OFF (.off)

The Object File Format: a minimal ASCII surface format — a vertex/face/edge count header, the vertex coordinates, then one line per face (leading vertex count + indices).

Format name	off
Extensions	.off
Read / Write	✓ / ✓
Extra dependencies	—

Reading & writing

import meshioplusplus

mesh = meshioplusplus.read("surface.off")
meshioplusplus.off.write("out.off", mesh)

write takes no keyword arguments.

File structure

OFF
<nverts> <nfaces> <nedges>
x y z                    # nverts lines
n i0 i1 ... i(n-1)       # nfaces lines: leading vertex count + indices

The first line must be exactly "OFF" (ReadError otherwise). The counts line's edge count is parsed but discarded. Faces are grouped by vertex count into triangle (3), quad (4), or polygon (any other count ≥ 3) cell blocks — a run of same-count faces stays in one block until the count changes, mirroring the OBJ reader. A leading count below 3 raises ReadError.

Cell types

triangle, quad, polygon (a polygon block written back out must be rectangular — every face in the block the same vertex count; the writer's Python reference also accepts a ragged polygon block, since it can iterate row-by-row). Any other cell type is skipped on write with a warning.

Data mapping

None — OFF carries no point_data, cell_data, or field_data; Mesh(points, cells) only.

Quirks & limitations

• Text-mode strictness: the Python reader requires a text-mode stream (raises if given bytes); the writer always opens the file itself in binary mode regardless of the caller's context.
• No boundary/edge data is ever produced — the edge count is read-and- discarded on read, and always written as 0.
• Faces are grouped into blocks purely by run (consecutive same-count faces), not by regrouping every triangle/quad/polygon in the file into one block each — a file that interleaves counts (e.g. tri, quad, tri) produces three separate blocks in file order, same as the OBJ reader.

Notes

• Fully handled by the C++ core.
• tests/python/meshes/off/cube_example.off (6 quad faces) / cube_example_as_triangs.off (the same cube pre-triangulated into 12 triangles) — a unit cube, once per representation (from issue #35, which reported that non-triangular faces were rejected outright); other tests round-trip synthetic tri_mesh/quad_mesh/polygon_mesh_one_cell fixtures and check .off/.0.off extension dispatch.