Closing a contour curve in OpenCV

Question

I'm using OpenCV (Canny + findCountours) to find external contours of objects. The curve drawn is typically almost, but not entirely, closed. I'd like to close it - to find the region it bounds.

How do I do this?

Things considered:

Dilation - the examples I've seen show this after Canny, although it would seem to me it makes more sense to do this after findContours
Convex hull - might work, though I'm really trying to complete a curve
Shape simplification - related, but not exactly what I want

[approxPoly](http://docs.opencv.org/modules/imgproc/doc/structural_analysis_and_shape_descriptors.html#approxpolydp) offers this feature on top of findContours() — berak, Jan 30 '14 at 23:25
Just duplicate the first element of the contour list at the end? — Martin Beckett, Jan 31 '14 at 00:08
MartinBeckett, berak, I think both comments should have been answers, +1 for both — Abid Rahman K, Jan 31 '14 at 08:20

score 1 · Accepted Answer · answered Aug 15 '17 at 09:04

Using PolyLine method to draw contours

cv2.PolyLine(img, points, is_closed=True, 255, thickness=1, lineType=8, shift=0)

Read the docs for further details: http://docs.opencv.org/2.4/modules/core/doc/drawing_functions.html

Mark answered if it resolved your problem. If not then let me know.

score 0 · Answer 2 · answered Jul 26 '17 at 01:09

A little late, but I just had to deal with this myself. If you perform an opening operation (erosion followed by dilation) prior to the Canny edge detection, the edges you detect should be more continuous, resulting in better contours. This method appears to be fairly robust. I mentioned this in another post and posted a code sample as part of a recent question.

Closing a contour curve in OpenCV

2 Answers2

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