meshio++

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Supported Formats ​

Format table ​

Each format name links to a detailed reference page (structure, options, data mapping, and the C++ vs Python behaviour).

Format nameExtensionsReadWriteExtra dependencies
abaqus.inp✓✓—
ansys.msh✓✓—
ansysInp.cdb, .inp✓✓—
avsucd.avs✓✓—
cgns.cgns✓✓h5py
dex.dex✓✓—
dolfin-xml.xml✓✓—
exodus.e, .exo, .ex2✓✓netCDF4
flac3d.f3grid✓✓—
flux.pf3✓✓—
freefem.msh✓✓—
gmsh / gmsh22.msh✓✓—
h5m.h5m✓✓h5py
hmf.hmf✓✓h5py
ip.ip✓✓—
mdpa.mdpa✓✓—
med.med✓✓h5py
medit.mesh, .meshb✓✓—
mff.mff✓✓—
mfm.mfm✓✓—
mphtxt.mphtxt✓✓—
nastran.bdf, .fem, .nas✓✓—
netgen.vol, .vol.gz✓✓—
neuroglancer(no extension)✓✓—
obj.obj✓✓—
off.off✓✓—
openfoam.foam✓——
permas.post, .post.gz, .dato, .dato.gz✓✓—
ply.ply✓✓—
stl.stl✓✓—
su2.su2✓✓—
svg.svg—✓—
tecplot.dat, .tec✓✓—
tetgen.ele / .node✓✓—
tikz.tikz—✓—
ugrid.ugrid✓✓—
unv.unv✓✓—
vtk / vtk42 / vtk51.vtk✓✓—
vtu.vtu✓✓—
wkt.wkt✓✓—
xdmf.xdmf, .xmf✓✓h5py (for HDF data)

Note on .msh: ansys, freefem, and gmsh all use .msh. When writing without an explicit file_format, meshio++ picks gmsh if the mesh carries gmsh-native tags (gmsh:physical/gmsh:geometrical/gmsh:dim_tags) or MED-derived tags (cell_tags/point_tags/med:*), else falls back to the first registered candidate (ansys). When reading, meshio++ tries the registered formats in order and uses the first that parses the file. Specify file_format explicitly (e.g. file_format="freefem") to avoid ambiguity either way.

Note on .inp: abaqus and ansysInp both use .inp. abaqus is registered first, so plain extension-based dispatch resolves to Abaqus by default; pass file_format="ansysInp" (or call meshioplusplus.ansysInp.read/write directly) to select the Ansys/APDL reader for a .inp file.

Note on tetgen: The format spans two files (.node + .ele). It cannot be read from or written to a buffer.

Note on svg: Write-only, 2D meshes only. C++ core with a Python fallback.

Note on tikz: Write-only, 2D meshes only; emits a standalone (directly pdflatex-compilable) LaTeX/TikZ document by default (standalone=False for a bare tikzpicture snippet). C++ core (byte-identical to the Python reference) with a Python fallback.

Note on openfoam: Read-only; a directory-based format (points/faces/owner/neighbour/boundary under constant/polyMesh), not a single file.

Note on mfm: Single element type per file (non-hybrid), linear elements only.

Note on FEconv-derived formats (unv, mfm, freefem, mphtxt, flux, mff, dex, ip): These readers/writers were implemented against the FEconv format documentation and public format specs (FEconv is GPL; no FEconv code or data is used — MIT-clean, with fixtures generated by round-trip). unv handles the parabolic mid-node "sandwich" ordering, maps permanent groups (datasets 2467/2477/2452/2435/2432/2430) to point_sets/cell_sets, and reads/writes field datasets (2414, and legacy 55/57 in Code-Aster mode) as point_data/cell_data; mphtxt and flux round-trip per-element region references as cell_data (mphtxt:geom, pf3:ref). Node orderings for higher-order elements round-trip losslessly but may differ from the originating tool's internal ordering for some element types.

Note on the field-only formats (mff, dex, ip): These carry result data, not geometry. They read into a geometry-less Mesh (no cells) with the field(s) in point_data; dex/ip also populate points from the coordinates in the file, while mff carries no coordinates (its points has zero columns and only the field values round-trip). To attach a field to a mesh, read the field file and the mesh file separately and copy the field Mesh's point_data onto the geometry Mesh — there is no fixed naming convention pairing a field file with its mesh (unlike TetGen's .node/.ele).

Native acceleration and fallbacks ​

meshio++ ships a C++ core (meshioplusplus._core, built with pybind11 + scikit-build-core). Most formats read and write through the C++ core with zero-copy numpy at the I/O boundary; each has a pure-Python fallback that is used automatically when the C++ path can't handle a file or when the extension was built without an optional dependency:

  • HDF5 (cgns, h5m, hmf, med, and XDMF data_format="HDF") — C++ when built with MESHIOPLUSPLUS_WITH_HDF5, otherwise h5py. For med, the C++ core covers the mesh-representation part (points, tags, families, metadata, node orientation, POG ragged polygons) and defers the field/bitmask/gmsh-bridging/multi-mesh constructs to the Python reference; see med.md.
  • netCDF (exodus) — C++ when built with MESHIOPLUSPLUS_WITH_NETCDF, otherwise netCDF4.
  • zlib (VTU zlib compression) — C++ when built with MESHIOPLUSPLUS_WITH_ZLIB, otherwise the Python stdlib.

Behaviour and file compatibility are identical either way; the native paths are only faster. Install the optional runtime deps with pip install meshioplusplus[all].

Format-specific write options ​

All writers are called as meshioplusplus.write(filename, mesh, file_format=..., **kwargs) or mesh.write(filename, **kwargs). The **kwargs depend on the format.

Gmsh (.msh) ​

python
meshioplusplus.gmsh.write(filename, mesh,
    fmt_version="4.1",   # "2.2", "4.0", or "4.1"
    binary=True,
    float_fmt=".16e",
)

Use file_format="gmsh22" to write version 2.2 via the generic meshioplusplus.write.

VTU (.vtu) ​

python
meshioplusplus.vtu.write(filename, mesh,
    binary=True,
    compression="zlib",   # "zlib", "lzma", or None
    header_type=None,     # "UInt32" or "UInt64"
)

VTK (.vtk) ​

python
meshioplusplus.vtk.write(filename, mesh,
    binary=True,
    # For version selection use file_format="vtk42" or "vtk51"
)

file_format="vtk" writes VTK 5.1. file_format="vtk42" or "vtk51" select specific versions.

XDMF (.xdmf, .xmf) ​

python
meshioplusplus.xdmf.write(filename, mesh,
    data_format="HDF",        # "HDF", "XML", or "Binary"
    compression="gzip",       # h5py compression filter (HDF only)
    compression_opts=4,       # compression level
)

With data_format="HDF", meshio++ writes a companion .h5 file alongside the .xdmf. With "XML", all data is embedded in the XML. With "Binary", data is written to separate .bin files.

Medit (.mesh) ​

python
meshioplusplus.medit.write(filename, mesh,
    float_fmt=".16e",
)

PLY (.ply) ​

python
meshioplusplus.ply.write(filename, mesh,
    binary=True,
)

STL (.stl) ​

python
meshioplusplus.stl.write(filename, mesh,
    binary=False,
)

MED (.med) ​

python
meshioplusplus.med.write(filename, mesh,
    med_version="4.1.0",   # MAJ.MIN.REL written to INFOS_GENERALES
)

MED does not support compression. meshioplusplus.med.read_med_multi/ write_med_multi read/write files containing several meshes — see med.md.

AnsysInp (.cdb, .inp) ​

meshioplusplus.ansysInp.read(filename) / meshioplusplus.ansysInp.write(filename, mesh) — no extra options. See the .inp note above for the Abaqus extension collision.

OpenFOAM (.foam, read-only) ​

meshioplusplus.openfoam.read(filename) — no extra options, no writer.

CGNS (.cgns) ​

python
meshioplusplus.cgns.write(filename, mesh,
    compression="gzip",
    compression_opts=4,
)

Nastran (.bdf) ​

python
meshioplusplus.nastran.write(filename, mesh,
    point_format="fixed-large",   # or "fixed-small", "free"
    cell_format="fixed-small",
)

FLAC3D (.f3grid) ​

python
meshioplusplus.flac3d.write(filename, mesh,
    float_fmt=".16e",
    binary=False,
)

SU2 (.su2) ​

meshioplusplus.su2.write(filename, mesh) — no extra options.

AVS-UCD (.avs) ​

meshioplusplus.avsucd.write(filename, mesh) — no extra options.

Abaqus (.inp) ​

meshioplusplus.abaqus.write(filename, mesh) — no extra options.

DOLFIN-XML (.xml) ​

meshioplusplus.dolfin.write(filename, mesh) — no extra options.

CLI format names ​

When using meshioplusplus convert -o <format>, use one of the format names from the first column of the table above (e.g. gmsh, gmsh22, vtk, vtk42, vtu, xdmf, …).

Released under the MIT License.

meshio++ contributors