# Integration over a polyhedron with Julia

Posted on December 3, 2022 by Stéphane Laurent
Tags: julia

The Julia function integrateOnSimplex provided in this gist allows to compute the integral of a function over a simplex or more generally a union of simplices (a simplex is a triangle in dimension 2, a tetrahedron in dimension 3). I will package it soon, under the name SimplicialCubature.

The Delaunay tessellation of a convex polyhedron in $$\mathbb{R}^n$$ provides a partition of this convex polyhedron into $$n$$-dimensional simplices. Therefore, once you have a way to integrate a function over a simplex, you are able to integrate over a convex polyhedron.

Suppose for example you have to evaluate the integral $\int_{-5}^4\int_{-5}^{3-x}\int_{-10}^{6-x-y} f(x,y,z) \,\mathrm{d}z\,\mathrm{d}y\,\mathrm{d}x$ for a certain function $$f$$.

The domain of integration is defined by the set of inequalities: $\left\{\begin{matrix} -5 & \leq & x & \leq & 4 \\ -5 & \leq & y & \leq & 3-x \\ -10 & \leq & z & \leq & 6-x-y \end{matrix} \right.$ which is equivalent to $\left\{\begin{matrix} -x & \leq & 5 \\ x & \leq & 4 \\ -y & \leq & 5 \\ x+y & \leq & 3 \\ -z & \leq & 10 \\ x+y+z & \leq & 6 \end{matrix} \right..$ This set of inequalities defines a convex polyhedron. We can get the vertices of this polyhedron with the Polyhedra package:

# define the polyhedron Ax <= b and get its vertices
using Polyhedra
A = [
-1  0  0;   # -x
1  0  0;   # x
0 -1  0;   # -y
1  1  0;   # x + y
0  0 -1;   # -z
1  1  1.0  # x + y + z
]
b = [5; 4; 5; 3; 10; 6.0]
H = hrep(A, b)
PH = polyhedron(H)
vertices = collect(points(PH))
## 8-element Vector{Vector{Float64}}
##  [4.0, -1.0, 3.0]
##  [-5.0, 8.0, 3.0]
##  [4.0, -1.0, -10.0]
##  [-5.0, 8.0, -10.0]
##  [4.0, -5.0, 7.0]
##  [-5.0, -5.0, 16.0]
##  [4.0, -5.0, -10.0]
##  [-5.0, -5.0, -10.0]

Then, to evaluate the integral, we proceed as follows:

• split the polyhedron into simplices (tetrahedra) with the Delaunay algorithm, implemented in the MiniQhull package;

• evaluate the integral over the union of these simplices with the integrateOnSimplex function.

Let’s go. We split the polyhedron into tetrahedra:

# decompose the polyhedron into simplices (tetrahedra)
using MiniQhull # to use delaunay()
indices = delaunay(hcat(vertices...))
_, ntetrahedra = size(indices)
tetrahedra = Vector{Vector{Vector{Float64}}}(undef, ntetrahedra)
for j in 1:ntetrahedra
ids = vec(indices[:, j])
tetrahedra[j] = vertices[ids]
end

Now we are ready to use the integrateOnSimplex function. We take as example the function $$f(x, y, z) = x + yz$$.

# function to be integrated
function f(x)
return x[1] + x[2]*x[3]
end
# integral
I = integrateOnSimplex(f, tetrahedra)
I.integral
## -5358.300000000004

Note that for this example we can do better. The function $$f$$ is polynomial. So we can get the exact value of its integral over a simplex; see the previous post.