title: What's the use of Canny before HoughLines (opencv)?
body: <p>I'm new to image processing and I'm working on detecting lines in a document image. I read the theory of Hough line transform but I can't see why I must use Canny before calling that function in opencv like being said in many tutorials. What's the point of finding edges in this case? The fact is that if I don't use Canny or threshold before HoughLines() the results will be very messy. I hope someone will explain for me the reason why.</p>
  
  <p>2 of the tutorials I've read:</p>
  
  <ol>
  <li><a href="http://opencv.willowgarage.com/documentation/cpp/imgproc_feature_detection.html#cv-houghlines" rel="noreferrer">Imgproc Feature Detection</a></li>
  <li><a href="http://opencv.itseez.com/doc/tutorials/imgproc/imgtrans/hough_lines/hough_lines.html" rel="noreferrer">Hough Line Transform</a></li>
  </ol>