FLUX mesh (.pf3)

The Altair FLUX .pf3 mesh format (as handled by FEconv). ASCII with French keyword headers.

Format name	flux
Extensions	.pf3
Read / Write	✓ / ✓
Extra dependencies	—

Reading & writing

import meshioplusplus

mesh = meshioplusplus.read("mesh.pf3")
meshioplusplus.flux.write("out.pf3", mesh)

write takes no keyword arguments.

File structure

Header lines are located by substring search for their French label (rather than by fixed line position), so header ordering is somewhat tolerant:

• dim — the line containing "NOMBRE DE DIMENSIONS" (leading integer).
• nel — the line containing "D'ELEMENTS" but not any of VOLUMIQUES, SURFACIQUES, LINEIQUES, PONCTUELS, MACRO (i.e. the grand-total line, not a per-category count).
• nnod — the line containing "NOMBRE DE POINTS" but not "INTEGRATION".

The element block starts after the line containing "DESCRIPTEUR DE TOPOLOGIE"; the coordinate block starts after "COORDONNEES DES NOEUDS". Between those markers, nel element records are parsed as one continuous token stream (line breaks inside a record are tolerated), each a 12-integer header followed by its connectivity:

field	meaning
0	element id (ignored on read)
1	desc1 (category — ignored on read)
2	desc2 (subtype — ignored on read)
3	region reference → cell_data["pf3:ref"]
4	node count (redundant with field 7)
5	unused (0)
6	desc3 (type code — selects the meshio++ type)
7	node count (drives how many ids follow)
8-11	unused (0 0 0 0)

followed by the element's 1-based node ids. After "COORDONNEES DES NOEUDS", nnod records of node_index x1 x2 ... x_dim follow; the leading index is read and discarded (rows are assumed already in file order).

Cell types

The desc3 field selects the type:

desc3	type	desc3	type
2	vertex	11	tetra10
3	line	12	wedge
4	line3	13	wedge15
5	triangle	15	hexahedron
6	triangle6	16	hexahedron20
7	quad	17	pyramid
8	quad8	10	tetra

meshio++ → (desc1, desc2, desc3) on write:

type	desc1, desc2, desc3	type	desc1, desc2, desc3
vertex	1, 1, 2	tetra10	5, 15, 11
line	2, 2, 3	wedge	6, 207, 12
line3	2, 3, 4	wedge15	6, 307, 13
triangle	3, 7, 5	hexahedron	7, 2202, 15
triangle6	3, 7, 6	hexahedron20	7, 3303, 16
quad	4, 202, 7	pyramid	8, 4202, 17
quad8	4, 303, 8	tetra	5, 4, 10

Hybrid meshes are supported.

Data mapping

• cell_data["pf3:ref"] — per-element region reference (from header field 3), one array per cell block.

Quirks & limitations

• Unlike UNV/gmsh/mphtxt, no node-order permutation table is applied — node ids pass through in file order directly. This round-trips losslessly through meshio++ but is not guaranteed to match FLUX's own internal node ordering convention for every element type.
• Header-line detection is French-text substring matching; a real FLUX file with reworded headers (not expected in practice, but possible) would break parsing.
• Region names (which FLUX may store as binary data alongside the mesh) are not read at all — only the numeric per-element reference.
• Several always-placeholder header fields on write: region counts are always 1/0/0/0/0/0, and both "max nodes per element" and "max integration points per element" fields are hardcoded to 20 regardless of actual mesh content.

Notes

• Fully handled by the C++ core.
• No reference fixture exists under tests/meshes/flux/; tests round-trip every supported linear and second-order type.