# Surface Centers

These routines compute geometric centers of point sets and distrubtions on surfaces.

A “center” c is a point on the surface which is a local minimum of the energy: $$ c = \underset{m}{\textrm{argmin}}\sum_{y \in \mathcal{Y}} d(m,y)^p $$ where \mathcal{Y} is a set of sites to take the average of and d(\cdot, \cdot) denotes the geodesic distance.

Centers on surfaces with p=2 (“means”) are known as *Karcher Means* or *Frechet Means*. Centers on surfaces with p=1 (“medians”) are known as *geometric medians*.

Centers are not unique!

In general, there will not be a single unqiue “center” of a point set or distribution on a surface. For nearby sites there may be a single center, but in general this may not be the case.

The routines in this section use random initialization to report *a center*. As such, the results may be different under repeated runs of the procedure.

`#include "geometrycentral/surface/surface_centers.h"`

## Centers of points

`SurfacePoint findCenter(IntrinsicGeometryInterface& geom, const std::vector<Vertex>& vertexPts, int p = 2)`

Find a center of a collection of points at vertices.

`p`

must be either 1 or 2.

`SurfacePoint findCenter(IntrinsicGeometryInterface& geom, VectorHeatMethodSolver& solver, const std::vector<Vertex>& vertexPts, int p = 2)`

Like the above method, but uses an existing solver object, which saves precomputation.

## Centers of distributions

`SurfacePoint findCenter(IntrinsicGeometryInterface& geom, const VertexData<double>& distribution, int p = 2)`

Find a center of a distribution at vertices.

Note that the input `distribution`

is treated as integrated values at vertices. If your distribution is “per-unit-area”, you should multiply times vertex area before passing it in.

`p`

must be either 1 or 2.

`SurfacePoint findCenter(IntrinsicGeometryInterface& geom, VectorHeatMethodSolver& solver, const VertexData<double>& distribution, int p = 2)`

Like the above method, but uses an existing solver object, which saves precomputation.

## Citation

These algorithms are described in The Vector Heat Method, the appropriate citation is:

@article{sharp2019vector, title={The Vector Heat Method}, author={Sharp, Nicholas and Soliman, Yousuf and Crane, Keenan}, journal={ACM Transactions on Graphics (TOG)}, volume={38}, number={3}, pages={24}, year={2019}, publisher={ACM} }