OpenCV - proper use of cv2.approxPolyDP ()

Question

I am trying to extract the approximation of an outline in an image using cv2.approxPolyDP() . Here is the image I'm using:

My code is trying to isolate the main island and identify and build a contour approximation and a contour body. I built the outline found in green, the approximation of red:

 import numpy as np import cv2 # load image and shrink - it 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 draws a green outline. The second graph shows the approximation in red - how do I draw this approximation as a continuous closed curve?

the documentation is not very clear and is not tutorial , but I understand that cv2.approxPolyDP() must define a continuous closed curve that I could build with cv2.drawContours() . It's right? If so, what am I doing wrong?

Answer 1

The problem is only in visualization: drawContours expects an array (a list in the case of python) of contours, and not just a single numpy array (which is returned from approxPolyDP ).

The solution is as follows: replacement

 cv2.drawContours(canvas, approx, -1, (0, 0, 255), 3) 

to

 cv2.drawContours(canvas, [approx], -1, (0, 0, 255), 3)