add example script for polygon approximation

doc/examples/plot_approximate_polygon.py

@@ -0,0 +1,35 @@
+"""
+====================
+Approximate Polygons
+====================
+
+This example shows how to approximate polygonal chains with the Douglas-Peucker
+algorithm.
+"""
+
+import numpy as np
+import matplotlib.pyplot as plt
+from skimage.draw import ellipse
+from skimage.measure import find_contours, approximate_polygon
+from skimage.morphology import erosion
+
+img = np.zeros((800, 800), 'int32')
+
+rr, cc = ellipse(250, 250, 180, 230, img.shape)
+img[rr, cc] = 1
+rr, cc = ellipse(600, 600, 150, 90, img.shape)
+img[rr, cc] = 1
+
+plt.gray()
+plt.imshow(img)
+
+strel = np.array([[0, 1, 0], [1, 1, 1], [0, 1, 0]], 'uint8')
+for contour in find_contours(img, 0):
+    coords = approximate_polygon(contour, 4)
+    plt.plot(coords[:, 1], coords[:, 0], '-r', linewidth=2)
+    coords = approximate_polygon(contour, 40)
+    plt.plot(coords[:, 1], coords[:, 0], '-g', linewidth=2)
+
+plt.axis((0, 800, 0, 800))
+plt.axis('off')
+plt.show()