {
+  let s = '';
+  for (let i = 0; i < bytes.length; i += 0x8000) {
+    s += String.fromCharCode.apply(null, bytes.subarray(i, i + 0x8000));
+  }
+  return btoa(s);
+};
+
+try {
+  const RAW_URL = new URLSearchParams(location.search).get('f') || 'http://127.0.0.1:5300/DSC03453.ARW';
+  const { initSkia } = await import('/src/engine/skiaShim.ts');
+  await initSkia({ locateFile: (file) => `/wasm/${file}` });
+  log('canvaskit up');
+  const { developRaw } = await import('/src/engine/rawDevelop.ts');
+
+  const bytes = new Uint8Array(await (await fetch(RAW_URL)).arrayBuffer());
+  log('raw bytes', bytes.length, RAW_URL);
+  // The develop hands its buffer to LibRaw's worker, which detaches it — one
+  // copy each, since the plane below is read out of a second decode.
+  const forPlane = bytes.slice();
+
+  const t0 = performance.now();
+  const jpeg = await developRaw(bytes);
+  log('develop ms', Math.round(performance.now() - t0), 'jpeg bytes', jpeg.length);
+  window.__dev = b64(jpeg);
+  document.getElementById('shot').src = URL.createObjectURL(new Blob([jpeg], { type: 'image/jpeg' }));
+
+  // The plane and the colour data behind that develop, so the shader can be
+  // replayed offline a stage at a time.
+  const { default: LibRaw } = await import('libraw-wasm');
+  const raw = new LibRaw();
+  // LibRaw hands the buffer to its worker, which detaches it — a copy, since
+  // the develop above already passed the original through.
+  await raw.open(forPlane, {
+    halfSize: true, outputBps: 16, outputColor: 0, noAutoScale: true,
+    useCameraWb: false, noAutoBright: true, gamm: [1, 1], userQual: 3, highlight: 0,
+  });
+  const meta = await raw.metadata(true);
+  const img = await raw.imageData();
+  const cd = meta.color_data;
+  const plane = new Uint8Array(img.data.buffer, img.data.byteOffset, img.data.byteLength);
+  log('plane', img.width, 'x', img.height, 'bytes', plane.length);
+  log('black', cd.black, 'maximum', cd.maximum, 'mul', cd.cam_mul, 'model', meta.normalized_model);
+  window.__dump = {
+    b64: b64(plane),
+    width: img.width, height: img.height,
+    black: cd.black, maximum: cd.maximum,
+    cam_mul: cd.cam_mul, rgb_cam: cd.rgb_cam,
+    model: meta.normalized_model, raw_bps: meta.raw_bps ?? null,
+  };
+  const thumb = await raw.thumbnailData().catch(() => null);
+  if (thumb?.data?.length) window.__thumb = b64(new Uint8Array(thumb.data));
+  log('thumb', thumb?.format, thumb?.width, 'x', thumb?.height);
+  raw.dispose();
+  log('DONE');
+} catch (e) {
+  log('ERROR', e?.message ?? String(e), (e?.stack ?? '').split('\n').slice(0, 6).join(' | '));
+  log('FAILED');
+}
+</script>