Add discussion of overlay extents

Author: ianhi

docs/examples/callbacks.ipynb

@@ -60,7 +60,7 @@
    "source": [
     "## Overlaying a mask\n",
     "\n",
-    "One potential usage of this is to overlay a mask over the images which changes for each image"
+    "One potential usage of this is to overlay a mask over the images which changes for each image. If the shape of image is changing then you will also need to adjust the `extent` of the overlay. If this is the case then uncomment the line in the `update_mask` function."
    ]
   },
   {
@@ -95,11 +95,23 @@
     "\n",
     "\n",
     "def update_mask(idx, image):\n",
-    "    overlay.set_data(gen_mask(idx, image))\n",
+    "    new_mask = gen_mask(idx, image)\n",
+    "    overlay.set_data(new_mask)\n",
+    "\n",
+    "    # if your image is changing shape uncomment the next line\n",
+    "    # overlay.set_extent((-0.5, new_mask.shape[1] - 0.5, new_mask.shape[0] - 0.5, -0.5))\n",
     "\n",
     "\n",
     "labeller.on_image_changed(update_mask)"
    ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "9f1e35b8-5b0e-442e-ae91-910d4ff04017",
+   "metadata": {},
+   "outputs": [],
+   "source": []
   }
  ],
  "metadata": {