schemaxismeshio: mesh interchange with explicit units
Convert a bounded, captured unstructured mesh between ASCII VTU and legacy VTK 4.2 while retaining ordered geometry, topology and point/cell arrays. This is a general mesh interchange route for inspection and geometry handoffs. It does not expose solver file formats or arbitrary meshio scripts.
Contract and units#
Use schemaxis.meshio/1, operation convert. Required fields are source
(absolute local regular .vtu/.vtk path), sourceFormat and targetFormat
(vtu or vtk), sourceUnits and targetUnits (m, cm, mm, in), and
lossPolicy: "strict". Same-format unit conversion is also useful. Every field
is required; no units are guessed from an extension or from field names.
Coordinates are multiplied by source metres-per-unit divided by target metres-per-unit, including the origin. For example mm to m multiplies XYZ by 0.001. Point/cell scalar and vector arrays retain their numeric values: vector fields are not implicitly displacement, velocity or length. Convert their physical units separately when needed. The summary labels tetrahedral volumes in target length units cubed. A unit declaration is a user assertion, not an independently measured property of the file.
Only triangle (VTK type 5) and tetra (type 10) cells are accepted, with exactly
three coordinates per point. Triangle cross-product magnitude must exceed
1e-12 times squared maximum vertex distance from its first vertex. A tetra's
signed determinant (b-a)·((c-a)×(d-a)) must exceed 1e-12 times that distance
cubed: inverted, degenerate and nearly degenerate tetrahedra are refused. This
is a bounded orientation/degeneracy check, not a general mesh quality metric.
Point order and flattened cell order/connectivity must remain exact, including
interleaved triangle/tetra blocks. XML comments, block grouping and numeric storage dtypes may
change. No points are merged, reordered, repaired or remeshed.
Data arrays require distinct names [A-Za-z_][A-Za-z0-9_]{0,63} and scalar or
three-vector tuples associated with every point/cell. Finite numbers are
required; integers are bounded by ±2^53 and compared exactly. Floating values
compare at relative 1e-11 and absolute 1e-14; byte formatting/dtypes are not a
preservation promise. Oriented triangle area vectors and signed tetra volumes
also compare at relative 1e-8 without an absolute floor, so small cells at a
large origin cannot pass from coordinate-relative tolerance alone. Float32 input values are interpreted at their declared IEEE
precision by the independent reader; integer values must fit the declared type.
meshio VTU ASCII uses about twelve significant digits, motivating the tolerance. XYZ magnitudes are at most 1e9 in both coordinate systems.
VTU XML bytes must be ASCII (no UTF-16 or NULs), containing a single UnstructuredGrid/Piece with inline ASCII DataArrays.
VTK must be 4.2 ASCII UNSTRUCTURED_GRID, using FIELD/FieldData point/cell
arrays. Binary/compressed/appended data, DTDs/entities, active
array attributes, extra metadata/sections, groups, sets, tensors, high-order
cells and unsupported arrays are refused before native execution. Bounds:
16 MiB source, 32 MiB output, 100,000 points/cells, 16 arrays per association.
The source plus output budget is consequently 48 MiB; retained JSON snapshots
also have a 32 MiB per-file bound. Native POSIX caps are 2 GiB address space,
32 MiB per file and 60 CPU seconds, within one configurable total deadline.
Running and inspecting#
Install meshio 5.3.5 plus NumPy in an isolated environment and configure its
absolute Python executable under application meshio, or pass --meshio.
The Schemaxis host does not import NumPy or meshio. The latest stable meshio
version is 5.3.5, checked against PyPI on
17 October 2026. Native read/write uses the public
meshio API.
Start from baseline or
reverse conversion, replacing
the source path with the supplied literal fixture's absolute path. Use normal
schemaxis plan INPUT, run INPUT --output-directory OUTPUT --meshio PYTHON
and verify OUTPUT. Studio offers editable conversion and reverse-conversion
starters, source forms and contextual Help.
The output includes mesh.vtk or mesh.vtu, mesh-summary.json, captured source,
authored/effective plans, native scripts/configuration, stdout/stderr/submissions,
consumed-input digests and a verification record. Inspect bounds, ordered counts,
array names and signed tetra volumes in the summary; use a compatible viewer for
geometry. Two isolated native processes perform conversion and fresh reopening.
Standard-library reference readers independently parse original and output bytes
and check the unit transform, ordered topology and data. Fresh meshio reopening
is an additional consistency check. Offline verification rechecks retained
artifacts/evidence without rerunning meshio and is not a signed anti-forgery proof.
Workflows, reuse and recovery#
Verified geometry artifacts can feed another meshio conversion through the normal workflow artifact reference; explicit format/units must agree with the producer. Source byte hashes govern reuse: editing a source at the same path invalidates reuse. Finite studies can vary targetUnits/targetFormat through the shared expression and axis model. No time-series, MPI, live native editing or interactive native inspection is exposed.
INVALID_MESH_SOURCE means choose an absolute matching regular file, with no
symlink. INVALID_MESH_CONTENT explains the unsupported section, numeric shape,
orientation or bound; export supported ASCII data instead of discarding it.
INTERPRETER_NOT_FOUND requires the configured isolated interpreter. Native
version errors require meshio 5.3.5. VERIFICATION_FAILED means inspect retained
logs and original source; a successful process or merely existing mesh does not
satisfy preservation. Cancellation and timeout retain failed/incomplete evidence
and terminate the owned subprocess tree.
Qualification uses literal fixtures with nonzero origin, interleaved cells, positive tetra volume 4 mm³, distinct scalar IDs/regions and three-vector point and cell arrays. Baseline converts mm to m; the reverse variant restores mm. The bounded release does not claim other meshio formats or binary-file support.