Adding Particle filter chapter.

This is an early, incomplete draft, but it is time to get it into
source control so I can track changes. Not ready for public
consumption.

code/book_plots.py

@@ -102,6 +102,62 @@ def plot_filter(xs, ys=None, c='#013afe', label='Filter', vars=None, **kwargs):
                      facecolor='yellow', alpha=0.2)
 
 
+
+
+def _blob(x, y, area, colour):
+    """
+    Draws a square-shaped blob with the given area (< 1) at
+    the given coordinates.
+    """
+    hs = np.sqrt(area) / 2
+    xcorners = np.array([x - hs, x + hs, x + hs, x - hs])
+    ycorners = np.array([y - hs, y - hs, y + hs, y + hs])
+    plt.fill(xcorners, ycorners, colour, edgecolor=colour)
+
+def hinton(W, maxweight=None):
+    """
+    Draws a Hinton diagram for visualizing a weight matrix.
+    Temporarily disables matplotlib interactive mode if it is on,
+    otherwise this takes forever.
+    """
+    reenable = False
+    if plt.isinteractive():
+        plt.ioff()
+
+    plt.clf()
+    height, width = W.shape
+    if not maxweight:
+        maxweight = 2**np.ceil(np.log(np.max(np.abs(W)))/np.log(2))
+
+    plt.fill(np.array([0, width, width, 0]),
+             np.array([0, 0, height, height]),
+             'gray')
+
+    plt.axis('off')
+    plt.axis('equal')
+    for x in range(width):
+        for y in range(height):
+            _x = x+1
+            _y = y+1
+            w = W[y, x]
+            if w > 0:
+                _blob(_x - 0.5,
+                      height - _y + 0.5,
+                      min(1, w/maxweight),
+                      'white')
+            elif w < 0:
+                _blob(_x - 0.5,
+                      height - _y + 0.5,
+                      min(1, -w/maxweight),
+                      'black')
+    if reenable:
+        plt.ion()
+
+if __name__ == "__main__":
+    hinton(np.random.randn(20, 20))
+    plt.title('Example Hinton diagram - 20x20 random normal')
+    plt.show()
+
 if __name__ == "__main__":
     p = [0.2245871, 0.06288015, 0.06109133, 0.0581008, 0.09334062, 0.2245871,
      0.06288015, 0.06109133, 0.0581008,  0.09334062]*2