title: Why HoughCircles returns 0 circles while trying to detect irises?
body: <p>I am trying to detect the eyes' irises but <code>HoughCircles</code> returns 0 circles.</p>
  
  <p>The input image(eyes) is:</p>
  
  <p><a href="https://i.stack.imgur.com/KNHGT.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/KNHGT.png" alt="Input image"></a></p>
  
  <p>Then I made the following things with this image:</p>
  
  <pre><code>cvtColor(eyes, gray, CV_BGR2GRAY);
  morphologyEx(gray, gray, 4,cv::getStructuringElement(cv::MORPH_RECT,cv::Size(3,3)));
  threshold(gray, gray, 0, 255, THRESH_OTSU);
  vector&lt;Vec3f&gt; circles;
  HoughCircles(gray, circles, CV_HOUGH_GRADIENT, 2, gray.rows/4);
  if (circles.size())
          cout &lt;&lt; "found" &lt;&lt; endl;
  </code></pre>
  
  <p>So the final gray image looks like this:</p>
  
  <p><a href="https://i.stack.imgur.com/4LXlT.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/4LXlT.png" alt="Output image"></a></p>
  
  <p>I've found this question <a href="https://stackoverflow.com/questions/17890838/using-houghcircles-to-detect-and-measure-pupil-and-iris">Using HoughCircles to detect and measure pupil and iris</a> but it didn't help me despite the similarity with my issue.</p>
  
  <p>So why does <code>HoughCircles</code> return 0 circles while trying to detect irises?
  If someone knows any better way to find irises, you are welcome.</p>