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     "ename": "SyntaxError",
     "evalue": "invalid syntax (<ipython-input-1-5f74d21e013e>, line 43)",
     "output_type": "error",
     "traceback": [
      "\u001b[1;36m  File \u001b[1;32m\"<ipython-input-1-5f74d21e013e>\"\u001b[1;36m, line \u001b[1;32m43\u001b[0m\n\u001b[1;33m    quad(v0=verts[y][x], v1=verts[y][x+1], v2=verts[y+1][x+1], v3=verts[y+1][x]], texture=textures.rug)\u001b[0m\n\u001b[1;37m                                                                               ^\u001b[0m\n\u001b[1;31mSyntaxError\u001b[0m\u001b[1;31m:\u001b[0m invalid syntax\n"
     ]
    }
   ],
   "source": [
    "from vpython import *\n",
    "\n",
    "scene.width = scene.height = 600\n",
    "scene.range = 0.6\n",
    "\n",
    "# A pulse ripples along a rug, demonstrating dynamic changea of shape\n",
    "# Bruce Sherwood, May 2012\n",
    "\n",
    "def display_instructions():\n",
    "    s = \"<b>Click to halt or run.</b> In VPython programs:\\n\"\n",
    "    s += \"    Rotate the camera by dragging with the right mouse button,\\n\"\n",
    "    s += \"         or hold down the Ctrl key and drag.\\n\"\n",
    "    s += \"    To zoom, drag with the left+right mouse buttons,\\n\"\n",
    "    s += \"         or hold down the Alt/Option key and drag,\\n\"\n",
    "    s += \"         or use the mouse wheel.\\n\"\n",
    "    s += \"Touch screen: pinch/extend to zoom, swipe or two-finger rotate.\"\n",
    "    scene.caption = s\n",
    "    \n",
    "display_instructions()\n",
    "\n",
    "# Construct a square WxH divided into little squares\n",
    "# There are (w+1)x(h+1) vertices\n",
    "# Center of rug is at 0,0,0\n",
    "\n",
    "H = W = 1\n",
    "w = 1\n",
    "h = 50\n",
    "dx = W/w\n",
    "dy = H/h\n",
    "\n",
    "# Create a grid of vertex objects covering the rug\n",
    "verts = []\n",
    "for y in range(h+1): # from 0 to h inclusive, to include both bottom and top edges\n",
    "    verts.append([])\n",
    "    for x in range(w+1): # from 0 to w inclusive, to include both left and right edges\n",
    "        verts[y].append(vertex(pos=vector(-0.5+x*dx,-0.5+y*dy,0), normal=vector(0,0,1), texpos=vector(x/w,y/h,0), shininess= 0))\n",
    "\n",
    "# Create quads (equivalent to two triangles) based on the vertex objects just created.\n",
    "# Note that a particular vertex may be shared by as many as 4 neighboring quads, and\n",
    "# changing one vertex affects all of the quads that use that vertex.\n",
    "for y in range(h): # from 0 to h, not including h\n",
    "    for x in range(w): # from 0 to w, not including w\n",
    "        quad(v0=verts[y][x], v1=verts[y][x+1], v2=verts[y+1][x+1], v3=verts[y+1][x], texture=textures.rug)\n",
    "\n",
    "#scene.waitfor('textures') # wait until the rug texture has been loaded\n",
    "\n",
    "Lpulse = 0.4 # length of half sine wave\n",
    "dy_pulse = Lpulse/50\n",
    "k = pi/(0.6*Lpulse)\n",
    "A = 0.05\n",
    "\n",
    "def pulse(z): # return the pulse height and normal\n",
    "    if z < 0.2*Lpulse: return 0\n",
    "    if z > 0.8*Lpulse: return 0\n",
    "    z -= 0.2*Lpulse\n",
    "    return A*sin(k*z)\n",
    "\n",
    "run = True\n",
    "\n",
    "def down(ev):\n",
    "    global run\n",
    "    run = not run\n",
    "\n",
    "scene.bind(\"mousedown\", down)\n",
    "\n",
    "y = -0.5-Lpulse-dy_pulse # bottom of pulse (starts below rug)\n",
    "while True:\n",
    "    rate(50)\n",
    "    while not run:\n",
    "        scene.waitfor('redraw')\n",
    "    y += dy_pulse\n",
    "    if y+Lpulse <= -0.5:\n",
    "        continue\n",
    "    if y >= 0.5:\n",
    "        y = -0.5-Lpulse\n",
    "        continue\n",
    "    \n",
    "    # Note: floor and ceil are floats in Python 2 but ints in Python 3\n",
    "    start = int(floor((y+0.5)/dy))     # lowest row of vertices in pulse\n",
    "    end = int(ceil((y+0.5+Lpulse)/dy)) # highest row of vertices in pulse\n",
    "    if start < 0:\n",
    "        if end <= 0: continue\n",
    "        start = 0\n",
    "    if end > h:\n",
    "        end = h\n",
    "    \n",
    "    yp = -0.5+start*dy\n",
    "    for s in range(start,end):\n",
    "        z0 = pulse(yp-y-dy_pulse)\n",
    "        z1 = pulse(yp-y)\n",
    "        z2 = pulse(yp+dy_pulse-y)\n",
    "        yp += dy # advance to next row\n",
    "        \n",
    "        # If slope of a line is dy/dz, normal to the line is in direction < -dz, +dy >\n",
    "        n1y0 = -(z1-z0)\n",
    "        n2y0 = -(z2-z1)\n",
    "        n1y = .5*(n1y0+n2y0) # average adjacent normals to smooth the lighting\n",
    "        n1z = dy\n",
    "        \n",
    "        vy = verts[s]\n",
    "        for vx in range(w+1):\n",
    "            vy[vx].pos.z = z1\n",
    "            vy[vx].normal = vector(0,n1y,n1z) "
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