IFC exchange

extensions/building/ifc/ writes the building model as an IFC4 STEP file and reads one back. The round-trip tests compare model fields directly and check the fingerprint. They cover the site, names, storeys, spaces, elements, openings, furniture, materials, constructions and rebuilt topology.

Modules

Module	What it gives
step	StepFile, StepValue, StepEntity, StepKind, parse, format_real and encode_string: the physical file of ISO 10303-21
ifc4	write_ifc, read_ifc and ifc_guid

Write and read a file

var text = write_ifc(building, "2026-01-01T00:00:00")
Path("office.ifc").write_text(text)
var back = read_ifc(text, Length64(1e-6, METER))

write_ifc validates the model first. The time stamp goes into the header. A fixed time stamp gives the same file for the same model. Each GlobalId comes from the model's fingerprint and a counter, so the ids are stable too.

read_ifc rebuilds the cell complex from the space outlines with assemble. It then gives each wall and slab the name and construction of the file element at the same place. It adds the columns, beams, doors, windows and furniture with their names.

What is written

Model	IFC4
Units	IfcSIUnit: metre, square metre, cubic metre and radian
Site	IfcSite with its latitude, longitude and elevation, and true north in the context
Building and storeys	IfcBuilding, IfcBuildingStorey with its elevation, and a GrossHeight quantity
Space	IfcSpace: an extruded polygon, with its use as the object type
Wall	IfcWall: an extruded rectangle in the wall's frame
Slab and roof	IfcSlab with FLOOR or ROOF: an extruded polygon, with voids where the face has holes
Column and beam	IfcColumn and IfcBeam: an extruded rectangle, I-shape or circle profile
Opening	IfcOpeningElement, IfcRelVoidsElement and IfcRelFillsElement
Door and window	IfcDoor and IfcWindow with their overall sizes
Glazing	Pset_WindowCommon and Pset_DoorWindowGlazingType
Furniture	IfcFurniture in its space: an extruded rectangle, with its kind as the object type
Material	IfcMaterial with Pset_MaterialCommon, Pset_MaterialMechanical and Pset_MaterialThermal
Construction	IfcMaterialLayerSet, associated with each wall and slab
Containment	IfcRelAggregates and IfcRelContainedInSpatialStructure

Beams hang below their axes. A circular beam profile is offset by half its diameter. Rectangle and I-shape profiles are offset by half their depth.

Four property sets keep values that IFC4 has no exact place for. ThreeMojo_Material keeps the strength and the look. ThreeMojo_Member keeps the exact plan points of a column or beam axis. ThreeMojo_Furnishing keeps the exact center and rotation of a piece of furniture. ThreeMojo_Site keeps the exact latitude, longitude and north angle. A reader that does not know them ignores them.

Read a file from another program

read_ifc reads a file when its spaces are extruded polygons on storeys. It follows placement chains, including unset axes and 2D profile placements. It checks each local placement reference and refuses a cycle. Traversal is iterative and bounded by the number of entities in the file.

Missing in the file	What the reader does
A storey's gross height	It uses the gap to the next storey, or the depth of the storey's spaces.
A storey's elevation	It uses the storey's placement.
A material's properties	It uses the library material that the name suggests: steel, timber, brick, gypsum, insulation, glass or aluminum. Any other name gives concrete.
A space's use	It uses OFFICE.
A material layer set	It makes one concrete construction, 0.2 m thick.
A window's glazing	It uses 1.6 W/(m² K), a solar heat gain coefficient of 0.4 and a visible transmittance of 0.7.

A wall or slab that matches no face of the rebuilt complex keeps the first construction. An opening in such a wall is skipped. The reader skips a piece of furniture that is not in a space or has no rectangle profile. It also skips one whose object type is not a furniture kind.

STEP files

parse reads a physical file. It refuses malformed text and gives the line of the error. StepFile.write writes it back. A real is written with the shortest digits that read back to the same number. A string escapes every character outside printable ASCII as \X2\ or \X4\.

STEP value nesting is limited to 256 containers. Lists and typed values share this limit. The outer attribute list of each header entry or entity counts as one container. The parser raises an Error with the line number before it descends past this limit. The limit applies independently to each attribute path.

Errors

Case	Function
The text is not a well-formed STEP file	parse, read_ifc
A STEP value exceeds 256 nested containers, including its outer attribute list	parse, read_ifc
The schema is not IFC4	read_ifc
The project or its unit assignment is missing or ambiguous	read_ifc
The assigned length unit is missing, repeated, prefixed, conversion-based or not meters	read_ifc
A local placement chain contains a cycle or a reference of the wrong kind	read_ifc
A space is not on a storey, or its profile is not a closed polyline	read_ifc
A column or beam is not on a storey, has no material, or has an unknown profile	read_ifc
A product has no extruded solid, or a profile curve is not a polyline	read_ifc
The model does not validate	write_ifc, read_ifc

Limits

• The reader supports only extruded solids. It does not read boundary representations, Boolean results, swept disks or mapped items.
• The reader rebuilds walls and slabs from spaces. A wall with no space on either side is not read.
• The file must have one project with one assigned length unit: unprefixed meters. The reader follows IfcProject.UnitsInContext to its IfcUnitAssignment. It does not convert feet or other length units. Unassigned units and units for other quantities do not change this check.
• Exact round trips apply to the supported order and layout made by assemble and add_*, with the section fields used by its shape. The file preserves customized names, site values, materials and glazing. It does not preserve arbitrary collection reordering or unused extra struct fields.
• The reader rebuilds topology with the supplied tolerance. Use the original model tolerance for an exact topology round trip. An empty model without constructions receives the documented default construction.
• The look of a material leaves IFC only through ThreeMojo_Material. The file has no IfcSurfaceStyle.
• The writer writes the Reference View subset of entities that this table lists. It is not a certified IFC exporter.

The mapping follows the IFC4 schema of buildingSMART International, published as ISO 16739-1:2018.