title: OpenCV houghLinesP parameters
body: <p>I am having difficulty finding the lines on a chessboard in this image using HoughLinesP with OpenCV in Python.</p>
  
  <p>In an attempt to understand the parameters of HoughLinesP, I have come up with the following code:</p>
  
  <pre><code>import numpy as np
  import cv2
  from matplotlib import pyplot as plt
  from matplotlib import image as image
  
  I = image.imread('chess.jpg') 
  G = cv2.cvtColor(I, cv2.COLOR_BGR2GRAY)
  
  # Canny Edge Detection:
  Threshold1 = 150;
  Threshold2 = 350;
  FilterSize = 5
  E = cv2.Canny(G, Threshold1, Threshold2, FilterSize)
  
  Rres = 1
  Thetares = 1*np.pi/180
  Threshold = 1
  minLineLength = 1
  maxLineGap = 100
  lines = cv2.HoughLinesP(E,Rres,Thetares,Threshold,minLineLength,maxLineGap)
  N = lines.shape[0]
  for i in range(N):
      x1 = lines[i][0][0]
      y1 = lines[i][0][1]    
      x2 = lines[i][0][2]
      y2 = lines[i][0][3]    
      cv2.line(I,(x1,y1),(x2,y2),(255,0,0),2)
  
  plt.figure(),plt.imshow(I),plt.title('Hough Lines'),plt.axis('off')
  plt.show()
  </code></pre>
  
  <p>The problem I am having is that this picks up only one line. If I reduce the maxLineGap to 1, it picks up thousands.</p>
  
  <p>I understand why this might be but how do I pick a suitable set of parameters to get all these co-linear lines to merge? Am I missing something?</p>
  
  <p>I would like to keep the code simple as I am using it as an example of this function in action.</p>
  
  <p>Thanks in advance for any help!</p>
  
  <p>Update: This works perfectly with HoughLines.</p>
  
  <p>And there doesn't seem to be edge detection issues as Canny is working just fine.</p>
  
  <p>However, I still need to get HoughLinesP to work. Any ideas??</p>
  
  <p>Images here: <a href="http://i.stack.imgur.com/gCB2P.jpg" rel="noreferrer">Results</a></p>