From dcbea756775f9e531861bd0df4a49480012277b3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Johannes=20Sch=C3=B6nberger?= <hannesschoenberger@gmail.com>
Date: Mon, 13 Aug 2012 22:21:40 +0200
Subject: [PATCH] improved polygon approximation example script

made existing example parameters more readable and add another example with
a more complex shape based on a subdivision algorithm.
---
 doc/examples/plot_approximate_polygon.py | 60 +++++++++++++++++++++---
 1 file changed, 53 insertions(+), 7 deletions(-)

diff --git a/doc/examples/plot_approximate_polygon.py b/doc/examples/plot_approximate_polygon.py
index cec98492..ec7e4f4d 100644
--- a/doc/examples/plot_approximate_polygon.py
+++ b/doc/examples/plot_approximate_polygon.py
@@ -9,6 +9,7 @@ algorithm.
 
 import numpy as np
 import matplotlib.pyplot as plt
+from scipy import signal
 from skimage.draw import ellipse
 from skimage.measure import find_contours, approximate_polygon
 
@@ -20,15 +21,60 @@ img[rr, cc] = 1
 rr, cc = ellipse(600, 600, 150, 90, img.shape)
 img[rr, cc] = 1
 
+hand = np.array([[1.64516129, 1.16145833],
+                 [1.64516129, 1.59375   ],
+                 [1.35080645, 1.921875  ],
+                 [1.375     , 2.18229167],
+                 [1.68548387, 1.9375    ],
+                 [1.60887097, 2.55208333],
+                 [1.68548387, 2.69791667],
+                 [1.76209677, 2.56770833],
+                 [1.83064516, 1.97395833],
+                 [1.89516129, 2.75      ],
+                 [1.9516129 , 2.84895833],
+                 [2.01209677, 2.76041667],
+                 [1.99193548, 1.99479167],
+                 [2.11290323, 2.63020833],
+                 [2.2016129 , 2.734375  ],
+                 [2.25403226, 2.60416667],
+                 [2.14919355, 1.953125  ],
+                 [2.30645161, 2.36979167],
+                 [2.39112903, 2.36979167],
+                 [2.41532258, 2.1875    ],
+                 [2.1733871 , 1.703125  ],
+                 [2.07782258, 1.16666667]])
+
+
+def doo_sabin_subdivision(points):
+    new_points = np.zeros((2 * (len(points) - 1), 2))
+    new_points[::2] = 3 / 4. * points[:-1] + 1 / 4. * points[1:]
+    new_points[1::2] = 1 / 4. * points[:-1] + 3 / 4. * points[1:]
+    return new_points
+
+new_hand = hand.copy()
+for _ in range(5):
+    new_hand = doo_sabin_subdivision(new_hand)
+
+appr_hand = approximate_polygon(new_hand, tolerance=0.02)
+
+print "Number of coordinates:", len(hand), len(new_hand), len(appr_hand)
+
+fig, (ax1, ax2) = plt.subplots(ncols=2, figsize=(9, 4))
+
+ax1.plot(hand[:, 0], hand[:, 1])
+ax1.plot(new_hand[:, 0], new_hand[:, 1])
+ax1.plot(appr_hand[:, 0], appr_hand[:, 1])
+
 plt.gray()
-plt.imshow(img)
+ax2.imshow(img)
 
 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)
+    coords = approximate_polygon(contour, tolerance=2.5)
+    ax2.plot(coords[:, 1], coords[:, 0], '-r', linewidth=2)
+    coords2 = approximate_polygon(contour, tolerance=39.5)
+    ax2.plot(coords2[:, 1], coords2[:, 0], '-g', linewidth=2)
+    print "Number of coordinates:", len(contour), len(coords), len(coords2)
+
+ax2.axis((0, 800, 0, 800))
 
-plt.axis((0, 800, 0, 800))
-plt.axis('off')
 plt.show()