Three-dimensional geometry (

The preferred way to specify a three-dimensional domain for meshing
is using a geometry (`.bdry`) file. This file format supports
simultaneous and consistent meshing of multiple sub-domains and is
extensible to non-polyhedral boundaries. This file format is conceptually
the same as the two-dimensional geometry file format, so most of the
description of the format is the same.

The first line of the file contains the number of vertices and number
of boundary sections in the file. The next `NVerts` lines give
vertex coordinates. The remainder of the file describes the boundary
geometry. A sample input file and a picture of the underlying geometry
are shown in Figure 3.6.

Each boundary data entry begins with a tag describing the kind of
data it contains. At present, the only valid tag is `polygon`.

Next, the relationship between the boundary entity (BE) and the domain is established. For entities lying on the bona fide domain boundary, a boundary condition must be specified, along with an integer tag (1 tag 126) identifying which interior region is adjacent to the BE. Entities lying within the domain necessarily separate two regions with different ID tags; both tags must be specified.

Finally, geometric data required to describe the BE must be given. This data is not restricted to a single line in the input file.

`polygon`geometric data-
A

`polygon`is specified by giving the number of vertices in the polygon, followed by the indices of the vertices, in order. These indices begin from 0.

Carl Ollivier-Gooch 2017-07-20