title: Perform Canny Edge Detection twice --> better line-detection?
body: <p>I'm working on a project with EmguCV (.NET-version of OpenCV) and I'm using the probabilistic Hough Transformation to find lines.</p>
  
  <p>So at first I was performing the canny-operator. Afterwards doing the Hough-transformation.</p>
  
  <pre><code>Gray cannyThreshold = new Gray(50);
  Gray cannyThresholdLinking = new Gray(300);
  
  Image&lt;Gray, Byte&gt; cannyEdges = gray.Canny(cannyThreshold, cannyThresholdLinking);
  
  LineSegment2D[] linesFound_temporary = cannyEdges.HoughLines
  (
      cannyThreshold,         // 1. Parameter
      cannyThresholdLinking,  // 2. Parameter
      1,                      // 3. Parameter
      Math.PI / 360.0,        // 4. Parameter
      gray.Width * 0.2,       // 5. Parameter
      gray.Width * 0.4,       // 6. Parameter
      gray.Width * 0.1        // 7. Parameter
  )[0];
  </code></pre>
  
  <p>Later I realised that the HoughLines-Method already integrated the canny edge detection.</p>
  
  <p>Nevertheless, my results in line-detection are better and more steady when I use the additional canny detection instead of leaving it out.</p>
  
  <p>Can anyone explain to me, why this happens? Or has anyone experienced the same?</p>