Netgen#

.vol read + write eager

Summary of the specification#

A Netgen .vol file opens with the word mesh3d and is then a sequence of named sections, each one a keyword line, a count line, and that many records. The leading fields differ per section and are what say which element an index belongs to: a surface element spells surfnr bcnr domin domout np before its nodes, while a volume element spells matnr np, so the node list starts at a different column in each. np is stated per record rather than fixed per section, because one section holds every element of its dimension whatever its order — a six-node triangle can sit next to a three-node one. The …gi and …uv spellings of the element sections carry extra geometry or parameter values after the nodes. materials, bcnames, cd2names and cd3names map the numeric indices back to names, and the file closes with endmesh.

Specification at a glance#

header	mesh3d, then dimension and geomtype
sections	points, edgesegments, surfaceelements, volumeelements, plus …gi / …uv spellings
surface record	surfnr bcnr domin domout np <nodes>
volume record	matnr np <nodes>
indices	1-based
name sections	materials, bcnames, cd2names, cd3names
terminator	endmesh

Netgen / NGSolve project ↗

Reading#

import polyxios as px

mesh = px.read("cube.vol")
mesh.element_tags      # bcnr / matnr indices, by name where the file names them

Writing#

px.write(mesh, "out.vol")
px.write(mesh, "out.vol", float_fmt=".17g")   # bit-exact coordinates

option	meaning
`float_fmt`	ASCII coordinate format specifier. Defaults to `.10g`, which does not name a float64 exactly; pass `.17g` for a bit-exact round trip, at the cost of file size.

Quirks worth knowing#

Elements are written section by section in the order the format fixes — faces, cells, edges, then point elements — so an element’s index changes unless the mesh already lay in that order.
Node references are 1-based in the file, 0-based in the PolyData; every type whose node order differs from polyxios’s is permuted both ways.
Index 0 means “unset” and is dropped rather than becoming a tag. A bc_<n>-style tag name keeps its number, unless that number is negative or past int64, which the format cannot spell.
A tag holding elements of more than one dimension is written into each dimension’s section, and comes back split one tag per dimension, because the sections number their indices independently.
Sections polyxios has no home for — identifications, face_colours, the singular_* family — are stepped over, with a warning when one is not empty.
lazy=True warns and loads eagerly; the format is ASCII.