python - Getting a bounded polygon coordinates from Voronoi cells

Question

I have points (e.g., lat, lon pairs of cell tower locations) and I need to get the polygon of the Voronoi cells they form.

from scipy.spatial import Voronoi

tower = [[ 24.686 ,  46.7081],
       [ 24.686 ,  46.7081],
       [ 24.686 ,  46.7081]]

c = Voronoi(towers)

Now, I need to get the polygon boundaries in lat,lon coordinates for each cell (and what was the centroid this polygon surrounds). I need this Voronoi to be bounded as well. Meaning that the boundaries don't go to infinity, but rather within a bounding box.

Answer 1

Given a rectangular bounding box, my first idea was to define a kind of intersection operation between this bounding box and the Vorono? diagram produce by scipy.spatial.Voronoi. An idea not necessarily great, since this requires to code a large number of basic functions of computational geometry.

However, here is the second idea (hack?) that came to my mind: the algorithms to compute the Vorono? diagram of a set of n points in the plane have a time complexity of O(n ln(n)). What about adding points to constraint the Vorono? cells of the initial points to lie in the bounding box?

Solution for a bounded Vorono? diagram

A picture is worth a great speech:

What I did here? That's pretty simple! The initial points (in blue) lie in [0.0, 1.0] x [0.0, 1.0]. Then I get the points (in blue) on the left (i.e. [-1.0, 0.0] x [0.0, 1.0]) by a reflection symmetry according to x = 0.0 (left edge of the bounding box). With reflection symmetries according to x = 1.0, y = 0.0 and y = 1.0 (other edges of the bounding box), I get all the points (in blue) I need to do the job.

Then I run scipy.spatial.Voronoi. The previous image depicts the resulting Vorono? diagram (I use scipy.spatial.voronoi_plot_2d).

What to do next? Just filter points, edges or faces according to the bounding box. And get the centroid of each face according to the well-known formula to compute centroid of polygon. Here is an image of the result (centroids are in red):

Some fun before showing code

Great! It seems to work. What if after one iteration I try to re-run the algorithm on the centroids (in red) rather than the initial points (in blue)? What if I try it again and again?

Step 2

Step 10

Step 25

Cool! Vorono? cells tend to minimize their energy...

Here is the code

import matplotlib.pyplot as pl
import numpy as np
import scipy as sp
import scipy.spatial
import sys

eps = sys.float_info.epsilon

n_towers = 100
towers = np.random.rand(n_towers, 2)
bounding_box = np.array([0., 1., 0., 1.]) # [x_min, x_max, y_min, y_max]

def in_box(towers, bounding_box):
    return np.logical_and(np.logical_and(bounding_box[0] <= towers[:, 0],
                                         towers[:, 0] <= bounding_box[1]),
                          np.logical_and(bounding_box[2] <= towers[:, 1],
                                         towers[:, 1] <= bounding_box[3]))


def voronoi(towers, bounding_box):
    # Select towers inside the bounding box
    i = in_box(towers, bounding_box)
    # Mirror points
    points_center = towers[i, :]
    points_left = np.copy(points_center)
    points_left[:, 0] = bounding_box[0] - (points_left[:, 0] - bounding_box[0])
    points_right = np.copy(points_center)
    points_right[:, 0] = bounding_box[1] + (bounding_box[1] - points_right[:, 0])
    points_down = np.copy(points_center)
    points_down[:, 1] = bounding_box[2] - (points_down[:, 1] - bounding_box[2])
    points_up = np.copy(points_center)
    points_up[:, 1] = bounding_box[3] + (bounding_box[3] - points_up[:, 1])
    points = np.append(points_center,
                       np.append(np.append(points_left,
                                           points_right,
                                           axis=0),
                                 np.append(points_down,
                                           points_up,
                                           axis=0),
                                 axis=0),
                       axis=0)
    # Compute Voronoi
    vor = sp.spatial.Voronoi(points)
    # Filter regions
    regions = []
    for region in vor.regions:
        flag = True
        for index in region:
            if index == -1:
                flag = False
                break
            else:
                x = vor.vertices[index, 0]
                y = vor.vertices[index, 1]
                if not(bounding_box[0] - eps <= x and x <= bounding_box[1] + eps and
                       bounding_box[2] - eps <= y and y <= bounding_box[3] + eps):
                    flag = False
                    break
        if region != [] and flag:
            regions.append(region)
    vor.filtered_points = points_center
    vor.filtered_regions = regions
    return vor

def centroid_region(vertices):
    # Polygon's signed area
    A = 0
    # Centroid's x
    C_x = 0
    # Centroid's y
    C_y = 0
    for i in range(0, len(vertices) - 1):
        s = (vertices[i, 0] * vertices[i + 1, 1] - vertices[i + 1, 0] * vertices[i, 1])
        A = A + s
        C_x = C_x + (vertices[i, 0] + vertices[i + 1, 0]) * s
        C_y = C_y + (vertices[i, 1] + vertices[i + 1, 1]) * s
    A = 0.5 * A
    C_x = (1.0 / (6.0 * A)) * C_x
    C_y = (1.0 / (6.0 * A)) * C_y
    return np.array([[C_x, C_y]])

vor = voronoi(towers, bounding_box)

fig = pl.figure()
ax = fig.gca()
# Plot initial points
ax.plot(vor.filtered_points[:, 0], vor.filtered_points[:, 1], 'b.')
# Plot ridges points
for region in vor.filtered_regions:
    vertices = vor.vertices[region, :]
    ax.plot(vertices[:, 0], vertices[:, 1], 'go')
# Plot ridges
for region in vor.filtered_regions:
    vertices = vor.vertices[region + [region[0]], :]
    ax.plot(vertices[:, 0], vertices[:, 1], 'k-')
# Compute and plot centroids
centroids = []
for region in vor.filtered_regions:
    vertices = vor.vertices[region + [region[0]], :]
    centroid = centroid_region(vertices)
    centroids.append(list(centroid[0, :]))
    ax.plot(centroid[:, 0], centroid[:, 1], 'r.')

ax.set_xlim([-0.1, 1.1])
ax.set_ylim([-0.1, 1.1])
pl.savefig("bounded_voronoi.png")

sp.spatial.voronoi_plot_2d(vor)
pl.savefig("voronoi.png")