TetGen#

.ele + .node read + write eager

Summary of the specification#

TetGen splits a mesh across sidecar files that share a basename. .node starts with a counts line - number of points, dimension, number of attributes, boundary-marker flag - then one line per point: an index, its coordinates, its attributes, and its marker if the flag is set. .ele starts with its own counts line - number of tetrahedra, nodes per tetrahedron, region-attribute flag - then one line per element: an index and its node indices, with an optional trailing region attribute. Indices may be 0- or 1-based; the first index in the file decides.

Specification at a glance#

.node header	npoints dim nattrs boundary_marker_flag
.node line	index x y z [attrs…] [marker]
.ele header	ntetrahedra nodes_per_tet region_attr_flag
.ele line	index n1 n2 n3 n4 [region]
index base	0 or 1, inferred from the first index
companions	.face, .edge, .neigh use the same convention

Read the full TetGen specification ↗

Reading#

import polyxios as px

mesh = px.read("model.ele")
mesh.vertices          # (n, 3)
mesh.element_types     # one type code per element

Writing#

px.write(mesh, "out.ele")
px.write(mesh, "out.ele", float_fmt=".17g")

Format-specific options#

Option	Default	Effect
`float_fmt`	`".10g"`	ASCII coordinate format specifier; pass `".17g"` for a bit-exact round trip.

Quirks worth knowing#

Pass either path - px.read("bar.ele") or px.read("bar.node") - and the paired file is located by basename.
Boundary markers become vertex tags (boundary_<marker>, marker 0 dropped as unmarked), node attributes become vertex_attrs attr_<k>, and region attributes become element attributes (region), so both labelling schemes are kept. Those attributes are REALs in the format and are read as float64, so a fractional region value keeps its fraction.
Node references resolve against the .node file’s own point numbers, so a file numbered from 0, from 1 or with gaps reads the same way. Both files are written numbered from 1.
Only tetrahedra reach the .ele file; every other element is skipped on write with a warning. A 10-node quadratic tetrahedron is read as a linear one, with a warning, since TetGen’s mid-edge order is not polyxios’s.
A .node file declaring 2 dimensions is padded with a zero z, so the vertex array is always (n, 3), and global_attrs["was_2d"] records the fact so the header goes back out declaring 2. A mesh whose vertices have since left the plane is written in three with a warning, and a mesh with tetrahedra to write keeps three whatever the flag says: the .ele file declares four nodes per element, and TetGen’s own 2-D mode fills the plane with triangles instead.