Skin extraction

meshioplusplus.extract_skin(mesh, linearize=False) derives the boundary surface ("skin") of a 3D volume mesh as a new surface Mesh. It implements the face-hashing algorithm of Kratos Multiphysics' SkinDetectionProcess: every face of every volume cell is keyed by its sorted corner-node ids, and a face whose key occurs exactly once is a boundary face — faces shared by two cells are interior and cancel out.

import meshioplusplus

vol = meshioplusplus.read("part.msh")        # tetra/hexa/wedge/pyramid mesh
skin = meshioplusplus.extract_skin(vol)      # triangle/quad/... surface mesh
meshioplusplus.write("skin.vtu", skin)

The STL and PLY writers call this automatically (by default) when handed a volume mesh, and the SVG/TikZ writers use it to render 3D meshes.

Supported cell types

volume type	boundary faces
tetra	4 × triangle
tetra10	4 × triangle6
hexahedron	6 × quad
hexahedron20	6 × quad8
hexahedron27	6 × quad9
wedge	2 × triangle + 3 × quad
wedge15	2 × triangle6 + 3 × quad8
pyramid	4 × triangle + 1 × quad
pyramid13	4 × triangle6 + 1 × quad8
pyramid14	4 × triangle6 + 1 × quad9

Face matching always uses the corner nodes only, so a mixed linear/quadratic mesh still cancels shared faces correctly. Boundary faces are wound outward (normals point away from the element interior), which is what makes the STL normals come out right.

Semantics

• Output block order is fixed: triangle, triangle6, quad, quad8, quad9 (only non-empty blocks appear), with faces in volume-cell enumeration order within each block.
• Points are compacted: only nodes referenced by a boundary face are kept, in ascending original-index order. point_data rows are subset the same way; cell_data, field_data, sets, and info are dropped (a boundary face has no canonical 1:1 source cell).
• linearize=True emits only the corner nodes of each face (triangle/quad output even for higher-order input); unused mid-edge/ face-center nodes then compact away. This is what the STL/PLY/SVG/TikZ writers use.
• Unsupported volume blocks (polyhedron*, ragged blocks, Lagrange and very-high-order types) are skipped with a warning. If the mesh has no supported volume block at all, ValueError is raised.
• Existing 0/1/2-D blocks are ignored — the skin is derived from the volume cells only. (This also means the writers' skin=True mode drops pre-existing surface blocks, with a warning: a volume mesh's surface blocks are usually its boundary already, and writing both would duplicate every facet.)
• Non-conforming interfaces (a face appearing more than twice) are treated as interior — only count == 1 survives.

For the general form that also handles 2D→edge extraction and records the parent cell id, see surface extraction.

Implementation

The C++ core implements the extractor in src/cpp/src/operations/surface.cpp — shared with extract_surface, which extract_skin calls in volume-only (face) mode — over the uniform mesh API (so it works under all three mesh backends), with the per-cell-type face topology tables in src/cpp/include/meshioplusplus/detail/cell_faces.hpp; a pure-numpy fallback (meshioplusplus._skin._extract_skin_py) carries a twin of the tables and produces identical output (same block order, connectivity, and point order — asserted by tests/python/test_skin.py). The public extract_skin tries the C++ core first and falls back to numpy, like the format shims. The face tables' outward winding is enforced by a gtest invariant (tests/cpp/test_skin.cpp): on each reference element, the Newell normal of every face must point away from the cell centroid.

extract_skin is exposed through every binding surface — Python, the C API (mio_extract_skin), Fortran (mesh%extract_skin()), and WASM (extractSkin) — alongside the STL/PLY writers' skin-by-default behavior.