Opencv: Hough lines not showing

Question

So I'm trying to obtain hough lines on a chessboard, but the algorithm results in only one line being detected. I'm using python 2.7 and opencv 3.0. Here's the code:

def applyHoughLineTransform():
    image1 = cv2.imread('pictures/board1.png',0)
    image2 = cv2.imread('pictures/board2.png',0)
    image3 = cv2.imread('pictures/board3.png')
    image4 = cv2.imread('pictures/board4.png')

    lines1 = cv2.HoughLines(image1,1,math.pi/180.0,5)
    lines2 = cv2.HoughLines(image2,1,math.pi/180.0,5)

    lines1 = lines1[0]
    lines2 = lines2[0]

    for rho,theta in lines1:
        print ('Rho and theta:',rho,theta)
        a = np.cos(theta)
        b = np.sin(theta)
        x0 = a*rho
        y0 = b*rho
        x1 = int(x0 + 1000*(-b))
        y1 = int(y0 + 1000*(a))
        x2 = int(x0 - 1000*(-b))
        y2 = int(y0 - 1000*(a))

        print (x1,y1)
        print (x2,y2)

        cv2.line(image3,(x1,y1),(x2,y2),(0,0,255),2)

    for rho,theta in lines2:
        a = np.cos(theta)
        b = np.sin(theta)
        x0 = a*rho
        y0 = b*rho
        x1 = int(x0 + 1000*(-b))
        y1 = int(y0 + 1000*(a))
        x2 = int(x0 - 1000*(-b))
        y2 = int(y0 - 1000*(a))

        cv2.line(image4,(x1,y1),(x2,y2),(0,0,255),2)

    cv2.imwrite('pictures/board1.png',image1)
    cv2.imwrite('pictures/board2.png',image2)

    cv2.imshow('hough line 1',image3)
    cv2.imshow('hough line 2',image4)

Here's the canny edge image on which i perform the hough line algorithm:

And here are the results:

As you can see, pretty lame. The canny algorithm seems to be providing really nice edges to operate on. I'm not entirely sure what I'm doing wrong. I imagine it has something to do with the arguments inputted into the houghLines function. If someone could point me in the right direction (or fix my problem entirely :) ) I would greatly appreciate it. Here's a link to the tutorial site I'm using: http://opencv-python-tutroals.readthedocs.org/en/latest/py_tutorials/py_imgproc/py_houghlines/py_houghlines.html

Answer 1

OPENCV 3.1.0

After checking the example from the package This is what you should use to get it right in Opencv 3.0.0 +

import cv2
import numpy as np
import math

image1 = cv2.imread('img.png')
gray=cv2.cvtColor(image1,cv2.COLOR_BGR2GRAY)
dst = cv2.Canny(gray, 50, 200)

lines= cv2.HoughLines(dst, 1, math.pi/180.0, 100, np.array([]), 0, 0)
#lines1 = cv2.HoughLines(image1,1,math.pi/180.0,5)
#lines2 = cv2.HoughLines(image2,1,math.pi/180.0,5)

#lines1 = lines1[0]
#lines2 = lines2[0]

a,b,c = lines.shape
for i in range(a):
    rho = lines[i][0][0]
    theta = lines[i][0][1]
    a = math.cos(theta)
    b = math.sin(theta)
    x0, y0 = a*rho, b*rho
    pt1 = ( int(x0+1000*(-b)), int(y0+1000*(a)) )
    pt2 = ( int(x0-1000*(-b)), int(y0-1000*(a)) )
    cv2.line(image1, pt1, pt2, (0, 0, 255), 2, cv2.LINE_AA)


cv2.imshow('image1',image1)
cv2.waitKey(0)
cv2.destoryAllWindows(0)

OUTPUT

Answer 2

The fix to this issue, was to switch from opencv 3.0 to 2.4. Now I get all the lines I want. Lesson learned... it's in beta for a reason! Here are the results:

Answer 3

I had the same issue with OpenCV 3.4. The culprit is in the numpy array lines:

[[[  7.99000000e+02   1.57079637e+00]] 
 [[  9.39000000e+02   1.57079637e+00]]    
 [[  1.57100000e+03   1.57079637e+00]]    
 [[  6.68000000e+02   1.57079637e+00]]    
 [[  5.46000000e+02   1.57079637e+00]]    
 [[  1.42700000e+03   1.57079637e+00]]
 ...
 [[  1.49100000e+03   1.57079637e+00]]]

Notice this is a 3D array, while the example code treats it as a 2D array. The fix is simply to extract rho and theta from the 3D array (only the first 2 lines are changed):

for line in lines:
    rho, theta = line[0]
    a = np.cos(theta)
    b = np.sin(theta)
    x0 = a*rho
    y0 = b*rho
    x1 = int(x0 + 1000*(-b))
    y1 = int(y0 + 1000*(a))
    x2 = int(x0 - 1000*(-b))
    y2 = int(y0 - 1000*(a))

    cv2.line(img,(x1,y1),(x2,y2),(0,0,255),2)