workshop: add Step 4b rotation curl plot

notebooks/berk_workshop_v1.ipynb

@@ -351,6 +351,55 @@
     ")"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "id": "e8050ac2",
+   "metadata": {},
+   "source": [
+    "## Step 4b — Scan-detector rotation from CoM curl\n",
+    "\n",
+    "`DirectPtychography` needs the rotation between the scan axes and the detector\n",
+    "axes. Find it by sweeping rotation angles and minimizing the curl of the\n",
+    "rotated CoM field. (Auto-estimated internally by `from_dataset4d` too — this\n",
+    "cell exposes the curve so you can see WHERE the minimum lands.)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "414e6538",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "import matplotlib.pyplot as plt\n",
+    "\n",
+    "# Fit origin (needed by estimate_detector_rotation) + run estimate.\n",
+    "com_model.fit_origin_background()\n",
+    "com_model.estimate_detector_rotation()\n",
+    "print(f\"detector rotation = {com_model.detector_rotation_deg:.2f}°\")\n",
+    "\n",
+    "# Recompute the curl-vs-angle curve to plot it.\n",
+    "angles_deg = torch.arange(-89, 90, 1, device=com_model.device, dtype=torch.float32)\n",
+    "com_t = com_model.origin_measured.view(*dset.shape[:2], 2)\n",
+    "com_n = com_t - com_t.mean(dim=(0, 1))\n",
+    "rot_rad = torch.deg2rad(angles_deg)[:, None, None]\n",
+    "rx = torch.cos(rot_rad) * com_n[None, ..., 0] - torch.sin(rot_rad) * com_n[None, ..., 1]\n",
+    "ry = torch.sin(rot_rad) * com_n[None, ..., 0] + torch.cos(rot_rad) * com_n[None, ..., 1]\n",
+    "grad_xy = torch.gradient(rx, dim=-1)[0]\n",
+    "grad_yx = torch.gradient(ry, dim=-2)[0]\n",
+    "curl = torch.mean(torch.abs(grad_yx - grad_xy), dim=(-2, -1)).cpu().numpy()\n",
+    "angles = angles_deg.cpu().numpy()\n",
+    "\n",
+    "fig, ax = plt.subplots(figsize=(6, 3.5))\n",
+    "ax.plot(angles, curl, lw=1.5)\n",
+    "ax.axvline(com_model.detector_rotation_deg, color=\"red\", lw=1, label=f\"{com_model.detector_rotation_deg:.1f}°\")\n",
+    "ax.set_xlabel(\"rotation angle (deg)\")\n",
+    "ax.set_ylabel(\"mean |curl(CoM)|\")\n",
+    "ax.set_title(\"Scan-detector rotation from CoM curl\")\n",
+    "ax.legend()\n",
+    "plt.tight_layout(); plt.show()"
+   ]
+  },
   {
    "cell_type": "markdown",
    "id": "6c89c61d",