title: feature descriptor for a given 2d point in an image
body: <p>Trying to get a descriptor for a predefined point using python opencv3. The goal is to provide a set of points for a given image and get their corresponding feature descriptors. I'm open to using SIFT, SURF, Brief, ORB, and basically any descriptor. However, I do not want to use any of the detection methods provided. I have created the following:</p>
  
  <pre><code>feat_object = cv2.xfeatures2d.BriefDescriptorExtractor_create()
  
  # define keypoint for a single 2d point
  pt = cv2.KeyPoint(point[0,0],point[1,0], 10)
  
  # create feature descriptor
  out = feat_object.compute(frame, pt)
  </code></pre>
  
  <p>However, I get the following error.</p>
  
  <blockquote>
    <p>---->  out = feat_object.compute(frame, pt)</p>
    
    <p>SystemError: error return without exception set</p>
  </blockquote>
  
  <p>Any suggestions?</p>