OpenCV - visualize polygonal curve(s) extracted with cv2.approxPolyDP()

Question

I want to visualize polygonal curve(s) extracted with cv2.approxPolyDP(). Here's the image I am using:

My code attempts to isolate the main island and define and plot the contour approximation and contour hull. I have plotted the contour found in green, the approximation in red:

import numpy as np import cv2  # load image and shrink - it's massive img = cv2.imread('../data/UK.png') img = cv2.resize(img, None,fx=0.25, fy=0.25, interpolation = cv2.INTER_CUBIC)  # get a blank canvas for drawing contour on and convert img to grayscale canvas = np.zeros(img.shape, np.uint8) img2gray = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)  # filter out small lines between counties kernel = np.ones((5,5),np.float32)/25 img2gray = cv2.filter2D(img2gray,-1,kernel)  # threshold the image and extract contours ret,thresh = cv2.threshold(img2gray,250,255,cv2.THRESH_BINARY_INV) im2,contours,hierarchy = cv2.findContours(thresh, cv2.RETR_TREE, cv2.CHAIN_APPROX_NONE)   # find the main island (biggest area) cnt = contours[0] max_area = cv2.contourArea(cnt)  for cont in contours:     if cv2.contourArea(cont) > max_area:         cnt = cont         max_area = cv2.contourArea(cont)  # define main island contour approx. and hull perimeter = cv2.arcLength(cnt,True) epsilon = 0.01*cv2.arcLength(cnt,True) approx = cv2.approxPolyDP(cnt,epsilon,True)  hull = cv2.convexHull(cnt)  # cv2.isContourConvex(cnt)  cv2.drawContours(canvas, cnt, -1, (0, 255, 0), 3) cv2.drawContours(canvas, approx, -1, (0, 0, 255), 3) ## cv2.drawContours(canvas, hull, -1, (0, 0, 255), 3) # only displays a few points as well.  cv2.imshow("Contour", canvas) k = cv2.waitKey(0)  if k == 27:         # wait for ESC key to exit     cv2.destroyAllWindows() 

Here are the resulting images:

The first image plots the contour in green. The second plots the approximation in red - how do I plot this approximation as a continuous closed curve?

The documentation isn't terribly clear and neither is the tutorial, but my understanding is that cv2.approxPolyDP() should define a continuous, closed curve, which I should be able to plot with cv2.drawContours(). Is that correct? If so, what am I doing wrong?

Answer 1

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)