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Statistical Distributions of Areas of Voronoi Cells

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POSTED BY: Vitaliy Kaurov
10 Replies
POSTED BY: Szabolcs Horvát
Posted 5 years ago
POSTED BY: BJ Miller
Posted 5 years ago

Thanks for sharing everybody! Maybe this can add some more information:

In the article "Statistical Distributions of Poisson Voronoi Cells in Two and Three Dimensions" by Masaharu Tanemura, mentioned by Marco Thiel, the distribution of the perimeter of the cells (and the number of edges etc.) is also analyzed. Here I adapted the code of Vitaly to perimeters and we see (as in the article) that now we have a normal distribution:

perimeters = Perimeter /@ vorInner;
hist = Histogram[perimeters, Automatic, "PDF", 
   PlotTheme -> "Detailed"];
dist = FindDistribution[perimeters]
Show[hist, Plot[PDF[dist, x], {x, 0, .1}]]

enter image description here

And for the number of edges, we find a BinomialDistribution with FindDistribution although the result look almost "normal" as we use EstimatedDitribution

numberOfEdges[poly_] := Length[First@poly]
edges = numberOfEdges /@ vorInner;
histE = Histogram[edges, Automatic, "PDF", PlotTheme -> "Detailed"];
distE = EstimatedDistribution[edges, 
  NormalDistribution[\[Mu], \[Sigma]]]
Show[histE, Plot[PDF[distE, x], {x, 0, 12}]]
distEB = FindDistribution[edges]
Plot[CDF[distEB, x], {x, 0, 12}, Filling -> Bottom]

enter image description here

POSTED BY: Erik Mahieu

Great, @Erik, thanks for exploring further and sharing!

POSTED BY: Vitaliy Kaurov

Dear All,

that is indeed interesting. I also believe that the exact distribution is unknown but it can be approximated by a GammaDistribution, see:

https://arxiv.org/pdf/1207.0608.pdf

https://arxiv.org/pdf/1612.02375.pdf

https://www.sciencedirect.com/science/article/pii/002608008990030X

http://www.scipress.org/journals/forma/pdf/1804/18040221.pdf

http://www.eurecom.fr/~arvanita/PVT.pdf

Interestingly, if you run the code in @Vitaliy Kaurov's cod for 50000 points you obtain:

dis = FindDistribution[areas]
(*GammaDistribution[3.2718, 0.0000197063]*)

I managed to get it done for 500000 points. I think that the edge effects should be getting smaller; as the area increases faster than the circumference.

So I obtain:

dis = FindDistribution[areas, 5]

(*
{MixtureDistribution[{0.658342, 0.341658}, {MaxwellDistribution[2.92588*10^-6], GammaDistribution[7.1867, 1.37091*10^-6]}], 

ExtremeValueDistribution[4.80446*10^-6, 2.73831*10^-6], 

GammaDistribution[2.94963, 2.17794*10^-6], 

BetaDistribution[2.94962, 459143.], 

MixtureDistribution[{0.687855, 0.312145}, {MaxwellDistribution[2.82857*10^-6], 
   LogNormalDistribution[-11.5031, 0.28619]}]}*)

This happens when we plot all of them:

Plot[Evaluate[PDF[#, x] & /@ dis], {x, 0, 0.00003}, PlotRange -> All, LabelStyle -> Directive[Bold, 16]]

enter image description here

If you compare that with the histogram:

Show[Histogram[areas, 200, "PDF"], Plot[Evaluate[PDF[#, x] & /@ dis], {x, 0, 0.00003}, PlotRange -> All,LabelStyle -> Directive[Bold, 16]]]

enter image description here

I would say that the second (ExtremeValueDistribution) fits best:

enter image description here

but it does not get the small areas quite right.

Regarding the boundary effects, one might also (similar to what Sander suggests) use the torus as the geometry:

disfun[x_, y_] := Sqrt[Min[Abs[x[[1]] - y[[1]]], 1 - Abs[x[[1]] - y[[1]]]]^2 + Min[Abs[x[[2]] - y[[2]]], 1 - Abs[x[[2]] - y[[2]]]]^2]

M = 50; nf = Nearest[Rule @@@ Transpose[{RandomReal[1, {M, 2}], Range[M]}], DistanceFunction -> disfun]

DensityPlot[First[nf[{x, y}]], {x, 0, 1}, {y, 0, 1}, PlotPoints -> 100, ColorFunction -> "TemperatureMap"]

enter image description here

Cheers,

Marco

POSTED BY: Marco Thiel

@Marco Thiel thank you for the references and insight! I believe FindDistribution, while finding correct distributions, needs some tune up to get the distribution parameters right. To test the waters, I tried using EstimatedDistribution and got a bit better results. Let's run this for 500,000 points:

pts=RandomPoint[Disk[],500000];
mesh=VoronoiMesh[pts,PerformanceGoal->"Speed"];
vor=MeshPrimitives[mesh,2]
vorInner=Select[vor,RegionWithin[Disk[],#]&];
areas=Area/@vorInner;
hist=Histogram[areas,200,"PDF",PlotTheme->"Detailed"];

and then find the parameters assuming distributions are known:

disGAM = EstimatedDistribution[areas, GammaDistribution[a, b]]

GammaDistribution[3.526166220777205, 1.7809806247800413*^-6]

disEXT = EstimatedDistribution[areas, ExtremeValueDistribution[a, b]]

ExtremeValueDistribution[4.775094275466527*^-6, 2.5746412541824736*^-6]

We see gamma (red) behaves a bit better now, both for zero and for the tail values:

Show[hist,Plot[{PDF[disEXT,x],PDF[disGAM,x]},{x,0,.00002},PlotStyle->{Blue,Red}]]

enter image description here

But while p-value of gamma is greater than extreme, it is still pretty low, not sure if there is a catch here somewhere:

DistributionFitTest[areas,disGAM]

0.000051399258187534436`

DistributionFitTest[areas,disEXT]

0.`

POSTED BY: Vitaliy Kaurov

I don't find it all that surprising that the result is a Gamma distribution. The 1-d case is well known as it is the distribution of interarrival times of a Poisson process and is exponentially distributed. The exponential distribution is a special case of the Gamma so maybe you're onto something.

POSTED BY: Carlo Barbieri

Very nice. Thanks for sharing. Some years ago I did this with 10^9 cells. If I recall correctly the left side is a bit higher (more probably) than a gamma distribution.

To remove any edge-effects, what people do is: take random points in a square, and then copy the square 9 times in a 3*3 grid, such as to emulate period boundary conditions. Then take only the polygons which center is in the center unit square…

Great stuff!

–SH

POSTED BY: Sander Huisman

Thanks! Awesome idea about edge-effects! Did you estimate any analytical models beyond Gamma or it was just pure empirical thing?

POSTED BY: Vitaliy Kaurov

I looked at different things: how many sides, angles, area, perimeter, and many conditionally averaged quantities. But area of the PDF as well. But no analytical shape is known for it. I believe that there is something known for perimeter.

POSTED BY: Sander Huisman
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