make_image_clip.py

import numpy as np
import matplotlib.pyplot as plt
from matplotlib import patches

rmax = 0.5
step = 2 * np.pi / 101
l = np.arange(0, 2 * np.pi + step, step)
x = np.cos(l) * rmax
y = np.sin(l) * rmax

hlim = (-0.5, 0.5)
vlim = (-0.5, 0.5)

fig, ax = plt.subplots(1, 1)

rng = np.random.RandomState(75)
data = rng.random_sample((100, 100))

ax.plot(x, y, color='k')

im = ax.imshow(data, extent=[hlim[0], hlim[1], vlim[0], vlim[1]],
               origin='lower', cmap='RdBu_r', interpolation='nearest')

patch = patches.Circle((0.5, 0.5), radius=0.5, clip_on=True,
                       transform=ax.transAxes)
fig.set_clip_path(patch)
im.set_clip_path(patch)
plt.show()