title: Convex hull in python for given set of points?
body: <p>I am trying to find convex hull in order to my get hands on opencv library and the code is:</p>
  
  <pre><code>import cv2
  import numpy as np
  a = [[0, 0], [1, 0], [0, 1], [1, 1], [0.5, 0.5]]
  cv2.convexHull(np.array(a))
  </code></pre>
  
  <p>I am getting an error which is -</p>
  
  <pre><code>Traceback (most recent call last):
    File "&lt;pyshell#10&gt;", line 1, in &lt;module&gt;
      cv2.convexHull(np.array(a))
  error: C:\projects\opencv-python\opencv\modules\imgproc\src\convhull.cpp:137: error: (-215) total &gt;= 0 &amp;&amp; (depth == 5 || depth == 4) in function cv::convexHull
  </code></pre>
  
  <p>How to fix it?</p>