In the lecture we have defined what we mean by a discrete minimal surface. The goal of this tutorial is to visualize such minimal surfaces.

Let \(\mathrm M\) be a discrete surface with boundary and let \(V, E, F\) denote the set of vertices, (unoriented) edges and faces. Once we have a discrete metric, i.e. a map \(\ell \colon E \to (0,\infty)\) such that the triangle inequalities hold,\[\ell_{ij} \leq \ell_{ik}+ \ell_{kj}, \quad \ell_{jk}\leq \ell_{ji} + \ell_{ik},\quad \ell_{ki} \leq \ell_{kj}+ \ell_{ji},\quad \textit{ for each }\sigma = \{i,j,k\},\]we can define the corresponding discrete Dirichlet energy \(E_D^\ell\):\[E_D^\ell (f) = \sum_{ij\in \tilde E} \omega_{ij}^\ell\,(f_j-f_i)^2,\quad \omega^\ell_{ij} = \tfrac{1}{2}(\cot \alpha_{ij} + \cot\beta_{ij}).\]Here \(\tilde E\) denote the positively oriented edges. Certainly the angles and thus the cotangent weights depend on the metric \(\ell\).

In particular, each map \(f\colon \mathrm M \to \mathbb R^3\), which maps thecombinatoric triangles to a non-degenerate triangles in \(\mathbb R^3\), yields an induced discrete metric,\[\ell_{ij} = |p_j – p_i|.\]We have seen in the lecture that a surface is *minimal*, i.e. a critical point of the area functional under all variations fixing the boundary, if and only if it solves the Dirichlet problem (with respect to its induced metric): For each interior vertex \(i\in\mathring V\) we have\[-\tfrac{1}{2}\sum_{\{i,j\}\in E}\,\omega_{ij}^\ell f_i +\tfrac{1}{2}\sum_{\{i,j\}\in E}\,\omega_{ij}^\ell f_j = 0,\]where \(\ell\) denotes the metric induced by \(f\).

Unfortunately, unlike the Dirichlet problem, this is a non-linear problem – the cotangent weights depend on \(\ell\) in a complicated way. Though, fortunately, we can solve it in many cases by iteratively solving a Dirichlet problem (here a paper that describes this). Since we know how to solve the Dirichlet problem we can implement this method without any effort.

The network above visualizes Plateau’s problem: Given a space curve \(\gamma\) compute a minimal disk with boundary \(\gamma\). Actually this corresponds to a soap film spanned into a wire.

In the discrete setup we start with some discrete space, say generated by a *curve node* (probably followed by a *resample node*), which we then stick into a *remesh node* to get a reasonably triangulated disk spanned in. Then we can apply the iteration to this input. Below one possible result:

For rendering we used here the following texture in the *mantra surface* material. To generate texture coordinates one can use the nodes *uvunwrap* or *uvtexture.*

The algorithm is not restricted to disks. We can stick in any surface we want. Another nice example is e.g. obtained by starting with a cylinder of revolution. Here the minimal surface becomes a part of a catenoid or, if the cylinder’s height becomes to large, it degenerates to a pair of flat disks (connected by a straight line). In the picture below the last two circles are already too far apart – the surface starts to splits into two parts.

As a last example we can start with the unit sphere into which we cut several holes. If we want nice symmetries we could use a *platonic solid node* to copy smaller spheres to the vertex positions of e.g. an octahedron. The area enclosed by the smaller spheres we then subtract from the unit sphere with help of a *cookie node*. If we run then the algorithm on this input we end up with nice symmetric minimal surface. For a certain radius of the holes the surface is close to the Schwarz P surface.

Again one can observe the same effect as for the cylinder – if the holes get to small the minimal surface degenerates to several spheres connected to the origin.

**Homework (due 12/14 July).** *Build a network that computes from a given discrete surface with boundary a minimal surface with the same boundary. Use this network to solve Plateau’s problem and to compute symmetric minimal surface from a sphere with holes as described above.*