OpenCV - Detect only a particular line in an image

Question

I'm trying to isolate a single line in the following image, I know of a few methods such as CannyEdge Detection that can detect all lines in an image but am struggling on how to get just the line I'm interested in.

Any information on tools in OpenCV that could help with this would be much appreciated.

The goal is to detect the top red out-line of the court (which I've outlined in blue)

Answer 1

In Python/OpenCV, you can threshold on the red color of the line, then get the biggest contour or on contours larger than some threshold in area, which is what I show below..

Input:

import cv2
import numpy as np

# read image as grayscale
img = cv2.imread('red_line.png')

# threshold on red color
lowcolor = (0,0,75)
highcolor = (50,50,135)
thresh = cv2.inRange(img, lowcolor, highcolor)


# apply morphology close
kernel = np.ones((5,5), np.uint8)
thresh = cv2.morphologyEx(thresh, cv2.MORPH_CLOSE, kernel)

# get contours and filter on area
result = img.copy()
contours = cv2.findContours(thresh, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
contours = contours[0] if len(contours) == 2 else contours[1]
result = img.copy()
for c in contours:
    area = cv2.contourArea(c)
    if area > 5000:
        cv2.drawContours(result, [c], -1, (0, 255, 0), 2)


# show thresh and result    
cv2.imshow("thresh", thresh)
cv2.imshow("result", result)
cv2.waitKey(0)
cv2.destroyAllWindows()

# save resulting images
cv2.imwrite('red_line_thresh.png',thresh)
cv2.imwrite('red_line_extracted.png',result)

Thresholded Image:

Resulting Contour: