OpenCV findContours with RETR_EXTERNAL not working

Following code is trying to extracting bounding rectangles from contours and merge rectangles which is contained in other rectangles

for i in xrange(len(find_contours)):
    cv2.drawContours(z2, find_contours, i, (100, 0, 0))

    rect = cv2.boundingRect(find_contours[i])

    cv2.rectangle(z1, (rect[0], rect[1]), (rect[0]+rect[2], rect[1]+rect[3]), (200,255,80), 1)

cv2.imshow('z1a',z1)
ret,z1=cv2.threshold(z1,50,255,cv2.THRESH_BINARY)
z1=np.uint8(z1)

find_thresh_img, find_contours, find_h = cv2.findContours(z1, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)

In above found contours, one inner contour is contained within an outer contour, but is not merged with outer contour. the inner one’s border is 2 pixel away from outer one’s border.

I checked the code multiple times, and not found something wrong.

Then I tried changed the drawing rectangle border thickness to 2, and it works..

cv2.rectangle(z1, (rect[0], rect[1]), (rect[0]+rect[2], rect[1]+rect[3]), (200,255,80), 2)

Checking it in PyCharm SciView Data View I found their border is connected. So I’m not sure why it’s working is caused by increased border width or connected shapes. (But in the past cv2.findContours with RETR_EXTERNAL will work for inner contours, so it’s likely caused by thin border)

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