ifc-syntax-geometry-brep

name	ifc-syntax-geometry-brep
description	Use when you need to model or read an IFC solid as boundary representation, CSG, or a clipping operation : faceted and advanced Breps, the shell / face / loop topology, Boolean results, CSG primitives, and half-space clipping of extruded elements. Prevents a non-watertight shell, a repeated first point in a poly-loop, a mixed loop-type shell, a clipping result with the wrong operator, a finite solid used as a clip, and a RepresentationType that does not match the item. Covers IfcManifoldSolidBrep, IfcFacetedBrep, IfcFacetedBrepWithVoids, IfcAdvancedBrep, IfcAdvancedBrepWithVoids, IfcClosedShell, IfcOpenShell, IfcFace, IfcFaceOuterBound, IfcAdvancedFace, IfcPolyLoop, IfcEdgeLoop, IfcBooleanResult, IfcBooleanClippingResult, IfcHalfSpaceSolid, IfcPolygonalBoundedHalfSpace, IfcCsgSolid, the IfcCsgPrimitive3D family, and IFC2x3 / IFC4 / IFC4.3 differences. Keywords: IFC Brep, boundary representation, IfcFacetedBrep, IfcAdvancedBrep, IfcClosedShell, IfcPolyLoop, watertight mesh, IfcBooleanClippingResult, clipping, half space, IfcHalfSpaceSolid, CSG, IfcBooleanResult, IfcCsgSolid, mesh geometry, my solid has holes, geometry not watertight, model renders inside out, opening not cut, how do I cut a wall, how do I import a mesh, Brep validation error, RepresentationType Brep, IFC2x3, IFC4, IFC4.3.
license	MIT
compatibility	Designed for Claude Code. Requires IFC IFC2x3, IFC4, IFC4.3.
metadata	{"author":"OpenAEC-Foundation","version":"1.0"}

IFC Syntax : Geometry Brep, CSG and Clipping

Boundary representation (Brep) describes a solid by explicitly listing the surfaces, edges and vertices that bound it, instead of by a parametric recipe such as an extrusion. Brep is the escape hatch of IFC geometry : imported meshes, free-form shapes, anything a tool cannot round-trip as a swept solid. This skill also owns CSG (Boolean combinations of solids) and clipping (the half-space subtract that cuts openings and sloped tops into extruded elements).

Applies to IFC2x3, IFC4, and IFC4.3. The faceted Brep, CSG and clipping families exist from IFC2x3 ; the advanced (curved-face) Brep family is IFC4 and later. See Version Differences.

Quick Reference

When to use Brep, CSG, or clipping

• A faceted Brep (IfcFacetedBrep) : an explicit watertight mesh of planar polygons. Use it to carry imported or irregular geometry. RepresentationType = 'Brep'.
• An advanced Brep (IfcAdvancedBrep, IFC4+) : a watertight solid whose faces may be curved (NURBS, cylinders, cones). Use it to round-trip true CAD free-form geometry. RepresentationType = 'AdvancedBrep'.
• CSG (IfcBooleanResult, IfcCsgSolid) : a solid built by Boolean union, intersection and difference of simpler solids and primitives. RepresentationType = 'CSG'.
• Clipping (IfcBooleanClippingResult) : the DIFFERENCE-with-a-half-space special case that cuts openings, recesses, chamfers and sloped tops into an extruded element. RepresentationType = 'Clipping'.

The Brep family tree

IfcGeometricRepresentationItem
└── IfcSolidModel
    └── IfcManifoldSolidBrep (abstract)   attribute : Outer : IfcClosedShell
        ├── IfcFacetedBrep                planar polygonal faces  (IFC2x3+)
        │   └── IfcFacetedBrepWithVoids   adds Voids : SET OF IfcClosedShell
        └── IfcAdvancedBrep               curved IfcAdvancedFace faces  (IFC4+)
            └── IfcAdvancedBrepWithVoids  adds Voids : SET OF IfcClosedShell

IfcManifoldSolidBrep defines exactly one attribute, Outer : IfcClosedShell : the watertight shell bounding the solid, with normals pointing away from the interior. Dim is derived and always 3.

IfcFacetedBrep : the workhorse

IfcFacetedBrep is a manifold solid Brep whose faces are all planar bounded polygons. It is the lowest-common-denominator Brep, present since IFC2x3. It adds no attributes : it inherits the single .