How can I detect the white lines alone from the above image using opencv python? which method can I use? or is there any inbuilt function available for this purpose?
Hough's didn't work well on this case.
The final output should like these:
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It would be helpful if you clarify in what manner you want to detect the lines and what exactly do you mean by 'detect'? Do you want an image with white lines only, or do you need to describe their position mathematically? Do you want to extract them because they are lines or because they are white? There are e.g. also black lines...a.smiet– a.smiet2018-01-16 16:58:56 +00:00Commented Jan 16, 2018 at 16:58
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Edited and added expected output.Perseus784– Perseus7842018-01-16 17:09:43 +00:00Commented Jan 16, 2018 at 17:09
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1 Answer
It seems like Hough's does actually do a good job. I took the example shown here and introduced only a small modification for the extraction of edges:
Load image and convert to grayscale:
import numpy as np
import cv2
import scipy.ndimage as ndi
img = cv2.imread(path)
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
Then I applied a median filter to get rid of some high frequency edges/noise like the net in the image:
smooth = ndi.filters.median_filter(gray, size=2)
Result looks like this:
Then I applied a simple threshold to extract the white lines. Alternatively, you could apply e.g. a Laplace filter to extract edges, however, simple thresholding will neglect e.g. the horizontal line at the back of the tennis court:
edges = smooth > 180
Result:
Then perform the Hough line transform similar to this example.
lines = cv2.HoughLines(edges.astype(np.uint8), 0.5, np.pi/180, 120)
for rho,theta in lines[0]:
print(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))
cv2.line(img,(x1,y1),(x2,y2),(0,0,255),2)
# Show the result
cv2.imshow("Line Detection", img)
You can play around with the accuracies and modify the result. I ended up choosing the parameters 0.5 and 120. Overall result:
1 Comment
Amit Amola
You need a nested loop to print all the lines, the output won't show up as it shows or at least mention that the code shows how to show only single line and rest can be looped over.