OpenCV - visualize polygonal curve(s) extracted with cv2.approxPolyDP()
The problem is in visualization only: drawContours expects array (list in case of python) of contours, not just one numpy array (which is returned from approxPolyDP).
Solution is the following: replacing
cv2.drawContours(canvas, approx, -1, (0, 0, 255), 3)to
cv2.drawContours(canvas, [approx], -1, (0, 0, 255), 3)