find_biggest_square

import cv2
import numpy as np

img = cv2.imread('sudoku.jpg')
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)

gray = cv2.GaussianBlur(gray, (7, 7), 0)
thresh = cv2.adaptiveThreshold(gray, 255, 1, 1, 7, 2)

contours, hierarchy = cv2.findContours(thresh, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)

biggest = None
max_area = 0
for i in contours:
	area = cv2.contourArea(i)
	if area > 100:
		peri = cv2.arcLength(i, True)
		approx = cv2.approxPolyDP(i, 0.02 * peri, True)
		if area > max_area and len(approx) == 4:
			cv2.drawContours(img,[i], 0, (0, 255, 0), 2)
			biggest = approx
			max_area = area

cv2.imshow('img', img)
cv2.imshow('tresh', thresh)
cv2.waitKey(0)
cv2.destroyAllWindows()