add entropy example

doc/examples/plot_entropy2.py

@@ -0,0 +1,38 @@
+"""
+=======
+Entropy
+=======
+
+The following example applies the local entropy measure to a noised image with a variable noise.
+
+"""
+import matplotlib.pyplot as plt
+import numpy as np
+
+from skimage.filters.rank import entropy
+from skimage.morphology import disk
+
+noise_mask = 28*np.ones((128,128),dtype=np.uint8)
+noise_mask[32:-32,32:-32] = 30
+
+noise = (noise_mask*np.random.random(noise_mask.shape)-.5*noise_mask).astype(np.uint8)
+img = noise + 128
+
+radius = 10
+e = entropy(img,disk(radius))
+
+plt.figure(figsize=[15,5])
+plt.subplot(1,3,1)
+plt.imshow(noise_mask,cmap=plt.cm.gray)
+plt.xlabel('noise mask')
+plt.colorbar()
+plt.subplot(1,3,2)
+plt.imshow(img,cmap=plt.cm.gray)
+plt.xlabel('noised image')
+plt.colorbar()
+plt.subplot(1,3,3)
+plt.imshow(e)
+plt.xlabel('image local entropy ($r=%d$)'%radius)
+plt.colorbar()
+
+plt.show()