import cv2
import numpy as np
import math

img = cv2.imread('TUP74.jpg', cv2.IMREAD_COLOR)
height, width = img.shape[:2]

_, thresh = cv2.threshold(cv2.cvtColor(img, cv2.COLOR_BGR2GRAY), 31, 255, cv2.THRESH_BINARY)

# Find top/bottom of the circle, to determine radius

row_info = cv2.findNonZero(cv2.reduce(thresh, 1, cv2.REDUCE_MAX))
first_y, last_y = row_info[0][0][1], row_info[-1][0][1]

diameter = last_y - first_y
radius = int(diameter / 2)
center_y = first_y + radius

# Repeat again, just on first column

row_info = cv2.findNonZero(thresh[:,0])
first_y, last_y = row_info[0][0][1], row_info[-1][0][1]

h = last_y - first_y
center_x = int(math.sqrt(radius**2 - (h/2)**2))

cv2.circle(img, (center_x,center_y), radius, (0,0,255), 2)

s = int(math.sqrt(radius**2 / 2))

tl = (center_x - s, center_y - s)
br = (center_x + s, center_y + s)

cv2.rectangle(img, tl, br, (255,0,0), 2)

cv2.imshow('', img)
cv2.waitKey()