simple and adaptive threshold

original image: 

import cv2 as cv
import numpy as np

img = cv.imread('sudoku.png', 0)
_, th1 = cv.threshold(img, 127, 255, cv.THRESH_BINARY)
th2 = cv.adaptiveThreshold(img, 255, cv.ADAPTIVE_THRESH_MEAN_C, cv.THRESH_BINARY, 11, 2);
th3 = cv.adaptiveThreshold(img, 255, cv.ADAPTIVE_THRESH_GAUSSIAN_C, cv.THRESH_BINARY, 11, 2);

cv.imshow("Image", img)
cv.imshow("th1", th1)
cv.imshow("th2", th2)
cv.imshow("th3", th3)

cv.waitKey(0)
cv.destroyAllWindows()