Detect closed contours

Question

This is example image:

and I use opencv to detect contours:

>>> fc = cv2.findContours(img, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)
>>> contours = fc[0]

For detecting closed contour I thought to check start and end points in each contour returned by opencv, while I noticed that regardless the shape of object opencv seem to outline each object, so I get this result:

>>> for contour in contours:
>>>    print(contour[0,:,:], contour[-1,:,:])
[[246  38]] [[247  38]]
[[92 33]] [[93 33]]

or each found contour has closed path.

I searched for additional constants to available methods in findContour() function, but it seems all return closed paths.

So is there some general way of detecting if found contour is closed?

---

I googled before asking and didn't got results, but I see good candidate in similar questions at the right side: How can i know if a contour is open or closed in opencv? where it is suggested I use cv2.isContourConvex(contour), but:

>>> for contour in contours:
>>>    print(cv2.isContourConvex(contour))
False
False

---

yet another update: contourArea looks like it may provide answer (at least for simple contours) but I didn't tested on anything else then above example image:

>>> for contour in contours:
>>>     print(cv2.contourArea(contour))
0.0
12437.5

Answer 1

I came across this problem myself and I found a work around...

Going through this..

for i in contours:
   print(cv2.contourArea(i), cv2.arcLength(i,True))

I've noticed that closed contours (e.g. circles) have a higher contourArea than arcLength, whilst open contours (e.g. lines) have a lower contourArea than arcLength, so you can filter them as such...

closed_contours = []
open_contours = []
for i in contours:
    if cv2.contourArea(i) > cv2.arcLength(i, True):
        closed_contours.append(i)
    else:
        open_contours.append(i)

I know this was asked over 3 years ago but for anybody else needing an idea this worked for me!