Is there a way to extract single line from Hough's Trasnform in OpenCV? - opencv

Question

Am using Hough's Transform to detect straight lines in an image. Transformation is done after a Canny edge detection and am able to get the lines, however, i need to display only the Left most line. Here is the section of code
cv::Mat Final, Canned;
HoughLines(Canned, lines, 1, CV_PI / 180, 150, 0, 0);
for (size_t i = 0; i < lines.size(); i++)
{
float rho = lines[i][0], theta = lines[i][1];
cv::Point pt1, pt2;
double a = cos(theta), b = sin(theta);
double x0 = a*rho, y0 = b*rho;
pt1.x = cvRound(x0 + 1000 * (-b));
pt1.y = cvRound(y0 + 1000 * (a));
pt2.x = cvRound(x0 - 1000 * (-b));
pt2.y = cvRound(y0 - 1000 * (a));
line(Final,pt1, pt2, Scalar(0, 0, 255), 1, CV_AA);
}
imshow("detected lines", Final);
Am attaching the image generated after applying Hough Transform
I need to display only the Left most line
here is the elements of Line vector.
[-386, 3.12414]
[-332, 3.08923]
[-381, 3.12414]
[-337, 3.10669]
[386, 0]
[-323, 3.05433]
[-339, 3.10669]
[-335, 3.08923]
[-330, 3.07178]
[383, 0]
[-317, 3.08923]

Answer 1

You can iterate though the lines and find the left most line by averaging the x-coordinates of the expoints of the line.
# Iterate through lines and find the x-position
xPositions = []
for line in lines:
cdst, pt1, pt2 = draw_line(line, cdst, (0,0,255))
xPositions.append((pt1[0]+pt2[0])/2)
Here is the output image:
Here is the source image:
Here is the complete code:
import math
import cv2 as cv
import numpy as np
src = cv.imread('/home/stephen/Desktop/lines.png', cv.IMREAD_GRAYSCALE)
dst = cv.Canny(src, 50, 200, None, 3)
# Copy edges to the images that will display the results in BGR
cdst = cv.cvtColor(dst, cv.COLOR_GRAY2BGR)
cdstP = np.copy(cdst)
# Find lines
lines = cv.HoughLines(dst, 1, np.pi / 180, 100, None, 0, 0)
# Function that draws line
def draw_line(line, img, color):
rho = line[0][0]
theta = line[0][1]
a = math.cos(theta)
b = math.sin(theta)
x0 = a * rho
y0 = b * rho
pt1 = (int(x0 + 1000*(-b)), int(y0 + 1000*(a)))
pt2 = (int(x0 - 1000*(-b)), int(y0 - 1000*(a)))
cv.line(cdst, pt1, pt2, color, 3, cv.LINE_AA)
return img, pt1, pt2
# Iterate through lines and find the x-position
xPositions = []
for line in lines:
cdst, pt1, pt2 = draw_line(line, cdst, (0,0,255))
xPositions.append((pt1[0]+pt2[0])/2)
# Find the left most line
leftMost = xPositions.index(min(xPositions))
# Draw only the left most line
cdst, pt1, pt2 = draw_line(lines[leftMost], cdst, (0,255,0))
cv.imshow('lines', cdst)
cv.waitKey()
cv.destroyAllWindows()