Extract single line contours from Canny edges

Question

I'd like to extract the contours of an image, expressed as a sequence of point coordinates.

With Canny I'm able to produce a binary image that contains only the edges of the image. Then, I'm trying to use findContours to extract the contours. The results are not OK, though.

For each edge I often got 2 lines, like if it was considered as a very thin area. I would like to simplify my contours so I can draw them as single lines. Or maybe extract them with a different function that directly produce the correct result would be even better.

I had a look on the documentation of OpenCV but I was't able to find anything useful, but I guess that I'm not the first one with a similar problem. Is there any function or method I could use?

Here is the Python code I've written so far:

def main():
    img = cv2.imread("lena-mono.png", 0)

    if img is None:
        raise Exception("Error while loading the image")

    canny_img = cv2.Canny(img, 80, 150)

    contours, hierarchy = cv2.findContours(canny_img, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)
    contours_img = cv2.cvtColor(img, cv2.COLOR_GRAY2BGR)

    scale = 10
    contours_img = cv2.resize(contours_img, (0, 0), fx=scale, fy=scale)

    for cnt in contours:
        color = np.random.randint(0, 255, (3)).tolist()
        cv2.drawContours(contours_img,[cnt*scale], 0, color, 1)

    cv2.imwrite("canny.png", canny_img)
    cv2.imwrite("contours.png", contours_img)

The scale factor is used to highlight the double lines of the contours. Here are the links to the images:

Lena greyscale
Edges extracted with Canny
Contours: 10x zoom where you can see the wrong results produced by findContours

Any suggestion will be greatly appreciated.

you should use HoughLines http://docs.opencv.org/modules/imgproc/doc/feature_detection.html#houghlines — Khashayar, Aug 06 '13 at 08:20
I don't think it's a matter of noise in the image. If you look the edges extracted with Canny they are OK. With HoughLines I wouldn't be able to handle curved lines in the image, wouldn't I? — Muffo, Aug 06 '13 at 09:11
yes of course you would, houghlines has some parameters that you should play with them and find your desired format, as in for curves you will get multiple lines! lines are basically start-end point! we are working with lane detection on a road and this is what we use as well. — Khashayar, Aug 06 '13 at 09:21

score 8 · Answer 1 · edited May 23 '17 at 12:25

If I understand you right, your question has nothing to do with finding lines in a parametric (Hough transform) sense.

Rather, it is an issue with the findContours method returning multiple contours for a single line.

This is because Canny is an edge detector - that means it is filter attuned to the image intensity gradient which occurs on both sides of a line.

So your question is more akin to: “how can I convert low-level edge features to single line?”, or perhaps: “how can I navigate the contours hierarchy to detect single lines?"

This is a fairly common topic - and here is a previous post which proposed one solution:

OpenCV converting Canny edges to contours

Extract single line contours from Canny edges

1 Answers1