The output of the CV2.Approxpolydp () function is a vertex coordinates of approximate polygon

The original image is

import cv2
import numpy as np
import matplotlib.pyplot as plt

def cv_show(name, img):
    cv2.imshow(name, img)
    cv2.waitKey(0)
    cv2.destroyAllWindows()


# Read the image
img = cv2.imread('contours.png')
cv_show('contours', img)

 # Gray degree and dual value
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
thresh = cv2.threshold(gray, 127, 255, cv2.THRESH_BINARY)[1]
cv_show('thresh', thresh)

 #
cnt = cv2.findContours(thresh, cv2.RETR_TREE, cv2.CHAIN_APPROX_NONE)[1]

After outline detection, the value of the output is CNT, the type is list, and the number of elements is 11, which contains 11 contour

C0 is indexed in CNT as 0, which represents the first outline, that is, the triangular outline

c0 = cnt[0]
res = cv2.drawContours(draw, [c0], -1, (0, 0, 255), 2)
cv_show('res', res)

The outline is similar below

epsilon = 0.1 * cv2.arcLength(c0, True)
approx = cv2.approxPolyDP(c0, epsilon, True)
draw_img = img.copy()
res = cv2.drawContours(draw_img, [approx], -1, (0, 0, 255), 2)
cv_show('res', res)

CV2.APAPROXPLOYDP function output APPROX, the number of elements is 3, that is, the three vertices of the triangle

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