Python cv2 HoughLines grid line detection

Question

I have a simple grid in an image, I am trying to determine the grid size, e.g. 6x6, 12x12, etc. Using Python and cv2.

enter image description here

I am testing it with the above 3x3 grid, I was planning to determine the grid size by counting how many vertical / horizontal lines there are by detecting them in the image:

import cv2
import numpy as np

im = cv2.imread('photo2.JPG')
gray = cv2.cvtColor(im,cv2.COLOR_BGR2GRAY)

imgSplit = cv2.split(im)
flag,b = cv2.threshold(imgSplit[2],0,255,cv2.THRESH_OTSU) 

element = cv2.getStructuringElement(cv2.MORPH_CROSS,(1,1))
cv2.erode(b,element)

edges = cv2.Canny(b,150,200,3,5)

while(True):

    img = im.copy()

    lines = cv2.HoughLinesP(edges,1,np.pi/2,2, minLineLength = 620, maxLineGap = 100)[0]

    for x1,y1,x2,y2 in lines:        
        cv2.line(img,(x1,y1),(x2,y2),(0,255,0),1)

    cv2.imshow('houghlines',img)

    if k == 27:
        break

cv2.destroyAllWindows()

My code detects the lines, as can be seen below, however there are multiple lines detected for each line in my image:

enter image description here

(there are two 1px green lines drawn for every line in the image)

I cannot simply divide the number of lines by two because (depending on the grid size) sometimes just the one line will be drawn.

How can I more accurately detect and draw a single line for every line detected in the original image?

I have tweaked threshold settings, reducing the image to black and white, yet I still get multiple lines. I assume this is because of the canny edge detection?

Problem may be because canny detects both edges of your line, so considered as two lines. One option is to iterate through the lines, and find the nearby lines. If near discard one. — Abid Rahman K
– Abid Rahman K, Commented Sep 28, 2013 at 4:03

StuR · Accepted Answer · 2013-09-28 15:04:15Z

10

I ended up iterating through the lines and removing lines that were within 10px of one another:

lines = cv2.HoughLinesP(edges,1,np.pi/180,275, minLineLength = 600, maxLineGap = 100)[0].tolist()

for x1,y1,x2,y2 in lines:
    for index, (x3,y3,x4,y4) in enumerate(lines):

        if y1==y2 and y3==y4: # Horizontal Lines
            diff = abs(y1-y3)
        elif x1==x2 and x3==x4: # Vertical Lines
            diff = abs(x1-x3)
        else:
            diff = 0

        if diff < 10 and diff is not 0:
            del lines[index]

gridsize = (len(lines) - 2) / 2

edited Sep 28, 2013 at 15:04

answered Sep 28, 2013 at 13:58

StuR

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Manoj · Accepted Answer · 2017-04-27 16:28:24Z

2

you can dilate the image with kernel = cv2.getStructuringElement(cv2.MORPH_CROSS, (2, 2)) dilated = cv2.dilate(edges, kernel, iterations=5) then apply cv2.HoughLinesP

answered Apr 27, 2017 at 16:28

Manoj

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john k · Accepted Answer · 2017-09-10 23:02:05Z

1

Doesn't the Hough function have a parameter that does exactly this? MaxLineGap? So if your lines were 2px thick, you set that parameter to 3? Does it not work?

answered Sep 10, 2017 at 23:02

john k

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