How to use .getObjPoints in CharucoBoard?

Python
1

When I generate a CharucoBoard. How do you find the positions of the Aruco Markers on the generated image?

Using getObjPoints does not work at all.
Figure_1

import sys, getopt
import time
import numpy as np


import cv2
from mpl_toolkits.mplot3d import Axes3D
import matplotlib.pyplot as plt
import matplotlib as mpl


def main():

    width = 1920
    height = 1080
    box_size = 200


    box_width_num = int(width/box_size)
    box_height_num = int(height/box_size)

    aruco_dict = cv2.aruco.getPredefinedDictionary(cv2.aruco.DICT_6X6_50)
    board = cv2.aruco.CharucoBoard((box_width_num, box_height_num), 2.0, 1.0, aruco_dict)
    img = board.generateImage( (width, height) )

    #print(board.getIds())
    squares = board.getObjPoints()

    img = cv2.cvtColor(img,cv2.COLOR_GRAY2RGB)

    for i in range(len(squares)):
        square = squares[i]
        minimum = (np.min(square, axis=0)*box_size*0.5).astype(int)
        maximum = (np.max(square, axis=0)*box_size*0.5).astype(int)

        img = cv2.rectangle(img, (minimum[0]  , minimum[1] ), (  maximum[0]  , maximum[1]  ), (0, 255, 0), 4)



    fig = plt.figure()
    ax = fig.add_subplot(1,1,1)
    plt.imshow(img,  interpolation = "nearest")
    ax.axis("off")
    plt.show()




if __name__ == "__main__":
   main()