Detecting multiple lines in OpenCV HoughLines - opencv

Question

I'm using OpenCV 4.4 and running the following code to detect lines of a grid. When I display the image it always detect one line as shown in the screenshot. How can I detect all vertical lines in the grid?
grid = cv2.imread('images/grid.jpeg')
grayscale = cv2.cvtColor(grid, cv2.COLOR_BGR2GRAY)
edges = cv2.Canny(grayscale, 50, 150, apertureSize=3)
lines = cv2.HoughLines(edges, 1, np.pi/180, 100)
for rho, theta in lines[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(grid, (x1, y1), (x2, y2), (255, 0, 0), 2)
cv2.imshow("Lines", grid)
cv2.waitKey(0)
cv2.destroyAllWindows()
Original Image:

Answer 1

You can use lineDetector algorithm.
Find the edges of your image, as #Micka suggested
img = cv2.imread("lines.png")
img_gry = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
img_cny = cv2.Canny(img_gry, 50, 200)
Result:
To detect the vertical edges, the difference between x-coordinates should be close to 0 Since only y-coordinates are changing.
if abs(x1 - x2) < 3:
cv2.line(img, pt1=(x1, y1), pt2=(x2, y2), color=(0, 0, 255), thickness=3)
Result:
Code:
import cv2
img = cv2.imread("lines.png")
img_gry = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
img_cny = cv2.Canny(img_gry, 50, 200)
lns = cv2.ximgproc.createFastLineDetector().detect(img_cny)
for ln in lns:
x1 = int(ln[0][0])
y1 = int(ln[0][1])
x2 = int(ln[0][2])
y2 = int(ln[0][3])
if abs(x1 - x2) < 3:
cv2.line(img, pt1=(x1, y1), pt2=(x2, y2), color=(0, 0, 255), thickness=3)
cv2.imshow("lns", img)
cv2.waitKey(0)