59 lines
1.4 KiB
Python
59 lines
1.4 KiB
Python
# Gift wrapping algorithm
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# https://en.wikipedia.org/wiki/Gift_wrapping_algorithm
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# Date: 2021-11-16
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import random
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import matplotlib.pyplot as plt
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def generate_points(n=100):
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X = []
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Y = []
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for _i in range(n):
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X.append(random.random())
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Y.append(random.random())
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plt.plot(X, Y, ".")
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return [(X[i], Y[i]) for i in range(n)]
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def cross_product(u, v):
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x1, y1 = u
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x2, y2 = v
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return x1 * y2 - x2 * y1
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def gift_wrapping(points):
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convex_hull = []
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x_sorted = sorted(points, key=lambda point: point[0])
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left_most = x_sorted[0]
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point_on_hull = left_most
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endpoint = None
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while endpoint != left_most:
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convex_hull.append(point_on_hull)
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endpoint = points[0]
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for point in points:
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cp = cross_product(
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(endpoint[0] - point_on_hull[0], endpoint[1] - point_on_hull[1]),
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(point[0] - point_on_hull[0], point[1] - point_on_hull[1]),
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)
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if endpoint == point_on_hull or cp < 0:
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endpoint = point
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point_on_hull = endpoint
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return convex_hull
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def draw_points(points, with_edge=False):
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X, Y = [], []
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for p in points:
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X.append(p[0])
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Y.append(p[1])
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plt.plot(X, Y, "." if not with_edge else "")
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pts = generate_points(50)
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print(pts)
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draw_points(pts)
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hull = gift_wrapping(pts)
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draw_points(hull + [hull[0]], True)
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plt.show()
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