Netgen (.vol, .vol.gz)

The Netgen neutral mesh format (.vol), plain text and keyword-block based, optionally gzip-compressed (.vol.gz).

Format name	netgen
Extensions	.vol, .vol.gz
Read / Write	✓ / ✓
Extra dependencies	—

Reading & writing

import meshioplusplus

mesh = meshioplusplus.read("mesh.vol")
meshioplusplus.netgen.write("out.vol", mesh, float_fmt=".16e")

• float_fmt — coordinate format.

File structure

The file starts with a literal mesh3d line, then keyword blocks in arbitrary order, each <keyword>\n<count>\n<count lines of data>; blank lines and #-comments are freely interspersed and skipped anywhere. Relevant blocks:

• dimension — a single int (2 or 3).
• geomtype — a single int; unexpected values just warn.
• points — <N> then N coordinate rows.
• pointelements — dim-0 (vertex) cells.
• edgesegments/edgesegmentsgi/edgesegmentsgi2 — dim-1 (line) cells (see the two-line variant below).
• surfaceelements/surfaceelementsgi/surfaceelementsuv — dim-2 cells; the node count is read from a fixed column position per row (variable per row: triangle=3, quad=4, triangle6=6, quad8=8).
• volumeelements — dim-3 cells; node count similarly variable (tetra=4, pyramid=5, wedge=6, hexahedron=8, tetra10=10, pyramid13=13, wedge15=15, hexahedron20=20).
• materials/bcnames/cd2names/cd3names — codimension-indexed name tables (codim 0/1/2/3 relative to mesh dimension) → field_data[name] = [idx, edim].
• identifications — periodic node-pair table, N rows of (node1, node2, identification_type_id).
• identificationtypes — a single row of N type ids.
• face_colours, singular_edge_left/right, singular_face_inside/outside, singular_points — read and discarded.

A single integer cell index (region/material marker) is stored per element across every block, exposed as cell_data["netgen:index"].

Cell types & node ordering

Node-count → type, per topological dimension:

dim	counts → types
0	1 → vertex
1	2 → line
2	3→triangle, 6→triangle6, 4→quad, 8→quad8
3	4→tetra, 5→pyramid, 6→wedge, 8→hexahedron, 10→tetra10, 13→pyramid13, 15→wedge15, 20→hexahedron20

Full Netgen→meshio++ node permutation table (meshio++[i] = netgen[table[i]]); the meshio++→Netgen direction uses the exact per-entry inverse:

type	permutation
triangle6	[0,1,2,5,3,4]
quad8	[0,1,2,3,4,7,5,6]
tetra	[0,2,1,3]
tetra10	[0,2,1,3,5,7,4,6,9,8]
pyramid	[0,3,2,1,4]
pyramid13	[0,3,2,1,4,7,6,8,5,9,12,11,10]
wedge	[0,2,1,3,5,4]
wedge15	[0,2,1,3,5,4,7,8,6,13,14,12,9,11,10]
hexahedron	[0,3,2,1,4,7,6,5]
hexahedron20	[0,3,2,1,4,7,6,5,10,9,11,8,16,19,18,17,14,13,15,12]

(line/triangle/quad/vertex use natural order.)

Data mapping

• cell_data["netgen:index"] — the single per-cell region/material marker; Netgen cannot store the field's name, so on write meshio++ prefers a netgen:index entry if present, else the first integer-dtype cell_data array found.
• field_data[name] = [idx, edim] — codimension-domain names (materials, boundary-condition names, co-dim-2/3 names).
• mesh.info["netgen:identifications"] (an (N,3) int array: node1/node2/type id) and mesh.info["netgen:identificationtypes"] (a (1,N) int array) — periodic identification data.

Quirks & limitations

• 1-based in file; -1/+1 applied on read/write.
• Row layout is column-position-dependent: surfaceelements reads its node count from a fixed column and node ids start at another fixed column; volumeelements uses different fixed positions — these encode Netgen's distinct per-dimension record schema (surfnr bcnr domin domout np p1 p2 ... for surfaces; matnr np p1 p2 ... for volumes).
• edgesegmentsgi2 has a two-physical-line variant, triggered by a header line exactly surf1 surf2 p1 p2 — each cell's data is then split across two lines instead of one. The C++ reader does not implement this two-line variant (nor identifications, materials/bcnames/etc., face_colours, or singular_*) — any of these tokens make the C++ reader throw and defer to Python.
• Only one integer cell-data array can be stored per file (a Netgen format limitation, not a meshio++ choice); when reading back, it is always named "netgen:index" regardless of its original name.
• The .vol.gz gzip container is handled entirely in Python (via gzip.open); the C++ reader/writer explicitly refuse the .gz suffix.
• identifications/identificationtypes are stored in mesh.info, which has no C++-core representation — any mesh carrying them, or with non-empty field_data (materials/bc names), is routed to the Python writer.

Notes

• tests/meshes/netgen/periodic_1d.vol, periodic_2d.vol, periodic_3d.vol — each carries identifications data (used to test the netgen:identifications/netgen:identificationtypes round-trip, which always forces the Python path per the rules above).
• The C++ core handles the common .vol path (points, cells, netgen:index) for both ascii read/write.