qhull_2d.py

#!/usr/bin/python

# Compute the convex hull of a set of 2D points
# A Python implementation of the qhull algorithm
# 
# Tested with Python 2.6.5 on Ubuntu 10.04.4

# Copyright (c) 2008 Dave (www.literateprograms.org)
#
# Permission is hereby granted, free of charge, to any person obtaining a copy 
# of this software and associated documentation files (the "Software"), to deal 
# in the Software without restriction, including without limitation the rights 
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell 
# copies of the Software, and to permit persons to whom the Software is 
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in 
# all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR 
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, 
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE 
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# IN THE SOFTWARE.

from __future__ import division
from numpy import *

link = lambda a,b: concatenate((a,b[1:]))
edge = lambda a,b: concatenate(([a],[b]))

def qhull2D(sample):
    def dome(sample,base): 
        h, t = base
        dists = dot(sample-h, dot(((0,-1),(1,0)),(t-h)))
        outer = repeat(sample, dists>0, 0)
        if len(outer):
            pivot = sample[argmax(dists)]
            return link(dome(outer, edge(h, pivot)),
                    dome(outer, edge(pivot, t)))
        else:
            return base
    if len(sample) > 2:
    	axis = sample[:,0]
    	base = take(sample, [argmin(axis), argmax(axis)], 0)
        return link(dome(sample, base), dome(sample, base[::-1]))
    else:
	return sample