import numpy as np
from matplotlib.lines import Path
from matplotlib.patches import PathPatch

import matplotlib.pyplot as plt

x = [0.3, 0.6, 0.6, 0.3, 0.3]
y = [0.3, 0.3, 0.6, 0.6, 0.3]

np.random.seed(12345)

image = np.random.random((12,12))

data = np.vstack([x, y]).transpose()
path = Path(np.vstack(data))
poly = PathPatch(path, edgecolor='red', facecolor='none')

fig = plt.figure()
ax = fig.add_subplot(1,1,1)
ax.add_patch(poly)
im = ax.imshow(image, extent=[0., 1., 0., 1.], interpolation='nearest')
im.set_clip_path(poly)
fig.savefig('test1.png')

fig = plt.figure()
ax = fig.add_subplot(1,1,1)
ax.add_patch(poly)
im = ax.imshow(image, extent=[0., 1., 0., 1.], interpolation='nearest')

display = ax.transData.transform(data)
path_display = Path(display)
poly_display = PathPatch(path_display, transform=None)

im.set_clip_path(poly_display)

fig.savefig('test2.png')