import cv2
import numpy as np

OPENCV_MAJOR_VERSION = int(cv2.__version__.split('.')[0])

# Create a black image.
img = np.zeros((300, 300), dtype=np.uint8)

# Draw a square in two shades of gray.
img[50:150, 50:150] = 160
img[70:150, 70:150] = 128

# Apply a threshold so that the square becomes uniformly white.
ret, thresh = cv2.threshold(img, 127, 255, 0)

# Find the contour of the thresholded square.
if OPENCV_MAJOR_VERSION >= 4:
    # OpenCV 4 or a later version is being used.
    contours, hier = cv2.findContours(thresh, cv2.RETR_TREE,
                                      cv2.CHAIN_APPROX_SIMPLE)
else:
    # OpenCV 3 or an earlier version is being used.
    # cv2.findContours has an extra return value.
    # The extra return value is the thresholded image, which (in
    # OpenCV 3.1 or an earlier version) may have been modified, but
    # we can ignore it.
    _, contours, hier = cv2.findContours(thresh, cv2.RETR_TREE,
                                         cv2.CHAIN_APPROX_SIMPLE)

color = cv2.cvtColor(img, cv2.COLOR_GRAY2BGR)
img = cv2.drawContours(color, contours, -1, (0,255,0), 2)
cv2.imshow("contours", color)
cv2.waitKey()
cv2.destroyAllWindows()