Added sections to gallery of examples

Modified travis_script.sh to account for the new structure of the gallery

Added README.txt files in directories of gallery examples

Fixed references to gallery images in user guide pages

Fixed broken links

doc/examples/transform/plot_ssim.py

@@ -0,0 +1,75 @@
+"""
+===========================
+Structural similarity index
+===========================
+
+When comparing images, the mean squared error (MSE)--while simple to
+implement--is not highly indicative of perceived similarity.  Structural
+similarity aims to address this shortcoming by taking texture into account
+[1]_, [2]_.
+
+The example shows two modifications of the input image, each with the same MSE,
+but with very different mean structural similarity indices.
+
+.. [1] Zhou Wang; Bovik, A.C.; ,"Mean squared error: Love it or leave it? A new
+       look at Signal Fidelity Measures," Signal Processing Magazine, IEEE,
+       vol. 26, no. 1, pp. 98-117, Jan. 2009.
+
+.. [2] Z. Wang, A. C. Bovik, H. R. Sheikh and E. P. Simoncelli, "Image quality
+       assessment: From error visibility to structural similarity," IEEE
+       Transactions on Image Processing, vol. 13, no. 4, pp. 600-612,
+       Apr. 2004.
+
+"""
+import numpy as np
+import matplotlib
+import matplotlib.pyplot as plt
+
+from skimage import data, img_as_float
+from skimage.measure import structural_similarity as ssim
+
+
+matplotlib.rcParams['font.size'] = 9
+
+
+img = img_as_float(data.camera())
+rows, cols = img.shape
+
+noise = np.ones_like(img) * 0.2 * (img.max() - img.min())
+noise[np.random.random(size=noise.shape) > 0.5] *= -1
+
+
+def mse(x, y):
+    return np.linalg.norm(x - y)
+
+img_noise = img + noise
+img_const = img + abs(noise)
+
+fig, (ax0, ax1, ax2) = plt.subplots(nrows=1, ncols=3, figsize=(8, 4), sharex=True, sharey=True, subplot_kw={'adjustable':'box-forced'})
+
+mse_none = mse(img, img)
+ssim_none = ssim(img, img, dynamic_range=img.max() - img.min())
+
+mse_noise = mse(img, img_noise)
+ssim_noise = ssim(img, img_noise,
+                  dynamic_range=img_const.max() - img_const.min())
+
+mse_const = mse(img, img_const)
+ssim_const = ssim(img, img_const,
+                  dynamic_range=img_noise.max() - img_noise.min())
+
+label = 'MSE: %2.f, SSIM: %.2f'
+
+ax0.imshow(img, cmap=plt.cm.gray, vmin=0, vmax=1)
+ax0.set_xlabel(label % (mse_none, ssim_none))
+ax0.set_title('Original image')
+
+ax1.imshow(img_noise, cmap=plt.cm.gray, vmin=0, vmax=1)
+ax1.set_xlabel(label % (mse_noise, ssim_noise))
+ax1.set_title('Image with noise')
+
+ax2.imshow(img_const, cmap=plt.cm.gray, vmin=0, vmax=1)
+ax2.set_xlabel(label % (mse_const, ssim_const))
+ax2.set_title('Image plus constant')
+
+plt.show()