title: Contours and convex hull in opencv2 for a thresholded 32 uint image
body: <p>I have the following code:</p>
  
  <pre><code>    import cv2
      import numpy as np
      import matplotlib
      import matplotlib.pyplot as plt
      from skimage import data
      from skimage import filter
      from skimage.filter import threshold_otsu
  
      matplotlib.rcParams['font.size'] = 9
  
      nomeimg = 'frame1_depth.png'
  
      i = cv2.imread(nomeimg, -1)
      #conversione da 16 a 32 uint
      img = np.array(i, dtype=np.uint32)
      img *= 65536
      print img.dtype
  
      #thresholding con il metodo Otsu 
      thresh = threshold_otsu(img)
      binary = img &gt; thresh
      print thresh
  
      plt.figure(1)
      fig, (ax1, ax2, ax3) = plt.subplots(1, 3, figsize=(8, 2.5))
      ax1.imshow(img)
      ax1.set_title('Original')
      ax1.axis('off')
  
      ax2.hist(img)
      ax2.set_title('Histogram')
      ax2.axvline(x=thresh, color='r', linestyle='dashed', linewidth=2)
  
      ax3.imshow(binary, cmap=plt.cm.gray)
      ax3.set_title('Thresholded')
      ax3.axis('off')
  
      plt.figure(2)
      f, ax = plt.subplots(figsize=(8, 2.5))
      ax.imshow(binary, cmap=plt.cm.gray)
      ax.set_title('Thresholded')
      ax.axis('off')
  
      plt.show()
  </code></pre>
  
  <p>I have a set of depth images from the XBOX kinect, and so after a data type conversion so that I can use some <code>opencv</code> functions which work only with 8 or 32 bit images, I've thresholded the image with the Otsu algorithm and showed the results.</p>
  
  <p>I have obtained a subplot in which I have my original image, the histogram and the thresholded black and white image.  Now I want to work only on this black and white image and I want to save it and compute contours, the convex hull and others geometrical features.  However, it's just thresholded, but with <code>otsu</code>. </p>
  
  <p>How can I compute this?</p>