CGNS (.cgns)

The CGNS (CFD General Notation System) format, in this implementation stored as a minimal, non-standard subset within an HDF5 container — a tetrahedra-only mesh, not the full CGNS/SIDS specification.

Format name	cgns
Extensions	.cgns
Read / Write	✓ / ✓
Extra dependencies	h5py (or a C++ build with HDF5)

Reading & writing

import meshioplusplus

mesh = meshioplusplus.read("mesh.cgns")
meshioplusplus.cgns.write("out.cgns", mesh, compression="gzip", compression_opts=4)

• compression / compression_opts — HDF5 gzip filter and level.

File structure

Base/
  Zone1/
    GridCoordinates/
      CoordinateX/" data"     # dataset name is literally " data" (leading space)
      CoordinateY/" data"
      CoordinateZ/" data"
    GridElements/
      ElementRange/" data"           -> [1, n_cells]   (1-based inclusive)
      ElementConnectivity/" data"    -> flat 1-based tetra connectivity

The leading-space dataset name " data" in every leaf group is not part of the real CGNS/HDF5 spec — it's this implementation's own ad hoc convention, consistent between the Python and C++ writers.

Cell types

tetra only — this is the only cell type the reader accepts and the only one the writer emits. Reading a connectivity array that doesn't reshape to exactly 4 columns raises ReadError("Can only read tetrahedra.").

Data mapping

None — no point_data, cell_data, or field_data is read or written by this format.

Quirks & limitations

• Read requires "Base" and "Base/Zone1" to be present, else ReadError('Expected "Base" in file. Malformed CGNS?') (and similarly for "Zone1").
• Indices are 1-based in the file; -1/+1 conversions are applied on read/write while preserving the connectivity array's original integer dtype.
• The writer only ever emits a "tetra" cell block — any other cell type present in mesh.cells is silently ignored (not warned).
• This is the least complete format meshio++ supports: no compression-aware reading (compression is a write-only concept here — HDF5 handles decompression transparently), no field/point/cell data at all.

Notes

• Read/written through the C++ core when built with MESHIO_WITH_HDF5, otherwise through h5py — behavior is identical either way.
• No reference fixture exists under tests/meshes/cgns/; the only test round-trips a synthetic tet_mesh.