STL (.stl)

The stereolithography format: a flat list of triangle facets (each with a normal and three vertices), in ASCII or binary. STL has no shared-vertex table — every facet repeats its own vertex coordinates.

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

Reading & writing

import meshioplusplus

mesh = meshioplusplus.read("part.stl")
meshioplusplus.stl.write("out.stl", mesh, binary=False)

# A 3D volume mesh (tetra/hexa/wedge/pyramid) writes its boundary skin:
vol = meshioplusplus.read("part.msh")
meshioplusplus.stl.write("skin.stl", vol)              # skin extraction (default)
meshioplusplus.stl.write("legacy.stl", vol, skin=False)  # legacy: drop volume cells

• binary — binary (True) or ASCII (False, the default — unlike most binary-capable formats in meshio++, which default to True).
• skin — when True (the default) and the mesh contains supported 3D volume cells (tetra, hexahedron, wedge, pyramid and their common higher-order variants), the boundary skin is extracted first (see extract_skin), quads are triangulated as (0,1,2)/(0,2,3), and only that skin is written. Any pre-existing surface blocks are dropped with a warning (a volume mesh's surface blocks are usually its boundary — writing both would duplicate every facet). skin=False restores the legacy behavior: volume cells are discarded and only existing triangle blocks are written.

File structure

ASCII:

solid [name]
 facet normal nx ny nz
  outer loop
   vertex x y z
   vertex x y z
   vertex x y z
  endloop
 endfacet
 ...
endsolid

Binary: an 80-byte free-form header, then a little-endian uint32 triangle count, then that many fixed 50-byte records: float32[3] normal, float32[3][3] facet vertices, int16 attribute byte count (conventionally 0, not enforced).

Binary-vs-ASCII detection: if the file is under 80 bytes, it's treated as ASCII. Otherwise, the 80-byte header and triangle count are read and the expected total size 84 + num_triangles*50 is compared against the actual file size; if they match, it's binary, otherwise the reader rewinds and parses as ASCII. This heuristic deliberately avoids the naive "does the file start with the literal word solid" check, since binary STL files sometimes also start with that word in their free-form header.

Cell types

triangle only on disk. On write, 3D volume cells (tetra, hexahedron, wedge, pyramid, tetra10, hexahedron20/27, wedge15, pyramid13/14) are accepted via the default skin extraction (linearized to corner nodes, quads triangulated).

Data mapping

• cell_data["facet_normals"] — if present, written verbatim; otherwise the writer computes normals from the cross product of two edge vectors.

Quirks & limitations

• Vertex de-duplication: all raw (possibly duplicate) triangle vertices are uniquified in first-occurrence order to build a shared point table — meaning point indices are not preserved across a round-trip, only the geometry.
• dtype: ASCII coordinates parse as float64; binary coordinates parse as float32 (matching the on-disk format) — this dtype difference is intentional, not a bug.
• Empty STL: a file with zero triangles produces a Mesh with no cell blocks at all, not an empty "triangle" block.
• Mixed cell types on write with skin=False (or no volume cells): a warning names the discarded types, and only triangles are written. With skin=True and volume cells present, only the extracted skin is written and pre-existing surface blocks are dropped with a warning.
• ASCII parsing uses a fast custom line reader that only looks at the last 3 whitespace-separated tokens per line (discarding any leading keyword like vertex), for performance over np.loadtxt.

Notes

• Fully handled by the C++ core (ASCII and binary).
• No reference fixture exists under tests/python/meshes/stl/; tests round-trip synthetic meshes only.