CAD import

Geometry does not have to be drawn in Magnelio. A model that already exists in a CAD system is read in with import_step() (or, for the kernel’s own dump, import_brep()), and what comes back is ordinary geometry: solids that carry a material, take part in Boolean operations, accept the chainable verbs and go into a GeometryModel like anything built with the primitives (io/cad.py, DD-178).

What an exchange format carries, and what it does not

A boundary-representation exchange file records finished solids. The construction history that produced them — the sketch, the parameters, the feature tree — is not part of the file and cannot be recovered from it. An imported solid is therefore not editable the way a Cylinder is: to change a dimension, change it in the CAD system and export again. What is stable across such a re-export are the part names, which is why they carry the material assignment (below).

Two formats are supported, and they differ in their metadata:

	STEP (.step, .stp)	BREP (.brep)
Geometry	exact	exact
Length unit	in the file	absent
Part names	yes	no
Display colours	yes	no
Assembly structure	yes	no

STEP is therefore the format to prefer. BREP is the geometry kernel’s native dump; it round-trips a shape without any conversion at all, but because it states no unit, import_brep() demands one from the caller:

horn = import_brep("horn.brep", unit="mm", material=pec)

Getting that argument wrong scales the model by a factor of a thousand and nothing in the file contradicts it — one more reason to use STEP, where the unit is read from the file and the geometry arrives in meters no matter what it was drawn in.

Materials are not carried by either format in any usable form. What a CAD system stores under “material” is a name for a parts list, not the permittivity, permeability and conductivity a field solver needs, so assigning materials on import is not a shortcoming of the reader — it is where the physics enters the model.

Only solids are imported

A material fills a volume, so only solid bodies can become part of a model. Surface bodies in the file (free faces, unstitched shells) are reported and skipped; if a file contains nothing else, the import fails rather than returning an empty model. Turn such bodies into solids in the CAD system first.

Assigning materials by name

materials accepts a single material, applied to every solid, or a dict keyed by the names the solids carry in the file:

parts = import_step(
    "connector.step",
    {
        "shell": pec,
        "pin": pec,
        "insulator*": ptfe,   # shell wildcards are allowed
    },
)

The rules are chosen so that a mapping that no longer fits the file says so instead of quietly doing something else:

• A literal name beats a wildcard, so a general rule plus an exception is written directly ({"*": ptfe, "pin": pec}).

• Two wildcards claiming the same solid for different materials are an error — name that solid literally to settle it.

• A key that matches no solid is an error, and the message lists the names the file actually contains. A renamed part is a typo the first time it happens; without this it would silently lose its material.

Solids that no key matched come back as construction bodies: they have no material, which makes them usable as Boolean operands but rejects them at add(). That is deliberate — a half-mapped assembly should not mesh as if the unmapped parts were vacuum. Importing without any mapping is the way to see what is in a file:

for solid in import_step("connector.step").members():
    print(solid.name, solid.bounding_box())

Assemblies

An assembly is flattened on import. Every solid comes back placed where it sits in the assembly, as a member of one Group; the tree structure itself is not reproduced, because a Group is what the rest of the API consumes — it distributes transformations over its members, keeps each member’s material, and is flattened again when the model takes it.

Names come from the component instance where there is one, and from the part it refers to otherwise. A part that contains several solids is split into <name>_1, <name>_2, …; a part with no name at all becomes solid_1, solid_2, … in file order.

Colours

A colour read from a STEP file is display information and nothing else: it selects the hue a solid is drawn with in plot() and in the ParaView export, and it survives a project store round-trip. It never touches the physics, and it never overrides a colour prescribed by the material — a material with an explicit color keeps it. Opacity stays with the material as well, so metals remain opaque and dielectrics translucent whatever the CAD system painted them.

Healing

CAD files cross kernel boundaries, and what survives the trip is not always a valid solid: tolerances disagree, faces do not quite close, orientations flip. import_step() therefore repairs each solid by default (heal=True), which is close to free on a file that was clean to begin with. import_brep does not, since a BREP file comes from this very kernel.

unify=True additionally merges neighbouring faces that lie on the same surface. Exporters routinely split what is geometrically one plane into many patches; merging them makes the solid simpler and the conformal mesh classification cheaper, but it edits the topology, so it stays opt-in.

If a solid is still invalid after the repair pass, the import warns and carries on, naming the solid. Meshing it may or may not give sensible results — the warning is there so a strange mesh has an explanation.

Limits

Sheet bodies, stitching surfaces into solids, and formats other than STEP and BREP (IGES, mesh formats) are not supported.