cad3d-ir-step

cad3d-ir-step imports STEP (AP203, AP214, AP242) and IGES files into cad3d-ir through Open CASCADE XDE. It preserves product definitions, assembly occurrences, local placements, names and surface colours, exports each reusable leaf shape as an external OCCT B-Rep resource and, on request, tessellates everything into glTF binary display meshes. Since 0.3.0 it also writes STEP: a STEP or IGES document re-exported in another schema or unit, or a faceted STEP built from the triangles of a visualization GLB.

The runtime uses the official cadquery-ocp-novtk binding because conversion does not require VTK or a visualization stack.

The distribution intentionally depends on cad3d-ir; the core distribution does not depend on this adapter or on Open CASCADE.

Usage

from pathlib import Path

from cad3d_ir import DirectoryResourceWriter
from cad3d_ir.importers import ImportOptions
from cad3d_ir_step import IgesImporter, StepImporter

output = Path("converted")
result = StepImporter().import_file(
    Path("assembly.step"),
    options=ImportOptions(include_visualization_mesh=True),
    resources=DirectoryResourceWriter(output),
)
preview = result.package.visualizations[0]  # preview/<package>.glb
IgesImporter().import_file(Path("part.igs"), options=ImportOptions(), resources=...)

The command-line entry point picks the importer from the file content and writes manifest.json plus geometry resources:

cad3d-ir-step assembly.step converted/            # B-Rep resources only
cad3d-ir-step assembly.step converted/ --mesh     # + per-body GLB + package GLB
cad3d-ir-step robot.stp converted/ --mesh --no-brep --deflection-ratio 0.002
cad3d-ir-step part.igs converted/ --mesh

Writing STEP

from cad3d_ir_step import MeshTooLargeError, export_step, mesh_to_step

# STEP or IGES -> STEP in another schema / unit. Definition colours are copied
# onto the component labels first, so instanced parts keep their colour.
result = export_step("assembly.igs", "assembly.step", schema="AP242", unit="mm")
print(result.products, result.components, result.colors_copied, result.byte_length)

# Triangles of a cad3d-ir GLB -> one sewn solid per placed body (planar
# triangular faces; opens anywhere, not editable). Capped at 20,000 triangles
# because sewing time grows with the square of the count.
try:
    mesh = mesh_to_step(open("part.glb", "rb").read(), "part.step")
except MeshTooLargeError as exc:
    print(exc.triangles, exc.limit)
else:
    print(mesh.solids, mesh.open_shells, mesh.volume_mm3, mesh.diagnostics[0].code)

export_step(verify=True) reads the written file back and reports fewer coloured shapes as STEP_COLORS_CHANGED; PMI that is not written is reported as STEP_PMI_NOT_WRITTEN. mesh_to_step(mode="tessellated") writes AP242 TRIANGULATED_FACE entities instead of solids (about 80 bytes per triangle instead of 2.4 KB), which not every CAD reads. Mesh results always carry STEP_MESH_SOLID_FACETED or STEP_MESH_TESSELLATED; open shells are reported as STEP_MESH_FREE_EDGES.

cad3d-ir-step-export assembly.step assembly-ap203.step --schema AP203
cad3d-ir-step-export part.igs part.step --unit inch --verify
cad3d-ir-step-export preview.glb part.step --max-triangles 20000
cad3d-ir-step-export preview.glb part.step --mode tessellated

Options

ImportOptions (core):

• include_visualization_mesh – write one mesh representation per body and one package-level Visualization (GLB). Off by default.
• include_brep – write exact OCCT B-Rep ASCII resources. On by default.
• mesh_linear_deflection_ratio – linear deflection as a ratio of the root bounding-box diagonal (default 0.001); mesh_angular_deflection in radians (default 0.3); mesh_include_edges for edge polylines.

StepImporter(require_ap242=False, body_mesh_resources=True):

• require_ap242=True restores the 0.1 behaviour of rejecting other schemas before any geometry is transferred. By default AP203/AP214 files are read and an informational STEP_SCHEMA_NOT_AP242 diagnostic is emitted.
• body_mesh_resources=False keeps only the package GLB when meshes are requested (what a viewer needs; halves the written bytes).

Current boundary

• STEP Part 21 files with any AP schema and IGES 5.x files are accepted; the mapping is the same XDE product graph for both.
• XDE product structure, names, local placements, visibility and definition surface colours are mapped. IGES carries little structure, so its packages are usually one part per shape.
• Leaf definition shapes are stored as OCCT B-Rep ASCII v1 resources (include_brep), as glTF binary meshes (include_visualization_mesh), or both.
• One resolved Default configuration is synthesized for every product.
• PMI/GD&T, layers, physical materials and external STEP references are not mapped; they produce *_NOT_MAPPED diagnostics instead of being dropped silently. Faces that OCCT cannot triangulate are counted in *_MESH_INCOMPLETE.
• Representation bounds come from BRepBndLib::Add (surface control points), which is conservative; the visualization bounds are exact mesh bounds.

Measured on a 47 MB industrial robot STEP (12,817 faces): read 28 s, meshing 4 s, extraction and GLB 5 s, peak RSS about 730 MB, 27.6 MB GLB with 700k triangles.

Fixtures

tests/fixtures/reference-assembly.step (AP242), reference-assembly-ap214.step and reference-assembly.iges are generated from primitive boxes and cylinders by scripts/generate_reference_fixture.py --schema {ap242,ap214,iges} with normalized dates and author fields. The abstraction test compares the AP242 assembly with an independent typed synthetic native-CAD source fixture.