title: greycomatrix for RGB image
body: <p>I am trying to find contrast in an image using greycomatrix here is the code: </p>
  
  <pre><code>import cv2
  import numpy as np
  from scipy import misc
  from skimage.feature import greycomatrix, greycoprops
  
  
  img=cv2.imread('leaf2.jpg')
  
  
  g=greycomatrix(img, [1], [0, np.pi/4, np.pi/2, 3*np.pi/4])
  print (g)
  
  contrast = greycoprops(g, 'contrast')
  print(contrast)
  </code></pre>
  
  <p>Here is the error: "The image must be a 2-D array"
  How to convert the image to a 2-D array, suitable for the function?</p>