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import numpy as np
import matplotlib.pyplot as plt

def createCircularMask(h, w, center=None, radius=None):
 if center is None:  # use the middle of the image
  center = [int(w/2), int(h/2)]
 if radius is None:  # use the smallest distance between the center and image walls
  radius = min(center[0], center[1], w-center[0], h-center[1])
 y, x = np.ogrid[:h, :w]
 dist_from_center = np.sqrt((x - center[0])**2 + (y-center[1])**2)

 mask = dist_from_center <= radius
 return mask

img = plt.imread("image.jpg")
h, w = img.shape[:2]
radius = h/4
mask = createCircularMask(h, w, radius=radius)
img[~mask] = 0
plt.imsave("image2.jpg", img, cmap="gray")