CODEWITHSUNDEEP
pythonopencv
Amarth Gûl
Amarth Gûl

Reputation: 1082

Strange lines detected by openCV python

I followed a tutorial of python openCV, and was trying to use HoughLinesP() to detect lines, here is the code:

imgLoc = '...\lines.jpg'

img = cv2.imread(imgLoc)
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
edges = cv2.Canny(gray, 50, 100)
minLineLength = 20; maxLineGap = 5
lines = cv2.HoughLinesP(edges, 1, np.pi/180, 100, \
    minLineLength, maxLineGap)
for [[x1, y1, x2, y2]] in lines:
    cv2.line(img, (x1, y1), (x2, y2), (0, 0, 255), 2)
cv2.imshow('line', img)
cv2.waitKey(); cv2.destroyAllWindows()

And here's what I get: floatingLines apparently very strange. there's so many lines in the sky which I never expected, and I thought there would be more vertical lines on the building, but there's not.

The tutorial doesn't give pictures demonstrating how the result should be, so I have no idea if that's normal or not.

So, Is there any problem in my code that lead to that wired image? If it does, can I make some change to let it detect more vertical line?

==========Update_1============

I followed the comment's suggestion, now more line can be detected:

#minLineLength = 20; maxLineGap = 5
lines = cv2.HoughLinesP(edges, 1, np.pi/180, 100, \
    minLineLength = 10, maxLineGap = 5)

lineDetection2.0 But still vertical lines are lacked. And the Canny() result: Canny

in the result of Canny() there ARE vertical edges, why would they disappeared after HoughLinesP()? (Or that's just visual error?)

==========Update_2============

I added a blur and the value of minLineLength:

gray = cv2.GaussianBlur(gray, (5, 5), 0)
...
lines = cv2.HoughLinesP(edges, 1, np.pi/180, 100, \
        minLineLength = 50, maxLineGap = 5)

The result is clearer, but still not much vertical lines... Edges detected And I started to wonder where does these slashes comes from

Upvotes: 0

Views: 737

Answers (1)

Totoro doesn't feed AI anymore
Totoro doesn't feed AI anymore

Reputation: 3408

It might be worth starting with some smoothing before performing Canny edge detection.

I suggest higher values for minLineLength (and maxLineGap, though this should not be very large). That should get rid of smaller lines and connect vertical line segments where detected. If that still does not bring out the vertical lines, we might have to look in to the threshold parameter.

Upvotes: 1

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