As of FME 2026.2, the new IFC_API Reader converts IFC geometry of type IfcFixedReferenceSweptAreaSolid into BRepSolids within FME (see IFC-to-FME Geometry Mapping). When these geometries are subsequently written using the IFC_API Writer, they are therefore written as IfcFacetedBrep.
I would like to suggest native support for reading, preserving and writing IfcFixedReferenceSweptAreaSolid.
The main issue with converting this type of geometry into a BRep is that a relatively compact parametric representation can become a very large explicit boundary representation. A swept-area solid can essentially be described using a cross-section, a directrix and an orientation rule, whereas a BRep needs to explicitly store the resulting vertices, loops and faces.
As a practical example, FME currently does not appear to provide a native geometry type for sweeping an arbitrary profile along a path. For a railway use case, I therefore created the rail geometry manually in a PythonCaller by placing the individual rail-profile points along the input line and connecting them to form the final solid. While this works geometrically, the resulting object has to be represented inside FME as a BRepSolid and is therefore written to IFC as IfcFacetedBrep. This can result in considerably larger IFC files compared with representing the same geometry as an IfcFixedReferenceSweptAreaSolid.
In one test containing only around 200 metres of rail geometry, the FME-generated IFC using IfcFacetedBrep was approximately 23 MB. After reconstructing the same rail geometry as IfcFixedReferenceSweptAreaSolid, the resulting IFC was only around 150 KB. While the converted file was created as a proof of concept through post-processing, the difference clearly illustrates the potential impact of the geometry representation on file size.
Ideally, FME could provide a native swept-profile geometry type that retains the relationship between the profile and the path, instead of immediately converting the result into an explicit BRep. A comparable concept already exists for IfcSweptDiskSolid, which can be represented in FME using the Pipe geometry type.
Such a geometry type could then allow the IFC_API Reader and Writer to preserve or create IfcFixedReferenceSweptAreaSolid geometry directly.
