STEP (.step, .stp)

ISO 10303-21, read through the optional OpenCASCADE backend and written without it. The writer is a hand-off of a closed surface to CAD: a faceted solid in the AP214 (AUTOMOTIVE_DESIGN) schema that FreeCAD, OpenCASCADE-based tools and commercial CAD systems open as a solid body, made of one planar face per facet. It is the CAD end of the skin-to-CAD workflow: the baseline a CAD user can measure, section and boolean against, where an STL import gives only an unconnected triangle soup.

Format name	step
Extensions	.step, .stp
Read / Write	✓ (with OpenCASCADE) / ✓
Extra dependencies	reading: OpenCASCADE 7.6 or later (-DMESHIOPLUSPLUS_WITH_OCCT=ON)

Reading

Reading STEP means evaluating its curves and surfaces, which is a CAD kernel's job: the reader runs on the optional OpenCASCADE backend that fit_cad introduced (-DMESHIOPLUSPLUS_WITH_OCCT=ON, LGPL-2.1 with the OCCT exception, bring your own, absent from the wheels, the WASM build and the release CLI binaries; see the OpenCASCADE decision). Without it, reading raises NotImplementedError (C++ Unsupported) naming the flag, and the round trip on the conformance page is checked only in a build with it. IGES is read the same way.

import meshioplusplus as mp

mesh = mp.read("part.step")                       # deflection 1e-3 of the bounding-box diagonal
mesh = mp.step.read("part.step", deflection=0.05, angular_deflection=0.3)
brep = mesh.info["brep"]                          # the faces, loops and edges as NURBS
mp.write("part.cad.json", mesh)                   # ... as Kratos CAD JSON

• The shape. STEPControl_Reader transfers every root into one shape, which BRepBuilderAPI_NurbsConvert turns into NURBS throughout. Converting each face's surface on its own would reparametrise a cylinder or a sphere and leave its trimming curves in the old parameters; converting the whole shape keeps every parameter curve on its converted surface.
• The mesh. BRepMesh_IncrementalMesh triangulates it at deflection (a length; at most 0 takes 1e-3 of the bounding-box diagonal) and angular_deflection (radians, 0.5 by default) into one triangle block, points shared between faces welded where they coincide. Cell data: cad:kind (0), cad:index and cad:id (the face), cad:span (the knot span of the triangle's centre in the face's parameters) and cad:trimmed (0), as for CAD JSON.
• The side channel. mesh.info["brep"] (C++ BrepInfo) holds every face's surface (degrees, knots, control net, weights; reversed faces flagged swapped_normal), its trimming loops (the outer one first, each trimming curve with its active range and direction, in the surface's own orientation) and every edge's 3-D curve and topology (the faces and trims it bounds). Faces have ids 1 .. F and edges F + 1 .. F + E, control points ids from 1, so the B-rep writes as Kratos CAD JSON that Kratos's CadIoModeler loads: a STEP model becomes Kratos IGA geometry with mp.write(...) or meshio++ convert part.step part.cad.json. Vertices are not carried.
• Errors. A file OpenCASCADE cannot read, or one that transfers no shape, is a ReadError; a non-finite deflection or a non-positive angular deflection a ValueError.

C++: read_step(path) and read_step(path, info, options) (formats/step.hpp, CadReadOptions in brep.hpp, cad_read_has_occt()). The C API's mio_read_with_info returns a "step" handle with the mio_brep_info_* accessors, which mio_write_with_info writes as CAD JSON; Fortran, Julia, R and the native CLI follow.

Writing

Writing takes no options:

import meshioplusplus as mp

mp.write("part.step", mp.read("part.stl"))       # a closed triangle skin
mp.write("block.stp", mp.read("block.msh"))       # a volume mesh: its skin is written

What is written

• The surface. triangle, quad and polygon blocks (quadratic surface cells contribute their corners); when the mesh has volume cells, the skin of those, as the stl writer takes it. Lines and vertices are dropped with a warning.
• One face per facet. Each facet is an ADVANCED_FACE on a PLANE, bounded by an EDGE_LOOP of ORIENTED_EDGEs over straight EDGE_CURVEs (LINEs) between VERTEX_POINTs. An edge two facets share is one EDGE_CURVE, used once in each direction, so the shell is topologically closed and needs no sewing. A facet of four or more corners that is not planar to 1e-9 of its size is fanned into triangles from its first corner, so every face lies exactly on its plane. Coplanar facets are not merged into one face.
• One solid per piece. Every edge-connected piece of the surface is a CLOSED_SHELL inside a MANIFOLD_SOLID_BREP; they are collected in one ADVANCED_BREP_SHAPE_REPRESENTATION under the usual product structure (PRODUCT, PRODUCT_DEFINITION, SHAPE_DEFINITION_REPRESENTATION). A piece wound inwards (negative signed volume) is written reversed, with a warning, so every solid has positive volume. A surface nested inside another (a void) becomes a second solid, not a cavity.
• Units. Millimetres, with a length uncertainty of 1e-7. Coordinates are written as they are: scale with transform first if the mesh is in metres.

The roadmap item that queued this writer described the faces as ADVANCED_FACEs with POLY_LOOP bounds. That combination is not valid AP214, since an ADVANCED_FACE must be bounded by EDGE_LOOPs; the writer emits the edge-loop form, which is also what lets the faces share edges.

Refusals

The writer refuses, with a WriteError, anything that is not the boundary of a solid:

• an open, non-manifold or inconsistently wound edge — the message gives the counts and names the first five as point-id pairs; run repair first;
• a degenerate facet (zero area), or an edge between two coincident points (run clean first);
• a non-finite coordinate, or a mesh with no surface facets at all.

C++ vs Python

The C++ core and the Python reference write identical bytes: entity ids are sequential, every number is written with %.17g (with the decimal point STEP requires, 3. and 1.E-05), the header time stamp is fixed at 1970-01-01T00:00:00, and the provenance lines go in a /* ... */ comment after ISO-10303-21;. FILE_NAME and PRODUCT carry the file's basename.

Validation

Checked with OpenCASCADE 8.0.1 (the cadquery-ocp 8.0.1.1 wheels): STEPControl_Reader reads every file tests/python/test_step.py writes — a subdivided cube, an inward-wound cube, two disjoint cubes, a tetrahedron's skin, a box with one warped (fanned) quad, a polygon-block cube and a torus — as the expected number of solids, BRepCheck_Analyzer finds each valid, and GProp gives the mesh's own volume to 1e-9. The test runs when OCP is importable (pip install cadquery-ocp) and skips otherwise. FreeCAD has not yet been checked directly; it reads STEP through the same OpenCASCADE reader. The reader is checked on the backend's own build (OpenCASCADE 7.6, the CI occt leg): a written box reads back as its faces, each trimmed by one loop and every edge bounding two of them, and its CAD JSON rewrite samples back onto the box (test_step.py, tests/cpp/test_cad_read.cpp).

Not supported

• Reading without the OpenCASCADE backend (see above), and a reader's vertices (STEP's VERTEX_POINTs are carried as edge ends only).
• Analytic faces — this writer is faceted by design. fit_cad writes the fitted cylinders, cones, spheres and tori, and B-spline faces for what no primitive explains, as CAD faces through the optional OpenCASCADE backend.
• Colours, layers, assemblies, and AP242 product manufacturing information.