DOC: add example of use of view_as_blocks with kmeans for simple dictionary learning

2026-07-14 11:18:06 +08:00 · 2012-02-14 00:16:00 -05:00
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+"""
+=======================================================
+Gabors / Primary Visual Cortex "Simple Cells" from Lena
+=======================================================
+
+(under construction)
+
+How to build a (bio-plausible) "sparse" dictionary (or 'codebook', or
+'filterbank') for e.g. image classification without any fancy math and
+with just standard python scientific librairies?
+
+Please find below a short answer ;-)
+
+This simple example shows how to get Gabor-like filters [1]_ using just
+the famous Lena image. Gabor filters are good approximations of the
+"Simple Cells" [2]_ receptive fields [3]_ found in the mammalian primary
+visual cortex (V1) (for details, see e.g. the Nobel-prize winning work of Hubel
+& Wiesel done in the 60s).
+
+Here we use McQueen's 'kmeans' algorithm [4]_, as a simple bio-plausible
+hebbian-like learning rule and we apply it (a) to patches of the
+original Lena image (retinal projection), and (b) to patches of an
+LGN-like [5]_ Lena image using a simple difference of gaussians (DoG)
+approximation.
+
+Enjoy ;-) And keep in mind that getting Gabors on natural image patches
+is not rocket science.
+
+.. [1] http://en.wikipedia.org/wiki/Gabor_filter
+.. [2] http://en.wikipedia.org/wiki/Simple_cell
+.. [3] http://en.wikipedia.org/wiki/Receptive_field
+.. [4] http://en.wikipedia.org/wiki/K-means_clustering
+.. [5] http://en.wikipedia.org/wiki/Lateral_geniculate_nucleus
+
+References
+----------
+D. H. Hubel and T. N. Wiesel Receptive Fields of Single Neurones in the
+Cat's Striate Cortex J. Physiol. pp. 574-591 (148) 1959
+
+D. H. Hubel and T. N. Wiesel Receptive Fields, Binocular Interaction and
+Functional Architecture in the Cat's Visual Cortex J. Physiol. 160 pp.
+106-154 1962
+"""
+
+import numpy as np
+from scipy import misc
+from scipy.cluster.vq import kmeans2
+import matplotlib.pyplot as plt
+
+from skimage.util.shape import view_as_windows
+from skimage.util.montage import montage2d
+from scipy import ndimage as ndi
+
+np.random.seed(42)
+
+patch_shape = 8, 8
+n_filters = 49
+
+lena = misc.lena() / 255.
+
+# -- filterbank1 on original Lena
+patches1 = view_as_windows(lena, patch_shape)
+patches1 = patches1.reshape(-1, patch_shape[0] * patch_shape[1])[::8]
+fb1, _ = kmeans2(patches1, n_filters, minit='points')
+fb1 = fb1.reshape((-1,) + patch_shape)
+fb1_montage = montage2d(fb1)
+
+# -- filterbank2 LGN-like Lena
+lena_dog = ndi.gaussian_filter(lena, .5) - ndi.gaussian_filter(lena, 1)
+patches2 = view_as_windows(lena_dog, patch_shape)
+patches2 = patches2.reshape(-1, patch_shape[0] * patch_shape[1])[::8]
+fb2, _ = kmeans2(patches2, n_filters, minit='points')
+fb2 = fb2.reshape((-1,) + patch_shape)
+fb2_montage = montage2d(fb2)
+
+# --
+plt.figure(figsize=(9, 3))
+
+
+plt.subplot(2, 2, 1)
+plt.imshow(lena, cmap=plt.cm.gray)
+plt.axis('off')
+plt.title("Lena (original)")
+
+plt.subplot(2, 2, 2)
+plt.imshow(fb1_montage, cmap=plt.cm.gray)
+plt.axis('off')
+plt.title("K-means filterbank (codebook) on Lena (original)")
+
+plt.subplot(2, 2, 3)
+plt.imshow(lena_dog, cmap=plt.cm.gray)
+plt.axis('off')
+plt.title("Lena (LGN-like DoG)")
+
+plt.subplot(2, 2, 4)
+plt.imshow(fb2_montage, cmap=plt.cm.gray)
+plt.axis('off')
+plt.title("K-means filterbank (codebook) on Lena (LGN-like DoG)")
+
+plt.show()