# Fortune's Voronoi algorithm implemented in C#

, 21 Apr 2013 MPL
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A C# implementation of the Fortune algorithm to compute 2D Voronoi graphs.

## Introduction

Given a set of two dimensional vectors (or data points), a Voronoi graph is a separation of those points into compartments where all points inside one compartment are closer to the contained data point than to any other data point. I won't give any demonstration here, but if you want to know more about Voronoi graphs, check out this.

The applications of Voronoi graphs are quite broad. Very useful for a lot of optimization problems (in most cases, the Delaunay Triangulation which can be easily derived from a Vononoi graph is used there), it ranges to computing topological maps from bitmaps.

[This is an article for freaks. After a rather painful experience writing the thing I hope it will benefit everyone who is looking for this algorithm in a civilized language (or simply does not want to use Fortune's original C implementation).]

In 1987, Steve Fortune described an algorithm to compute such a graph by using a sweep line in combination with a binary tree. A PowerPoint explanation of the algorithm (the one I used to implement it) can be found here. Note that I did not use the linked data structure to represent a graph - I think that is an unnecessary difficulty in the age of `ArrayList`s and `HashSet`s.

## The Implementation

Data points are represented by my own `Vector` class. It can do much more than needed here (but there was no reason to strip it before bringing it) but I won't explain it here. The most important fact is that although working with `double`s the Vector class automatically rounds values to 10 digits (or whatever is set in the `Vector.Precision` field). Yes, sadly, this is very important if you want to sort of compare `double`s.

A `VoronoiGraph` is a class that only contains a `HashSet` of vertices (as 2D vectors) and a `HashSet` of `VoronoiEdge`s - each with references to the left and right data point and (of course) the two vertices that bound the edge. If the edge is (partially or completely) unbounded, the vector `Fortune.VVUnknown` is used.

`BinaryPriorityQueue` is used for the sweep line event queue.

## Usage

The algorithm itself (`Fortune.ComputeVoronoiGraph(IEnumerable)`) takes any `IEnumerable` containing only two dimensional vectors. It will return a `VoronoiGraph`. The algorithm's complexity is O(n ld(n)) with a factor of about 10 microseconds on my machine (2GHz).

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Software Developer (Senior)
Germany
I did my diploma in Dresden and Sydney where I dealt with algorithms, agents and other cool AI stuff. Now I moved to Frankfurt to work on my PhD dealing with software structures for artificial intelligence systems. If I can, I do things in C# and ASP.NET, but if I have to, my C++, Java and SQL are not that bad.
Long Live .NET.

 Re: My vote of 5 Member 8683705 14-May-12 22:38
 Re: My vote of 5 maurice.calvert 15-May-12 5:56
 Voronoi Cells C-Bl 28-Jul-09 0:08
 3 data points case SOAD_ 21-Jun-09 7:08
 Re: 3 data points case rhill-ca 26-Jun-09 7:45
 Another wrong vertex Sunil Terkar 11-May-09 3:04
 Ben,   For following another set of points, algorithum misses the vertex by almost 250 mt.   NODE_ID EASTING NORTHING 6 10494.00016 10092.53629 7 10290.7531 10684.97519 8 9947.773702 11043.95948 9 9776.284008 11059.26798 10 10991.95541 10217.30055 11 10057.01902 10092.5363   It gives 8 edges and 3 vertex, but due to wrong location of vertex 3, egde no 6,7 and 8 have not come out properly. Actual location of vertex 3 should be 9819.40,10578.40. I strongly feel that it should have 4 vertexes.   Could you please look into it. Thanks, Sunil Terkar
 Re: Another wrong vertex BenDi 11-May-09 11:03
 Re: Another wrong vertex Sunil Terkar 12-May-09 1:37
 Re: Another wrong vertex Sunil Terkar 12-May-09 6:09
 Re: Another wrong vertex rhill-ca 26-Jun-09 7:34
 Short of one edge and one vertex Sunil Terkar 6-May-09 0:11
 Re: Short of one edge and one vertex rhill-ca 26-Jun-09 7:31
 medial axis led 16-Apr-09 10:38
 Thanks! Reconstructivism 30-Mar-09 3:48
 Rays Wayne Romer 22-Mar-09 21:55
 Re: Rays Reconstructivism 30-Mar-09 3:56
 Re: Rays BenDi 30-Mar-09 9:57
 Re: Rays Pavel Vladov 29-Apr-09 1:54
 Re: Rays BenDi 29-Apr-09 10:49
 Re: Rays Pavel Vladov 30-Apr-09 0:39
 Re: Rays BenDi 30-Apr-09 9:04
 Re: Rays Pavel Vladov 30-Apr-09 23:36
 Re: Rays Slagh 14-May-09 23:10
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