Visual demonstration of what the disk structuring element in scikit-image is and how it scales.

disk_strel.py

import numpy as np
import matplotlib.pyplot as plt
from skimage.morphology import disk

N = 5
fig, axes = plt.subplots(1, N, figsize=(16, 3))
for n, ax in enumerate(axes):
    np1 = n + 1
    ax.imshow(np.pad(disk(np1), N-n, 'constant'), interpolation='nearest')
    c = plt.Circle((np1 + N - n, np1 + N - n), radius=np1, fill=False, lw=4, color='b')
    ax.add_artist(c)
    ax.set_xlim(0, 2 * N + 2)
    ax.set_ylim(0, 2 * N + 2)
    
plt.show()