Unable to display bounding rectangles around contours in OpenCV (Python)

Question

I have written this code to draw rectangular boxes around contours drawn in an image, however on running, I get everything right except the boxes, which I don't see at all. What's the error?

for cnt,heir in zip(contours, hierarchy):
    (x,y,w,h) = cv2.boundingRect(cnt);
    cv2.rectangle(im2,(x,y),(x+w,y+h),(0,255,0),2)

cv2.drawContours(im2, contours, -1, (255,255,255), 2);   
cv2.imshow("Contours",im2);

PS. I use OpenCV 3.1.0 and Python 2.7

EDIT: I tried iterating through each contour and in order to check it I modified the code as follows:

for cnt,heir in zip(contours, hierarchy):
    print ('Contour Area:',cv2.contourArea(cnt));
    (x,y,w,h) = cv2.boundingRect(cnt);
    print (x,y,h,w)
    cv2.putText(im2,'worm',(x+w,y+h), cv2.FONT_HERSHEY_SIMPLEX, 0.5, (255,255,255), 2, cv2.LINE_AA);
    cv2.rectangle(im2,(x,y),(x+w,y+h),(255,0,0),2);

I printed the contour area for each, the values of (x,y,w,h) for each and putting text "worm" for each contour and drawing the rectangular boxes around each contour. However I get just 1 output:

for an image like:

I need to display the text "worm" at each of the worm-like creature. However I just get it once. What are the issues?

use print() to see what you have in variables (ie. print(contours)) and which part of code is executed (ie. print("I'm inside for-loop")). Maybe OpenCV returns empty list with contours BTW: OpenCV if can't do something doesn't raise error but returns None and you don't know that there is a problem. — furas
– furas, Commented Jan 9, 2017 at 7:26
stackoverflow.com/questions/22137511/… - they use different color because (0,255,0) is invisible on their image. — furas
– furas, Commented Jan 9, 2017 at 7:30
@furas This is what I get : i.imgur.com/dSS3YRy.png - A very large list of nested arrays with some numbers. Also, I changed the colour, doesn't work ! — singhuist
– singhuist, Commented Jan 9, 2017 at 10:15
did you check what you have in (x,y,w,h) ? maybe you have w=0, h=0 — furas
– furas, Commented Jan 9, 2017 at 10:52
can you upload the image you are working with? and what are you trying to detect? — Jeru Luke
– Jeru Luke, Commented Jan 9, 2017 at 11:09

thewaywewere · Accepted Answer · 2017-01-09 16:24:29Z

3

I used to use below code to draw rectangles on the contours detected. Hope it helps.

for contour in contours:
    # get rectangle bounding contour
    [x,y,w,h] = cv2.boundingRect(contour)

    # draw rectangle around contour on original image
    cv2.rectangle(img,(x,y),(x+w,y+h),(255,0,255),2)

answered Jan 9, 2017 at 16:24

thewaywewere

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singhuist Over a year ago

coincidentally I just tried by removing the hierarchy part - an it does work! Thank you for your help :)

singhuist · Accepted Answer · 2017-01-09 17:24:03Z

0

Simply write:

for c in contours:
    (x,y,w,h) = cv2.boundingRect(c);
    cv2.putText(im2,'worm',(x+w,y+h), cv2.FONT_HERSHEY_SIMPLEX, 0.5, (255,255,255), 2, cv2.LINE_AA);
    cv2.rectangle(im2,(x,y),(x+w,y+h),(255,0,0),2);

cv2.drawContours(im2, contours, -1, (255,255,255), 2);   
cv2.imshow("Contours",im2);

As suggested by furas, zip(contours,hierarchy) will return only one pair if hierarchy has only one. In this case a simple loop over contours list works.

answered Jan 9, 2017 at 17:24

singhuist

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