ifc-impl-geometry-extraction

name	ifc-impl-geometry-extraction
description	Use when extracting the geometry of an IFC element from an existing file in order to render it, mesh it, or measure it: resolving IfcProduct.Representation to its shape items, picking the right representation, walking the IfcLocalPlacement chain to a world transformation matrix, and decoding each RepresentationType (SweptSolid, Brep, Tessellation, CSG/Clipping, MappedRepresentation). Prevents every extracted element piling up at the origin because the placement chain was skipped, a blocky model because the Box representation was read instead of Body, scrambled meshes from treating IFC indices as zero-based, empty geometry because IfcMappedItem instances were not resolved, and wrong volumes from treating a tessellated approximation as an exact solid. Covers the product-to-shape resolution path, RepresentationIdentifier and RepresentationType, the placement-to-matrix computation, mapped-item transforms, and the IFC2x3 versus IFC4 versus IFC4.3 differences. Keywords: extract IFC geometry, IfcShapeRepresentation, RepresentationType, RepresentationIdentifier, Body, IfcLocalPlacement, world transformation matrix, IfcExtrudedAreaSolid, IfcFacetedBrep, IfcTriangulatedFaceSet, IfcMappedItem, IfcRepresentationMap, IfcCartesianTransformationOperator, tessellation, swept solid, mesh, render IFC, elements at origin, everything stacked at zero, model is blocky, geometry is empty, how do I get the geometry out of an IFC file, how do I render an IFC model.
license	MIT
compatibility	Designed for Claude Code. Requires IFC IFC2x3, IFC4, IFC4.3.
metadata	{"author":"OpenAEC-Foundation","version":"1.0"}

Extracting Geometry from an IFC File

An IFC element keeps its shape and its position apart. The geometry items sit in a LOCAL coordinate system; the element's place in the world is carried separately by ObjectPlacement. Extracting usable geometry means doing two things ALWAYS together: decoding the shape items, and resolving the placement chain to a world transformation matrix.

Verified against the buildingSMART IFC4.3 specification, with IFC4 ADD2 TC1 and IFC2x3 TC1 deltas noted. See references/methods.md for full entity signatures, references/examples.md for worked STEP snippets and matrix computation, and references/anti-patterns.md for the extraction mistakes that produce wrong or empty geometry.

Quick Reference

The resolution path

IfcProduct.Representation
  -> IfcProductDefinitionShape.Representations  (a list)
    -> IfcShapeRepresentation                  (pick by RepresentationIdentifier)
      -> Items                                 (the geometry primitives)
        -> IfcExtrudedAreaSolid / IfcFacetedBrep / IfcTriangulatedFaceSet / ...

In parallel, ALWAYS resolve placement:

IfcProduct.ObjectPlacement
  -> IfcLocalPlacement.PlacementRelTo  (walk up to the root)
    -> compose each RelativePlacement into one world matrix

World geometry = world matrix applied to the local geometry items. NEVER render or measure the raw items without applying the placement.

RepresentationIdentifier: which representation kind

Identifier	Holds	Use for
Body	3D solid / surface geometry	rendering, meshing, volume
Axis	2D/3D axis line	wall/beam centreline analysis
FootPrint	2D ground projection	plan drawings, site layout
Box	bounding box solid	fast culling, spatial queries
Profile	3D profile outline	section display
Surface	analytical surface	structural analysis
Reference	non-Body reference geometry	opening reference shapes