---
author: Stéphane Laurent
date: '2022-01-19'
highlighter: 'pandoc-solarized'
output:
  html_document:
    highlight: kate
    keep_md: no
  md_document:
    preserve_yaml: True
    variant: markdown
rbloggers: yes
tags: 'geometry, R, rgl, graphics, C, haskell'
title: Funny 3D Voronoï diagrams
---

Four years ago I wrapped the C library **Qhull** in a [Haskell
package](https://github.com/stla/qhull). Thanks to **Qhull**, this
package can do some convex hulls and some Delaunay tessellations. The
**Qhull** library also allows to do some Voronoï tessellations, but I
never understood how that works. So I programmed the Voronoï
tessellation in Haskell, by deriving it from the Delaunay tessellation.

Recently, I released the R package **tessellation**. It uses the C code
of my Haskell package to contruct Delaunay tessellations, and I
programmed the Voronoï tessellation in R.

I discovered something interesting with my Haskell package: take a
classical polyhedron (e.g. a tetrahedron, or a cube), enclose it with
two or three circles (I mean with some points on the circles), and do
the Voronoï diagram of all these points (the vertices of the polyhedron
and the points on the circles). Then you get a funny Voronoï diagram.
You can see some examples on [the website I made for the
**tessellation** package](https://stla.github.io/tessellation/) (thanks
to the **pkgdown** package).

Let's make another example here. This time, I take a cuboctahedron and I
enclose it with three circles:

``` {.r}
library(rgl) # provides `cuboctahedron3d()`
cuboctahedron_vertices <- t(cuboctahedron3d()$vb[-4L, ])
xi_ <- seq(0, 2*pi, length.out = 91)[-1L]
R <- 1.5
circle1 <- t(vapply(xi_, function(xi) R*c(cos(xi), sin(xi), 0), numeric(3L)))
circle2 <- t(vapply(xi_, function(xi) R*c(cos(xi), 0, sin(xi)), numeric(3L)))
circle3 <- t(vapply(xi_, function(xi) R*c(0, cos(xi), sin(xi)), numeric(3L)))
enclosedCuboctahedron <- 
  rbind(cuboctahedron_vertices, circle1, circle2, circle3)
```

Here is the picture:

``` {.r}
open3d(windowRect = c(50, 50, 562, 562))
view3d(20, zoom = 0.65)
shade3d(cuboctahedron3d(), color = "darkorange", shininess = 10)
wire3d(cuboctahedron3d(), color = "darkslategray4", lwd = 4)
spheres3d(rbind(circle1, circle2, circle3), radius = 0.04)
```

![](figures/enclosedCuboctahedron.png)

Now let's make the Voronoï diagram with the **tessellation** package:

``` {.r}
library(tessellation)
del <- delaunay(enclosedCuboctahedron, degenerate = TRUE)
v <- voronoi(del)
## Voronoï diagram with twelve bounded cells.
```

I have not checked whether the option `degenerate = TRUE` is necessary,
but it is often necessary in order to get a correct Voronoï diagram, and
I never understood why. Now let's plot the Voronoï diagram:

``` {.r}
library(paletteer) # provides many color palettes
open3d(windowRect = c(50, 50, 562, 562))
bg3d("palegoldenrod")
plotVoronoiDiagram(v, colors = paletteer_c("grDevices::Dark 3", 12L))
```

I animated it. Here is the result:

![](figures/voronoi_enclosed_cuboctahedron.gif)

This is not my favorite one (I prefer the one derived from the cube,
that you can see on the **tessellation** website), but nevertheless it
is nice.