Medit binary#

.meshb read + write always mmapped

Summary of the specification#

.meshb is the binary form of the INRIA Medit mesh format. The file is a stream of keyword-indexed fields: an integer keyword code, the byte position of the next field, then that field’s data. Codes identify the version and dimension first, then typed entity blocks - Vertices, Edges, Triangles, Quadrilaterals, Tetrahedra, Hexahedra - each a count followed by fixed-width records of node indices plus a trailing reference (tag) integer. The version code fixes whether floats are 32- or 64-bit.

Specification at a glance#

layout	keyword code, next-field offset, then the field payload
header codes	MeshVersionFormatted, Dimension
entity blocks	Vertices, Edges, Triangles, Quadrilaterals, Tetrahedra, Hexahedra
record shape	node indices (1-based) + one reference integer
float width	set by the version code: 32-bit or 64-bit
ascii sibling	.mesh - the same keywords in text form

Read the full Medit binary specification ↗

Reading#

import polyxios as px

mesh = px.read("model.meshb")
mesh.vertices          # (n, 3)
mesh.element_types     # element groups found in the file

A path is memory-mapped rather than loaded, always - there is no eager mode to ask for and no lazy= to pass:

mesh = px.read("big.meshb")

Writing#

px.write(mesh, "out.meshb")

This codec takes no format-specific options.

Quirks worth knowing#

The trailing reference integer on each entity becomes element_attrs["ref"] and one element_tags["ref_<n>"] group per distinct value, which is how Medit files carry surface and region labels. On write, references come from that attribute, failing that from the ref_<n> groups; a group named anything else has nowhere to go in a record that carries a number, so it is reported rather than numbered on the caller’s behalf.
A record spells its reference in one signed 32-bit field, so a label outside that range is reported rather than written narrowed - narrowing one wraps it onto another label. vertex_attrs["ref"] is checked the same way, and a float or wrong-length column is refused rather than truncated or left to a broadcast error.
A Dimension 2 file’s vertices are padded with a zero z, so the mesh is 3-D like every other one polyxios holds, and global_attrs["was_2d"] records the fact so the writer restores it. A mesh whose vertices have since left the plane is written in three with a warning, and a flat mesh of solid cells keeps three whatever the flag says, a Tetrahedra section under Dimension 2 being a file no reader loads.
Edges, Prisms and Pyramids are decoded alongside the triangles, quadrilaterals, tetrahedra and hexahedra.
The higher-order sections (TrianglesP2, TetrahedraP2, HexahedraQ2, …) are stepped over. The format fixes no node ordering for high-order elements - libMeshb’s own documentation defers it to a companion *Ordering section - so reading one would mean guessing a permutation, and a silently bent element is worse than a skipped one.
Field offsets are validated against the file size before allocation, so a truncated or hostile file raises instead of over-allocating.
A path is mapped and a file object read into memory, whichever lazy= says, so lazy=True warns and changes nothing while lazy=False is the default and passes without comment. The format is binary throughout - there is no ASCII flavour under this extension to load eagerly, which is what the flag would otherwise choose between.