title: OpenCV DescriptorExtractor returns empty
body: <p>I am trying to do object detection using OpenCV on iOS. I'm using <a href="http://docs.opencv.org/doc/user_guide/ug_features2d.html" rel="nofollow">this code sample from the documentation</a>. </p>
  
  <p>Here's my code:</p>
  
  <pre><code>Mat src = imread("src.jpg");
  Mat templ = imread("logo.jpg");
  
  Mat src_gray;
  cvtColor(src, src_gray, CV_BGR2GRAY);
  
  Mat templ_gray;
  cvtColor(templ, templ_gray, CV_BGR2GRAY);
  
  int minHessian = 500;
  
  OrbFeatureDetector detector(minHessian);
  
  std::vector&lt;KeyPoint&gt; keypoints_1, keypoints_2;
  
  detector.detect(src_gray, keypoints_1);
  detector.detect(templ_gray, keypoints_2);
  
  OrbDescriptorExtractor extractor;
  
  Mat descriptors_1, descriptors_2;
  
  extractor.compute(src_gray, keypoints_1, descriptors_1);
  extractor.compute(templ_gray, keypoints_2, descriptors_2);
  </code></pre>
  
  <p>The problem is on the line <code>extractor.compute(src_gray, keypoints_1, descriptors_1);</code> which leaves <code>descriptors_1</code> always empty.</p>
  
  <p><code>src</code> and <code>templ</code> are not empty.</p>
  
  <p>Any thoughts?</p>
  
  <p>Thanks</p>