title: Python How to detect vertical and horizontal lines in an image with HoughLines with OpenCV?
body: <p>I m trying to obtain a threshold of the calibration chessboard. I cant detect directly the chessboard corners as there is some dust as i observe a micro chessboard.
  I try several methods and HoughLinesP seems to be the easiest approach. But the results are not good, how to improve my results?</p>
  
  <pre><code>import numpy as np
  import cv2
  
  img = cv2.imread('lines.jpg')
  gray = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)
  edges = cv2.Canny(gray,50,150,apertureSize = 3)
  print img.shape[1]
  print img.shape
  minLineLength=100
  lines = cv2.HoughLinesP(image=edges,rho=0.02,theta=np.pi/500, threshold=10,lines=np.array([]), minLineLength=minLineLength,maxLineGap=100)
  
  a,b,c = lines.shape
  for i in range(a):
      cv2.line(img, (lines[i][0][0], lines[i][0][1]), (lines[i][0][2], lines[i][0][3]), (0, 0, 255), 3, cv2.LINE_AA)
      cv2.imwrite('houghlines5.jpg',img)
  </code></pre>
  
  <p>As you can see on figure below, i cant obtain my chessboard, the lines are plotted in a lot of directions... (the original picture : <a href="https://s22.postimg.org/iq2b91xq9/droite_Image_00000.jpg" rel="noreferrer">https://s22.postimg.org/iq2b91xq9/droite_Image_00000.jpg</a>)</p>
  
  <p><a href="https://i.stack.imgur.com/Qmksl.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/Qmksl.jpg" alt="enter image description here"></a></p>