Weighted Triangulations

In this tutorial, we demonstrate how to construct a weighted Delaunay triangulation. For more information about weighted Delaunay triangulations, see the math details here. To summarise, weighted Delaunay triangulations associate with each vertex $p_i$ a scalar weight $w_i$. To pass weights into triangulate, use the weights keyword argument, which by default is ZeroWeight().

Let's first consider a simple example. We use a triangulation with random weights and compare it to the standard unweighted Delaunay triangulation.

using DelaunayTriangulation, CairoMakie
A, B, C, D, E, F, G, H,
I, J, K, L, M, N = (-1.0, 3.0), (1.0, 3.0),
(2.0, 2.0), (2.0, 0.0), (1.0, -1.0), (-1.0, -1.0),
(-2.0, 0.0), (-2.0, 2.0), (-1.0, 2.0),
(0.0, 1.5), (1.0, 2.5), (-1.0, 0.5),
(1.0, 0.0), (1.5, 1.0)
points = [A, B, C, D, E, F, G, H, I, J, K, L, M, N]
weights = randn(length(points))
tri = triangulate(points; weights)

Delaunay Triangulation.
   Number of vertices: 12
   Number of triangles: 14
   Number of edges: 25
   Has boundary nodes: false
   Has ghost triangles: true
   Curve-bounded: false
   Weighted: true
   Constrained: false

tri_unweighted = triangulate(points)

Delaunay Triangulation.
   Number of vertices: 14
   Number of triangles: 18
   Number of edges: 31
   Has boundary nodes: false
   Has ghost triangles: true
   Curve-bounded: false
   Weighted: false
   Constrained: false

fig = Figure()
ax = Axis(fig[1, 1], width = 400, height = 400, title = "Unweighted", titlealign = :left)
triplot!(ax, tri_unweighted, strokecolor = :black)
ax2 = Axis(fig[1, 2], width = 400, height = 400, title = "Weighted", titlealign = :left)
triplot!(ax2, tri, strokecolor = :red)
resize_to_layout!(fig)
fig

The triangles are of course not the same. Also, note that not all vertices appear in the weighted triangulation. To see the effect of the weights, let's reset all the weights to zero and see how the triangulation changes as we vary the weight of the point at $(x, y) = (0, 1.5)$, starting with a weight of $-10$.

w = Observable(-10.0)
weights = @lift (wts = zeros(length(points)); wts[10] = $w; wts)
tri = @lift triangulate(points; weights = $weights)
weight_itr_base = LinRange(-10, 10, 30*5)
weight_itr = vcat(weight_itr_base, reverse(weight_itr_base))
title_obs = lift(w -> L"w_{10} = %$(round(w, sigdigits = 4))", w)
fig, ax, sc = triplot(tri,
axis = (title = title_obs, titlealign = :left),
figure = (fontsize = 24,))
scatter!(ax, [J], color = :red, markersize = 13)
record(fig, "varying_weight.mp4", weight_itr; framerate = 30) do _w
    w[] = _w
end;

See that, once the weight gets so large, it essentially dominates the triangulation.

Just the code

An uncommented version of this example is given below. You can view the source code for this file here.

using DelaunayTriangulation, CairoMakie
A, B, C, D, E, F, G, H,
I, J, K, L, M, N = (-1.0, 3.0), (1.0, 3.0),
(2.0, 2.0), (2.0, 0.0), (1.0, -1.0), (-1.0, -1.0),
(-2.0, 0.0), (-2.0, 2.0), (-1.0, 2.0),
(0.0, 1.5), (1.0, 2.5), (-1.0, 0.5),
(1.0, 0.0), (1.5, 1.0)
points = [A, B, C, D, E, F, G, H, I, J, K, L, M, N]
weights = randn(length(points))
tri = triangulate(points; weights)

tri_unweighted = triangulate(points)

fig = Figure()
ax = Axis(fig[1, 1], width = 400, height = 400, title = "Unweighted", titlealign = :left)
triplot!(ax, tri_unweighted, strokecolor = :black)
ax2 = Axis(fig[1, 2], width = 400, height = 400, title = "Weighted", titlealign = :left)
triplot!(ax2, tri, strokecolor = :red)
resize_to_layout!(fig)
fig

w = Observable(-10.0)
weights = @lift (wts = zeros(length(points)); wts[10] = $w; wts)
tri = @lift triangulate(points; weights = $weights)
weight_itr_base = LinRange(-10, 10, 30*5)
weight_itr = vcat(weight_itr_base, reverse(weight_itr_base))
title_obs = lift(w -> L"w_{10} = %$(round(w, sigdigits = 4))", w)
fig, ax, sc = triplot(tri,
axis = (title = title_obs, titlealign = :left),
figure = (fontsize = 24,))
scatter!(ax, [J], color = :red, markersize = 13)
record(fig, "varying_weight.mp4", weight_itr; framerate = 30) do _w
    w[] = _w
end;

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