Medit / GMF (.mesh, .meshb)

The Medit mesh format, also known as the INRIA libMeshb GMF (Gamma Mesh Format): keyword sections in ASCII (.mesh) or a binary, position-indexed record stream (.meshb). Dispatch is purely by whether the filename ends in "b".

Format name	medit
Extensions	.mesh, .meshb
Read / Write	✓ / ✓
Extra dependencies	—

Reading & writing

import meshioplusplus

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

• float_fmt (default ".16e") — coordinate format for the ASCII writer (the binary path's float width is dictated by the file's MeshVersionFormatted version, not by this kwarg).

File structure

ASCII (.mesh)

Whitespace/#-comment tokenized, keyword-driven:

MeshVersionFormatted <v>      # 0/1 -> float32 coords, 2 -> float64
Dimension <d>
Vertices
<n>
x1 x2 [x3] ref                # n rows
Triangles
<n>
v0 v1 v2 ref                  # n rows, 1-based
...
End

Recognized element keywords: Edges, Triangles, Quadrilaterals, Tetrahedra, Prisms, Pyramids, Hexahedra (and the alternate spelling Hexaedra). Every keyword listed above is followed by a count then that many rows of <node ids> <ref>. Several other keywords are recognized but discarded on read: Corners, Normals, NormalAtVertices, SubDomainFromMesh, VertexOnGeometricVertex/Edge, EdgeOnGeometricEdge, Identifier, Geometry, RequiredVertices, TangentAtVertices, Tangents, Ridges.

Binary (.meshb, libMeshb GMF)

A leading int32 magic code — 1 for native byte order, or its byte-swap (16777216) to signal the opposite endianness, which flips every subsequent typed read. Then an int32 version (1-4), which fixes the integer/float/ position field widths: v1 → 4-byte int, 4-byte float; v2 → 4-byte int, 8-byte float; v3 → 4-byte int, 8-byte float, 8-byte positions; v4 → 8-byte int, 8-byte float, 8-byte positions. The rest of the file is a sequence of (keyword_code, [position], [count], payload) records, each keyed by a numeric GMF field code (from a large internal table of ~200 known libMeshb field codes — only a handful are meaningful to meshio++: GmfVertices (4), GmfEdges (5), GmfTriangles (6), GmfQuadrilaterals (7), GmfTetrahedra (8), GmfPrisms (9), GmfPyramids (49), plus GmfMeshVersionFormatted (1), GmfDimension (3) and GmfEnd (54)). Records with any other code are skipped with a warning.

Cell types

keyword	meshio++ type	nodes
Edges	line	2
Triangles	triangle	3
Quadrilaterals	quad	4
Tetrahedra	tetra	4
Prisms	wedge	6
Pyramids	pyramid	5
Hexahedra / Hexaedra	hexahedron	8

Data mapping

• point_data["medit:ref"] — the per-vertex trailing reference integer.
• cell_data["medit:ref"] — the per-element trailing reference integer, one array per cell block.

Quirks & limitations

• Only one integer point-data array and one integer cell-data array can be written (Medit's single-ref-column limitation): if more than one candidate exists, the first is used and the rest silently dropped (a warning is emitted for the dropped ones).
• Coordinate dtype is version-driven, not user-selectable in binary mode: ASCII MeshVersionFormatted 0 or 1 → float32, 2 → float64; binary version 1 → float32, versions 2-4 → float64. The writer auto-upgrades to binary version 4 if any cell block's connectivity needs 8-byte integers.
• Binary record headers embed absolute byte offsets (pos fields) that the writer must track precisely while emitting records — a strict requirement for libMeshb compatibility, not merely a convenience field.
• Corners/Normals/etc. sections are read-but-discarded in the ASCII path; no equivalent handling exists in the C++ reader beyond simple token-skipping.
• The C++ core implements only the ASCII .mesh variant. The binary .meshb GMF format (including its little/big-endian variants) always falls back to the Python implementation.

Notes

• tests/meshes/medit/cube86.mesh (ascii) — 39 points, 72 triangles, 86 tets, boundary tag counts {1:14, 2:14, 3:14, 4:8, 5:14, 6:8}.
• sphere_mixed.1.meshb (binary) — 3270 points, 864 triangles, 3024 wedges, 9072 tets, tag counts {1:432, 2:216, 3:216}.
• hch_strct.4.meshb / hch_strct.4.be.meshb (binary, little/big-endian pair, exercising the endian-swap path) — 306 points, 12 triangles, 178 quads, 96 wedges, 144 hexahedra, tag counts {1:15, 2:15, 3:160}.
• All four originate from UGRID files converted with UGC (simcenter.msstate.edu).