title: Calculating descriptors for dense keypoints
body: <p>In OpenCV4Android, I'm using the DENSE feature detector that lays a grid of points over the image. Next, I want to compute the descriptors for those keypoints. For this I tried to use the ORB descriptor extractor.</p>
  
  <pre><code>    mDetector = FeatureDetector.create(FeatureDetector.DENSE);
      mExtractor = DescriptorExtractor.create(DescriptorExtractor.ORB);
  
      MatOfKeyPoint pointsmat0 = new MatOfKeyPoint();
      Mat descriptors0 = new Mat();
  
      mDetector.detect(image0, pointsmat0);
      mExtractor.compute(image0, pointsmat0, descriptors0);
  </code></pre>
  
  <p>Now, when outputting <code>pointsmat0.total</code> and <code>descriptors0.rows()</code> these amounts should be equal, because the descriptor extractor should remove keypoints for which no descriptor could be computed. However, this is not the case.</p>
  
  <p>I get:</p>
  
  <pre><code>pointsmat0.total() around 10000
  descrpitors0.rows() around 8000
  </code></pre>
  
  <p>I've tried using the BRIEF descriptor extractor, but this has the same problem. So, DENSE+ORB / DENSE+BRIEF has this problem.</p>
  
  <p>When I run this sample with ORB+ORB the number of keypoints is equal to the number of descriptors (500 both). So, the question: Which descriptor extractor can be used with DENSE?</p>