[R] Voronoi-Diagrams in R

[Rolf Turner]

I just tried the following:

require(deldir)
require(geometry)
set.seed(42)
bm <- data.frame(x=sample(1:82,800,TRUE),y=sample(1:82,800,TRUE))
del <- deldir(bm)
tri <- triang.list(del)
geodel <- delaunayn(bm)
length(tri) # Got [1] 1481
dim(geodel) # Got [1] 1481    3

So all seems to be in harmony in the universe.

Something is weird about your "bm"; hard to say what without seeing it.

One thought:  Are there any duplicated points in your "bm"?  (Upon 
re-reading your message and noting the problem with "ptNum", I think 
this might be the case.)

Try doing something like:

newbm <- bm[!duplicated(bm),]

and then hit "newbm" with both deldir() and delaunayn().

cheers,

Rolf Turner

On 23/05/14 03:36, Raphael Päbst wrote:
> Hello again,
> I have found further depths of confusion concerning delaunay
> triangulations to explore.
>
> Here is the code I'm using to create the confusing results:
>
> bm <- getbm(x) # a data.frame with 2 columns and 800 rows, values are
> integers ranging from 1 to 82
>
> del <- deldir(bm) # creating an object of type deldir with the 800
> coordinates from bm
> tri <- triang.list(del) # creating a list of length n, holding the n
> triangles created by the delaunay triangulation with deldir(bm) as
> data.frames
>
> geodel <- delaunayn(bm) # creating a matrix with 3 columns and m rows
> for the m triangles created by the delaunay triangulation using
> delaunayn(bm) from geometry-package
>
> now, the following is what I would expect: n and m should be the same,
> since both triangulations should give me the same number of triangles,
> right?
>
> but this is what I get:
>
> nrow(geodel) = 1584
> length(tri) = 1186
>
> This confuses me and I have the feeling I have left out some very
> important parameter either in delaunayn() or deldir() to create the
> different results.
> There is however more confusion to come, this time in the result of
> triang.list(del):
>
> Every element in that list is a data.frame holding information on one
> triangle. It has 3 columns and 3 rows with column headers 'ptNum', 'x'
> and 'y'. Now I would expect, that 'ptNum' would give me the index at
> which I can find the coordinates 'x' and 'y' as a row in my original
> data.frame of coordinates, bm. And for example
>
> bm[1,] gives me the same 'x' and 'y' that are listed in data.frame
> tri[1] as 'ptNum' = 1.
> But for some other points this does not work and for example the 'x'
> and 'y' of 'ptNum' = 129 do not match bm[129, ]. Have I totally
> misunderstood the meaning of 'ptNum' in this case or does my mistake
> lie somewhere else?
>
> Once again please excuse my slightly chaotic description of my
> problems and my faulty english and thanks for any help you can give!