Python How to detect vertical and horizontal lines in an image with HoughLines with OpenCV?
<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="nofollow">https://s22.postimg.org/iq2b91xq9/droite_Image_00000.jpg</a>)</p>

<p><a href="http://i.stack.imgur.com/Qmksl.jpg" rel="nofollow"><img src="http://i.stack.imgur.com/Qmksl.jpg" alt="enter image description here"></a></p>

<p>I would rather write this as a comment but unfortunately I can't. You should change the minLineLength and minLineGap. Or what if its just sqaures that you have to find, I would get all the lines and check the angles between them to get lines only along squares. I have worked with HoughLineP before and it is pretty much based on the above two arguments. Additionally, try using Bilateral filtering. I really helps when the sharpening using median filter doesn't help. </p>

<p><a href="http://docs.opencv.org/2.4/doc/tutorials/imgproc/gausian_median_blur_bilateral_filter/gausian_median_blur_bilateral_filter.html" rel="nofollow">Bilateral Filter</a></p>

python opencv camera-calibration hough-transform