EnSight Gold (.case / .geo)

The EnSight Gold format: a small .case index file plus a Gold geometry file (conventionally .geo), the de-facto exchange format for EnSight/ParaView post-processing. meshio++ handles the mesh-geometry subset in both ASCII and C-binary form.

Format name	ensight
Extensions	.case, .geo
Read / Write	✓ / ✓
Extra dependencies	—

Reading & writing

import meshioplusplus

mesh = meshioplusplus.read("out.case")          # or the .geo directly
meshioplusplus.ensight.write("out.case", mesh, binary=True)

• binary — write C-binary geometry (default True); False writes ASCII (%12.5e floats — EnSight caps usable precision at 6 significant digits).

Either path (.case or .geo) selects the sibling pair on write; both files are always written. On read, a .case path is parsed for its GEOMETRY/model: entry (resolved relative to the case file's directory), while any other path is treated as a Gold geometry file directly.

File structure

.case: only the FORMAT (must declare type: ensight gold) and GEOMETRY (model: [ts] [fs] <file>.geo) sections are consumed; VARIABLE/TIME sections are ignored (mesh-only scope). Transient wildcard geometry names (model: name.****.geo) are rejected.

.geo (Gold): 2 description records, node id <off|given|assign|ignore>, element id <...>, an optional extents block, then per part: part, part number, description, coordinates, node count, optional node-id array, and the X, Y, Z coordinate arrays (blocked), followed by element sections (tria3, tetra4, ..., each with a count, an optional element-id array, and connectivity). Binary files start with an 80-char "C Binary" record; all strings are 80-char records and all numbers 32-bit ints/floats in the writer's native byte order. "Fortran Binary" files are rejected.

Per the Gold specification, connectivity is positional (1-based index into the part's coordinate list); given/ignore id arrays are present in the file but skipped — the same interpretation VTK's EnSight readers use.

The writer emits a single part (id 1, description Mesh) with node id assign / element id assign (no id arrays) and always writes three coordinate components (z padded with 0 for 2D meshes).

Cell types

EnSight keyword	meshio++ type
point	vertex
bar2 / bar3	line / line3
tria3 / tria6	triangle / triangle6
quad4 / quad8	quad / quad8
tetra4 / tetra10	tetra / tetra10
pyramid5 / pyramid13	pyramid / pyramid13
penta6 / penta15	wedge / wedge15
hexa8 / hexa20	hexahedron / hexahedron20
nsided	polygon (read only)
nfaced	polyhedron<N> (read only)

Node ordering matches meshio for every type except penta15, which differs from wedge15 by the involution {0,2,1,3,5,4, 8,7,6, 11,10,9, 12,14,13} (the same map VTK's EnSight readers apply; note that Kratos's penta15/hexa20 permutations fix Kratos-specific ordering and do not apply here). Ghost-cell sections (g_*) are read as their base type.

Data mapping

• Multi-part files are concatenated into one point array; every per-part element section becomes its own cell block.
• cell_data["ensight:part"] — the owning part number per cell (Int64), emitted only when the file has two or more parts, so single-part round-trips stay clean. The writer ignores it (always one part).
• The writer drops point_data/cell_data/field_data with a warning — EnSight variables live in separate per-variable files, which are out of scope (v1).

Quirks & limitations

• The format spans two files and cannot be read from or written to a buffer.
• Foreign-endian binaries are auto-detected: the reader checks the plausibility of the part-number/count records (preferring the smaller of two plausible interpretations at the tiny part-number record) and byte-swaps all subsequent reads.
• Writing nsided/nfaced (polygon/polyhedron) blocks raises WriteError (v1); reading them is supported (nfaced cells are grouped by node count into polyhedron<N> blocks, the openfoam convention).
• Transient/moving geometry (change_coords_only, filename wildcards) and EnSight 5/6 files are not supported.
• The .geo extension is also used by Gmsh script files — those are not meshes and not claimed by meshio++'s gmsh reader, but a stray Gmsh .geo passed to ensight will simply fail to parse.

Notes

• tests/python/meshes/ensight/simple.case/simple.geo — hand-authored two-part ASCII example (non-sequential node id given ids, tetra4 + tria3 + nsided) exercising part concatenation, positional connectivity, and ensight:part tagging.
• Fully handled by the C++ core (ASCII + binary, both endiannesses); the pure-Python reference implements the identical feature set for fallback platforms.