User:MWinter4/Wachspress coordinates

In geometric modeling, Wachspress coordinates form a system of generalized barycentric coordinates (GBCs) on convex polytopes. For a convex polytope $P\subset {\mathbb {R}}^{d}$ with vertices $v_{1},...,v_{n}$ and a point $x\in P$, the Wachspress coordinates provide a canonical choice for convex coefficients $\alpha _{1}(x),...,\alpha _{n}(x)\geq 0$ for $x$, that is,

$\sum _{i}\alpha _{i}(x)=1\;$(normalization)$\;\;$ and $\;\;\sum _{i}\alpha _{i}(x)v_{i}=x\;$(linear precision).

Wachspress coordinates were initially introduced by Eugene Wachspress on polygons in dimension two, and later generalized to polytopes of higher dimension and general combinatorics by Joe Warren.

Wachspress coordinates have a number of properties not shared by most other GBCs. They are of particular interest for theoretical considerations since their existence is a strong statement about the geometry of convex polytopes.

Wachspress coordinates are rational coordinates, which makes them objects of intrinsic algebro-geometry interest. At the same time they can be defined in terms of convex geometry, spectral graph theory or rigidity theory and also emerge in mathematical physics and algebraic statistics. Their ubiquity makes them a source for surprising interactions between these domains.

Wachspress coordinates are rational coordinates, that is, each coordinate is given as a rational function over the polytope:

$\alpha _{i}(x)={\frac {p_{i}(x)}{q(x)}},$

where the $p_{i}$ and $q$ are polynomials and $\textstyle q(x)=\sum _{i}p_{i}(x)$ is required for normalization. Wachspress showed that generalized barycentric coordinates can in general not be polynomials, and so Wachspress coordinates are in a sense as simple as possible. In fact, Warren showed that they are the unique rational generalized barycentric coordinates of lowest possible degree. The degree of $p_{i}$ is exactly $m-d$, where $m$ is the number of facets of the polytope, and $d$ is its dimension. The degree of $q$ is $m-d-1$.

Wachspress coordinates are affine invariant, which is best seen from their definition via relative cone volumes.

Rational generalized barycentric coordinates

Wachspress coordinates are rational functions: there are polynomials $p_{i},q\in {\mathbb {R}}[X_{1},...,X_{d}]$ so that

$\alpha _{i}(x)={\frac {p_{i}(x)}{q(x)}}.$

The polynomial $\textstyle q=\sum _{i}p_{i}$ guarantees normalization. It is also known as the adjoint polynomial of the polytope and plays a significant role in the study of positive geometries.

The degree of the Wachspress coordiantes, that is, the degree of the $p_{i}$, is precisely $m-d$, where $m$ is the number of facets of $P$ and $d$ is the dimension of the polytope. It was shown by Warren (199?) that this is the lowest possible degree for GBCs on a polytope and that the Wachspress coordinates are the unique GBCs of this degree.

Applications

• Positive geometry
• Algebraic statistics
• Finite element basis
• ...

Properties

• ...

Construction via cone volumes

Assume that $P$ contains the origin in its interior. To compute the Wachspress coordinates of the origin in the polytope let $P^{\circ }$ be its polar dual. For a vertex $v_{i}$ in $P$, let $F_{i}$ be the facet of $P^{\circ }$ dual to $v_{i}$, and $C_{i}$ the cone over $F_{i}$ with apex at $x$. The Wachspress coordinate $\alpha _{i}(0)$ of the origin is the volume of this cone relative to the volume of the polar dual:

$\alpha _{i}(0)={\frac {\mathrm {vol} (C_{i})}{\mathrm {vol} (P^{\circ })}}={\frac {\mathrm {vol} _{d-1}(F_{i})}{||v_{i}||\cdot \mathrm {vol} (P^{\circ })}}.$

The cone volumes clearly add up to the volume of $P^{\circ }$ and so $\alpha _{1}(0)+\cdots +\alpha _{n}(0)=1$. To compute the Wachspress coordinates for any other interior point $x$ of the polytope, perform the above computation for the translate $P-x$. Since relative volumes are affinely invariant, the Wachspress coordinates too are affinely invariant (i.e. they do not change if the polytope and the point are transformed by the same affine transformation).

Relation to Colin de Verdière matrices

Suppose that $P$ contains the origin in its interior. For a vector $\mathbf {c} =(c_{1},...,c_{n})\in {\mathbb {R}}^{n}$ the generalized polar dual is

$P^{\circ }(\mathbf {c} )=\{x\in \mathbb {R} ^{d}\mid \langle x,v_{i}\rangle \leq c_{i}{\text{ for all }}i\in \{1,...,n\}\}$

...

$\mathrm {vol} (P^{\circ }(\mathbf {c} ))\,=\,\mathrm {vol} (P)\,+\,(\mathbf {c} -\mathbf {1} )^{\top }{\tilde {\alpha }}\,+\,(\mathbf {c} -\mathbf {1} )^{\top }{\tilde {M}}(\mathbf {c} -\mathbf {1} )\,+\,o(\|\mathbf {c} -\mathbf {1} \|^{2}).$

Wachspres variety

The Wachspress coordinates describe a map from $P$ to the standard simplex $\Delta _{n}$. The image of this map is the graph of a rational function in $\Delta _{n}$ and hence an affine variety, the Wachspress variety. Its ideal is called the Wachspress ideal. The Wachspress variety is smooth (in $\Delta _{n}$) and of codimension $n-d$. It is cut out by $n$ polynomials of degree $m-d$:

$...$

External links

• www.example.com