title: matplotlib's pcolormesh throws ValueError: too many values to unpack
body: <p>I have an image that I want to show using <code>pcolormesh</code> but I do not really understand how this should work exactly. I have the X and corresponding Y for a certain color but if I enter a normal array as C in <code>pcolormesh</code> I get an error.</p>
  
  <p>My code:</p>
  
  <pre><code># load image
  img = cv2.imread('Distorted_resized_50.jpg')
  img_array = np.asarray(img)
  height, width, channels = img.shape
  gray_img = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
  
  # create vector matrix
  U, V = np.meshgrid(range(gray_img.shape[1]),
                     range(gray_img.shape[0]))
  UV = np.vstack((U.flatten(),
                  V.flatten())).T
  
  H, mask = cv2.findHomography(UV_cp, XYZ_gcp)
  UV_warped = cv2.perspectiveTransform(np.array([UV]).astype(np.float32), H)
  UV_warped = UV_warped[0]
  UV_warped = UV_warped.astype(np.int)
  X_warped = UV_warped[:,0].reshape((height, width))
  Y_warped = UV_warped[:,1].reshape((height, width))
  
  fig, axs = plt.subplots(figsize=(15,10))
  axs.pcolormesh(X_warped, Y_warped, img_array)
  </code></pre>
  
  <p>Anybody that can help me? The explanation on the website isn't very clear to me.
  It all works fine until <code>fig, axs = plt.subplots(figsize=(15,10))</code></p>
  
  <p>Traceback:</p>
  
  <pre class="lang-none prettyprint-override"><code>Traceback (most recent call last):
    File "C:\Users\Yorian\Desktop\TU\Stage Shore\python_files\Rectificatie dmv foto thuis\rectify.py", line 53, in &lt;module&gt;
      ax.pcolormesh(X_warped, Y_warped, img_array)
    File "C:\Python27\lib\site-packages\matplotlib\axes.py", line 7734, in pcolormesh
  X, Y, C = self._pcolorargs('pcolormesh', *args, allmatch=allmatch)
    File "C:\Python27\lib\site-packages\matplotlib\axes.py", line 7350, in _pcolorargs
  numRows, numCols = C.shape
  ValueError: too many values to unpack
  </code></pre>