Drawing Bounding box around given size Area contour

Question

I want to draw a bounding box around each closed contour of an area larger than some threshold, not just the biggest contour. How can I go about doing this? So far this is what I have tried:

contours, _ = cv2.findContours(thresh, cv2.RETR_LIST, cv2.CHAIN_APPROX_SIMPLE)
for c in contours:
    rect = cv2.boundingRect(c)
    if rect[2] < 100 or rect[3] < 100: continue
    print cv2.contourArea(c)
x,y,w,h = cv2.boundingRect(c)
cv2.rectangle(im,(x,y),(x+w,y+h),(0,255,0),2)
cv2.putText(im,'Moth Detected',(x+w+10,y+h),0,0.3,(0,255,0))
cv2.imshow("Show",im)
cv2.waitKey()  
cv2.destroyAllWindows()      

Answer 1

Remember, your indentation level matters in Python. It's also worth noting that your code doesn't necessarily draw a box around the largest contour, it draws a box around the last element of contours . Fortunately, the fix is simple. You just need to indent your calls to cv2.rectangle() and cv2.putText() so they evaluate on every loop iteration. You can also eliminate a call to cv2.boundingRect() by expanding rect into x,y,w,h . Your code would then be:

contours, _ = cv2.findContours(thresh, cv2.RETR_LIST, cv2.CHAIN_APPROX_SIMPLE)
for c in contours:
    rect = cv2.boundingRect(c)
    if rect[2] < 100 or rect[3] < 100: continue
    print cv2.contourArea(c)
    x,y,w,h = rect
    cv2.rectangle(im,(x,y),(x+w,y+h),(0,255,0),2)
    cv2.putText(im,'Moth Detected',(x+w+10,y+h),0,0.3,(0,255,0))
cv2.imshow("Show",im)
cv2.waitKey()  
cv2.destroyAllWindows()