How to do it...

1. Import the modules:

import cv2
import numpy as np
import matplotlib.pyplot as plt

2. Draw a test image—a black circle (without filling) on a white background:

image = np.full((480, 640), 255, np.uint8)
cv2.circle(image, (320, 240), 100, 0)

3. Compute the distance from every point to the circle:

distmap = cv2.distanceTransform(image, cv2.DIST_L2, cv2.DIST_MASK_PRECISE)

4. Visualize the results:

plt.figure()
plt.imshow(distmap, cmap='gray')
plt.show()