DOC: Canny filter. Improved docstring + example for the gallery

doc/examples/plot_canny.py

@@ -0,0 +1,57 @@
+"""
+===================
+Canny edge detector
+===================
+
+The Canny filter is a multi-stage edge detector. It uses a filter based on the
+derivative of a Gaussian in order to compute the intensity of the gradients.The
+Gaussian reduces the effect of noise present in the image. Then, potential
+edges are thinned down to 1-pixel curves by removing non-maximum pixels of the
+gradient magnitude. Finally, edge pixels are kept or removed using hysteresis
+thresholding on the gradient magnitude.
+
+The Canny has three adjustable parameters: the width of the Gaussian (the 
+noisier the image, the greater the width), and the low and high threshold for
+the hysteresis thresholding. 
+"""
+import numpy as np
+import matplotlib.pyplot as plt
+from scipy import ndimage
+from skimage import filter
+
+# Generate noisy image of a square
+im = np.zeros((128, 128))
+im[32:-32, 32:-32] = 1
+
+im = ndimage.rotate(im, 15, mode='constant')
+im = ndimage.gaussian_filter(im, 4)
+im += 0.2 * np.random.random(im.shape)
+
+# Compute the Canny filter for two values of sigma
+edges1 = filter.canny(im)
+edges2 = filter.canny(im, sigma=3)
+
+# display results
+plt.figure(figsize=(10, 4))
+
+plt.subplot(131)
+plt.imshow(im, cmap=plt.cm.jet)
+plt.axis('off')
+plt.title('noisy image', fontsize=20)
+
+plt.subplot(132)
+plt.imshow(edges1, cmap=plt.cm.gray)
+plt.axis('off')
+plt.title('Canny filter, $\sigma=1$', fontsize=20)
+
+plt.subplot(133)
+plt.imshow(edges2, cmap=plt.cm.gray)
+plt.axis('off')
+plt.title('Canny filter, $\sigma=3$', fontsize=20)
+
+plt.subplots_adjust(wspace=0.02, hspace=0.02, top=0.9,
+                    bottom=0.02, left=0.02, right=0.98)
+
+
+plt.show()
+