[R] Voronoi-Diagrams in R

[Raphael Päbst]

Ok, now I'm scared. I copied the exact code from your post and got
length(tri) = 1481 and
dim(geodel) = 1577    3

You are right though, there seem to bee duplicates in my original bm,
but removing them still got diffeent results. I fear there's something
weird going on with my R installation.

Cheers

Raphael


On 5/23/14, Rolf Turner <r.turner at auckland.ac.nz> wrote:
>
> 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!
>
>