diff --git a/python/mjspec.ipynb b/python/mjspec.ipynb
index 2eb89566..925da6bc 100644
--- a/python/mjspec.ipynb
+++ b/python/mjspec.ipynb
@@ -270,37 +270,6 @@
     "We'll start with an \"arena\" xml, containing only a plane and light, and define some utility functions."
    ]
   },
-  {
-   "cell_type": "code",
-   "execution_count": 0,
-   "metadata": {
-    "id": "AgYLwOhs1Msn"
-   },
-   "outputs": [],
-   "source": [
-    "#@title arena model\n",
-    "arena_xml = \"\"\"\n",
-    "<mujoco>\n",
-    "  <visual>\n",
-    "    <headlight diffuse=\".5 .5 .5\" specular=\"1 1 1\"/>\n",
-    "    <global elevation=\"-10\" offwidth=\"2048\" offheight=\"1536\"/>\n",
-    "    <quality shadowsize=\"8192\"/>\n",
-    "  </visual>\n",
-    "\n",
-    "  <asset>\n",
-    "    <texture type=\"skybox\" builtin=\"gradient\" rgb1=\"1 1 1\" rgb2=\"1 1 1\" width=\"10\" height=\"10\"/>\n",
-    "    <texture type=\"2d\" name=\"groundplane\" builtin=\"checker\" mark=\"edge\" rgb1=\"1 1 1\" rgb2=\"1 1 1\" markrgb=\"0 0 0\" width=\"300\" height=\"300\"/>\n",
-    "    <material name=\"groundplane\" texture=\"groundplane\" texuniform=\"true\" texrepeat=\"5 5\" reflectance=\"0\"/>\n",
-    "  </asset>\n",
-    "\n",
-    "  <worldbody>\n",
-    "    <geom name=\"floor\" size=\"5 5 0.01\" type=\"plane\" material=\"groundplane\"/>\n",
-    "    <light pos=\"0 0 3\"/>\n",
-    "  </worldbody>\n",
-    "</mujoco>\n",
-    "\"\"\""
-   ]
-  },
   {
    "cell_type": "code",
    "execution_count": 0,
@@ -403,7 +372,7 @@
     "    spec.option.density = 1.294\n",
     "\n",
     "    # Defaults for joint and geom\n",
-    "    main = spec.default()\n",
+    "    main = spec.default\n",
     "    main.geom.type = mj.mjtGeom.mjGEOM_CAPSULE\n",
     "    main.joint.type = mj.mjtJoint.mjJNT_BALL\n",
     "    main.joint.springdamper = [0.003, 0.7]\n",
@@ -482,6 +451,27 @@
    "outputs": [],
    "source": [
     "#@title Make video\n",
+    "arena_xml = \"\"\"\n",
+    "<mujoco>\n",
+    "  <visual>\n",
+    "    <headlight diffuse=\".5 .5 .5\" specular=\"1 1 1\"/>\n",
+    "    <global elevation=\"-10\" offwidth=\"2048\" offheight=\"1536\"/>\n",
+    "    <quality shadowsize=\"8192\"/>\n",
+    "  </visual>\n",
+    "\n",
+    "  <asset>\n",
+    "    <texture type=\"skybox\" builtin=\"gradient\" rgb1=\".5 .5 .5\" rgb2=\"0 0 0\" width=\"10\" height=\"10\"/>\n",
+    "    <texture type=\"2d\" name=\"groundplane\" builtin=\"checker\" mark=\"edge\" rgb1=\"1 1 1\" rgb2=\"1 1 1\" markrgb=\"0 0 0\" width=\"300\" height=\"300\"/>\n",
+    "    <material name=\"groundplane\" texture=\"groundplane\" texuniform=\"true\" texrepeat=\"5 5\" reflectance=\"0.3\"/>\n",
+    "  </asset>\n",
+    "\n",
+    "  <worldbody>\n",
+    "    <geom name=\"floor\" size=\"5 5 0.01\" type=\"plane\" material=\"groundplane\"/>\n",
+    "    <light pos=\"0 0 3\" diffuse=\"1 1 1\" specular=\"1 1 1\"/>\n",
+    "  </worldbody>\n",
+    "</mujoco>\n",
+    "\"\"\"\n",
+    "\n",
     "spec = procedural_tree(spec=mj.MjSpec.from_string(arena_xml))\n",
     "model = spec.compile()\n",
     "data = mj.MjData(model)\n",
@@ -489,7 +479,7 @@
     "duration = 3  # (seconds)\n",
     "framerate = 60  # (Hz)\n",
     "frames = []\n",
-    "with mj.Renderer(model, width=1920 // 2, height=1080 // 2) as renderer:\n",
+    "with mj.Renderer(model, width=1920 // 3, height=1080 // 3) as renderer:\n",
     "  while data.time < duration:\n",
     "    # Add rightward wind.\n",
     "    wind = 40 * unit_bump(data.time, .2 * duration, .7 * duration)\n",
@@ -755,7 +745,103 @@
     "  while data.time < duration:\n",
     "    mj.mj_step(model, data)\n",
     "    if len(frames) < data.time * framerate:\n",
-    "      cam.azimuth = 20 + 30 * (1 - np.cos(np.pi*data.time / duration))\n",
+    "      cam.azimuth = 20 + 20 * (1 - np.cos(np.pi*data.time / duration))\n",
+    "      renderer.update_scene(data, cam)\n",
+    "      pixels = renderer.render()\n",
+    "      frames.append(pixels)\n",
+    "\n",
+    "media.show_video(frames, fps=framerate )"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {
+    "id": "92PjTlVCprxn"
+   },
+   "source": [
+    "## Mesh\n",
+    "\n",
+    "Random meshes can be easily created by sampling random vertices on the unit sphere and letting MuJoCo create the corresponding convex hull."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 0,
+   "metadata": {
+    "id": "702z1GBRprxq"
+   },
+   "outputs": [],
+   "source": [
+    "#@title Add \"rock\" mesh\n",
+    "def add_rock(spec=None, scale=1, name=\"rock\", pos=[0, 0, 0]):\n",
+    "  if spec is None:\n",
+    "    spec = mj.MjSpec()\n",
+    "\n",
+    "  # Defaults\n",
+    "  main = spec.default\n",
+    "  main.mesh.scale = np.array([scale]*3 , dtype = np.float64)\n",
+    "  main.geom.type = mj.mjtGeom.mjGEOM_MESH\n",
+    "\n",
+    "  # Random gray-brown color\n",
+    "  gray = np.array([.5, .5, .5, 1])\n",
+    "  light_brown = np.array([200, 150, 100, 255]) / 255.0\n",
+    "  mix = np.random.uniform()\n",
+    "  rgba = light_brown*mix + gray*(1-mix)\n",
+    "\n",
+    "  # Create mesh vertices\n",
+    "  mesh = np.random.normal(size = (20, 3))\n",
+    "  mesh /= np.linalg.norm(mesh, axis=1, keepdims=True)\n",
+    "\n",
+    "  # Create Body and add mesh to the Geom of the Body\n",
+    "  spec.add_mesh(name=name, uservert=mesh.flatten())\n",
+    "  body = spec.worldbody.add_body(pos=pos, name=name, mass=1)\n",
+    "  body.add_geom(meshname=name, rgba=rgba)\n",
+    "  body.add_freejoint()\n",
+    "\n",
+    "  return body"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 0,
+   "metadata": {
+    "id": "8BR4mlFdprxx"
+   },
+   "outputs": [],
+   "source": [
+    "#@title Video\n",
+    "spec = add_hfield(mj.MjSpec.from_string(arena_xml))\n",
+    "\n",
+    "# Add lights\n",
+    "for x in [-15, 15]:\n",
+    "  for y in [-15, 15]:\n",
+    "    spec.worldbody.add_light(pos = [x, y, 10], dir = [-x, -y, -15])\n",
+    "\n",
+    "# Add rocks\n",
+    "for x in np.linspace(-8, 8, 8):\n",
+    "  for y in np.linspace(-8, 8, 8):\n",
+    "    pos = [x, y, np.random.uniform(4, 14)]\n",
+    "    rock = add_rock(spec = spec,\n",
+    "                      scale = np.random.uniform(.3, 1),\n",
+    "                      name = f\"rock_{x}_{y}\",\n",
+    "                      pos = pos)\n",
+    "model = spec.compile()\n",
+    "data = mj.MjData(model)\n",
+    "\n",
+    "cam = mj.MjvCamera()\n",
+    "mj.mjv_defaultCamera(cam)\n",
+    "cam.lookat = [0, 0, 0]\n",
+    "cam.distance = 30\n",
+    "cam.elevation = -30\n",
+    "\n",
+    "duration = 6  # (seconds)\n",
+    "framerate = 60  # (Hz)\n",
+    "frames = []\n",
+    "with mj.Renderer(model, width=1920 // 3, height=1080 // 3) as renderer:\n",
+    "  while data.time < duration:\n",
+    "    mj.mj_step(model, data)\n",
+    "    if len(frames) < data.time * framerate:\n",
+    "      cam.azimuth = 20 + 20 * (1 - np.cos(np.pi*data.time / duration))\n",
     "      renderer.update_scene(data, cam)\n",
     "      pixels = renderer.render()\n",
     "      frames.append(pixels)\n",