# Deldir

deldir is an R package for computing Delaunay triangulations and Voronoi/Dirichlet tesselations. This Julia package wraps the Fortran code from deldir.

## Installation

Switch to `Pkg`

mode in Julia with `]`

and run

add Deldir

## Usage

The coordinates of the generators are specified as two vectors that are fed to `deldir`

, the main function of this package:

x = rand(8)
y = rand(8)
del, vor, summ = deldir(x, y)

The output from `deldir`

are three DataFrames:
One for the topology of the Delaunay triangulation; one for topology of the Voronoi tesselation; one with a summary mainly related to the area of the triangles and Voronoi cells.

By default, `deldir`

works with points in the unit rectangle, but other bounding rectangles can be specified as a third argument.

The area of the Voronoi cells are also available directly with the function `voronoiarea`

.

Two functions are available to extract the edges of the Delaunay triangles and Voronoi cells in a "plot friendly" manner:

Dx, Dy = edges(del)
Vx, Vy = edges(vor)

Using the results from above this can be plotted using e.g. the Plots package:

using Plots
scatter(x, y, xlim = (0,1), ylim = (0,1), markersize = 6, label = "generators")
plot!(Dx, Dy, label = "Delaunay")
plot!(Vx, Vy, style = :dash, label = "Voronoi")

One realization looks like the following.

# Details

There are other Julia package for interacting with Voronoi cells and Delaunay triangulations. The deldir R package has been around for a long time and therefore it is my hope that the Fortran code give correct results. Furthermore, deldir has two qualities I value:

- It interacts well with the bounding box.
- It returns the area of the Voronoi cells
*in the same order as the input generators*.

I am also the author of the pure Julia package VoronoiCells with similar functionalities.
The *VoronoiCells* package executes *much* faster.
Consider the time taken to run the `voronoiarea`

functions of both packages with an increasing number of points:

The script generating this output is available in the `examples`

folder.
The comparison plot is made with

julia> versioninfo()
Julia Version 1.5.0
Commit 96786e22cc (2020-08-01 23:44 UTC)
Platform Info:
OS: Linux (x86_64-pc-linux-gnu)
CPU: Intel(R) Core(TM) i5-8265U CPU @ 1.60GHz
WORD_SIZE: 64
LIBM: libopenlibm
LLVM: libLLVM-9.0.1 (ORCJIT, skylake)
(@v1.5) pkg> status
Status `~/.julia/environments/v1.5/Project.toml`
[64db5801] Deldir v1.3.1
[e3e34ffb] VoronoiCells v0.2.0

## Compiled code

To make it easier to install *Deldir* the Fortran code is cross-compiled using the BinaryBuilder package and hosted in the Julia Packaging organization.

I am not using the latest version of the Fortran code, because there is a tighter coupling with R from version 0.1-16 and onwards.
In particular, the use of the subroutines `fexit`

and `rexit`

from R cause problems even when linking against R's libraries.

## Limitations

Not all features of the R package are available. I have e.g. chosen to ignore options regarding dummy points.

Are you missing anything important? Check out the manual for the R package to see if the Fortran library supports it.

# Acknowledgement

Rolf Turner is author of the deldir package for R as well as all Fortran code used in this package.