COMSOL text mesh (.mphtxt)

The COMSOL Multiphysics .mphtxt text mesh format (as handled by FEconv). ASCII, storing a version, tag/type name tables and one or more mesh objects; comments run from # to end of line.

Format name	mphtxt
Extensions	.mphtxt
Read / Write	✓ / ✓
Extra dependencies	—

Reading & writing

import meshioplusplus

mesh = meshioplusplus.read("mesh.mphtxt")
meshioplusplus.mphtxt.write("out.mphtxt", mesh)

write takes no keyword arguments.

File structure

The whole file (after stripping #-comments) is read as a flat token stream:

1. Version major, version minor (2 ints, discarded).
2. n_tags length-prefixed tag-name strings (discarded).
3. n_types length-prefixed type-name strings — this count is also the number of "object" records that follow.
4. For each object (only the first is actually parsed — the reader breaks after it):
   • 3 ints (object type indices, discarded), a class-name string (expected "Mesh"), an object-version int, sdim (spatial dimension), n_points, lowest (the file's lowest node index — normally 1, but not assumed to be).
   • n_points * sdim floats — node coordinates, row-major (n_points, sdim).
   • n_eltypes element-type blocks, each:
     • a COMSOL type-name string ("tet", "tri2", …);
     • n_nodes, n_elem;
     • n_elem * n_nodes ints — connectivity, shifted by -lowest to become 0-based;
     • n_par_per, n_par, then n_par * n_par_per discarded parameter tokens;
     • n_geom, then n_geom ints — one geometric entity index per element → cell_data["mphtxt:geom"];
     • n_ud, then n_ud * 2 discarded up/down-pair ints.

Cell types

Hybrid (multi-type) meshes are supported. COMSOL ↔ meshio++ type map:

COMSOL	meshio++	COMSOL	meshio++
vtx	vertex
edg	line	edg2	line3
tri	triangle	tri2	triangle6
quad	quad	quad2	quad9
tet	tetra	tet2	tetra10
prism	wedge	prism2	wedge18
pyr	pyramid
hex	hexahedron	hex2	hexahedron27

Node-order permutation (COMSOL ↔ meshio++, self-inverse swaps — identity for every other type):

type	permutation
quad	[0, 1, 3, 2]
hexahedron	[0, 1, 3, 2, 4, 5, 7, 6]

Data mapping

• cell_data["mphtxt:geom"] — per-element geometric entity index, one array per cell block.

Quirks & limitations

• Only the first mesh object in the file is read; subsequent objects are ignored entirely — a hard limitation for multi-mesh .mphtxt files.
• Element parameter values (n_par) and up/down topology-linking pairs (n_ud) are always discarded on read and always written as empty/zero on write — any parametrization or entity-linking data COMSOL stores there does not round-trip.
• The connectivity shift uses the file's actual lowest value (not a hardcoded 1), so files with an unusual lowest-index convention are handled correctly.
• Unsupported cell types on write: the Python writer warns and skips them; the C++ writer raises WriteError, which forces the Python fallback.

Notes

• Fully handled by the C++ core.
• No reference fixture exists under tests/meshes/mphtxt/; tests round-trip linear, second-order, and hybrid (tri_quad_mesh) meshes.