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Basix 0.4.1

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basix::lattice Namespace Reference

Lattices of points. More...


enum  type {
  equispaced = 0, gll = 1, chebyshev = 2, gl = 4,
  chebyshev_plus_endpoints = 10, gl_plus_endpoints = 11
enum  simplex_method { none = 0, warp = 1, isaac = 2, centroid = 3 }


xt::xtensor< double, 2 > create (cell::type celltype, int n, lattice::type type, bool exterior, lattice::simplex_method simplex_method=lattice::simplex_method::none)

Detailed Description

Lattices of points.

Enumeration Type Documentation

◆ simplex_method

The method used to generate points inside simplices.

lattice::simplex_method::none can be used when no method is needed (eg when making points on a quadrilateral, or when making equispaced points).

lattice::simplex_method::warp will use the warping defined in Hesthaven and Warburton, Nodal Discontinuous Galerkin Methods, 2008, pp 175-180 (

lattice::simplex_method::isaac will use the method described in Isaac, Recursive, Parameter-Free, Explicitly Defined Interpolation Nodes for Simplices, 2020 (

lattice::simplex_method::centroid will place points at the centroids of the grid created by putting points on the edges, as described in Blyth and Pozrikidis, A Lobatto interpolation grid over the triangle, 2001 (

◆ type

enum basix::lattice::type

The type of point spacing to be used in a lattice.

lattice::type::equispaced represents equally spaced points on an interval and a regularly spaced set of points on other shapes.

lattice::type::gll represents the GLL (Gauss-Lobatto-Legendre) points.

lattice::type::chebyshev represents the Chebyshev points.

lattice::type::gl represents the GL (Gauss-Legendre) points.

lattice::type::chebyshev_plus_endpoints represents the Chebyshev points plus the endpoints of the interval. lattice::type::gl_plus_endpoints represents the Chebyshev points plus the endpoints of the interval. These points are only intended for internal use.

Function Documentation

◆ create()

xt::xtensor< double, 2 > basix::lattice::create ( cell::type  celltype,
int  n,
lattice::type  type,
bool  exterior,
lattice::simplex_method  simplex_method = lattice::simplex_method::none 

Create a lattice of points on a reference cell optionally including the outer surface points

For a given celltype, this creates a set of points on a regular grid which covers the cell, eg for a quadrilateral, with n=2, the points are: [0,0],[0.5,0],[1,0],[0,0.5],[0.5,0.5],[1,0.5],[0,1],[0.5,1],[1,1] If the parameter exterior is set to false, the points lying on the external boundary are omitted, in this case for a quadrilateral with n=2, the points are: [0.5,0.5]. The lattice type can be chosen as "equispaced" or "gll". The "gll" lattice has points spaced along each edge at the Gauss-Lobatto-Legendre quadrature points. These are the same as "equispaced" when n<3.

celltypeThe cell::type
nSize in each direction. There are n+1 points along each edge of the cell.
typeA lattice type
exteriorIf set, includes outer boundaries
simplex_methodThe method used to generate points on simplices
Set of points