Maybe this sample work for you.
import numpy as np
import matplotlib.pyplot as plt


I = np.zeros((500, 500))
radius = 100
x, y = np.ogrid[-radius: radius, -radius: radius]
cy = 200
cx = 300
index = x**2 + y**2 <= radius**2
a = np.sqrt((radius - abs(y))**2 + (radius - abs(x))**2)
a = (a / a.max()) * 255
I[cy-radius:cy+radius, cx-radius:cx+radius][index] = a[index]
plt.imshow(I)
plt.show()