ifc-syntax-geometry-tessellation

name	ifc-syntax-geometry-tessellation
description	Use when you need to author, read, or debug tessellated (faceted) geometry in an IFC4 or IFC4.3 model : triangle meshes and polygon meshes built from a shared indexed point pool, the lightweight geometry that the Reference View MVD relies on. Prevents the tessellation errors that break meshes : 0-based instead of 1-based indices, a triangle row with the wrong number of indices, a PnIndex indirection applied twice, a Closed flag that lies about whether the mesh is a solid, and using tessellation entities in an IFC2x3 file where they do not exist. Covers IfcCartesianPointList3D and IfcCartesianPointList2D, IfcTessellatedFaceSet, IfcTriangulatedFaceSet (Coordinates, Normals, Closed, CoordIndex, PnIndex), IfcPolygonalFaceSet, IfcIndexedPolygonalFace, IfcIndexedPolygonalFaceWithVoids, the PnIndex indirection layer, the RepresentationType Tessellation mapping, and the IFC4 / IFC4.3 version differences. Keywords: IFC tessellation, IfcTriangulatedFaceSet, IfcPolygonalFaceSet, IfcTessellatedFaceSet, IfcCartesianPointList3D, IfcCartesianPointList2D, IfcIndexedPolygonalFace, IfcIndexedPolygonalFaceWithVoids, CoordIndex, PnIndex, Normals, Closed, triangle mesh, polygon mesh, faceted geometry, indexed mesh, mesh looks wrong, holes in mesh, flipped triangles, geometry not rendering, how do I store a mesh, Reference View, RepresentationType Tessellation, IFC4, IFC4.3.
license	MIT
compatibility	Designed for Claude Code. Requires IFC IFC4, IFC4.3.
metadata	{"author":"OpenAEC-Foundation","version":"1.0"}

IFC Syntax : Geometry Tessellation

Tessellation is faceted geometry : a mesh of flat triangles or polygons. It was introduced in IFC4 and is the geometric backbone of the Reference View MVD, because a consuming application can render a mesh with almost no implementation effort. IFC2x3 has no tessellation entities : its faceted equivalent is IfcFacetedBrep.

The design is index-based : one shared point pool holds every vertex once, and every face is a short list of integer indices into that pool. This makes a tessellated model far more compact than thousands of repeated point instances.

This skill covers IFC4 and IFC4.3. The differences between those two versions are small and listed in the Version Differences section.

Quick Reference

The indexed model

IfcCartesianPointList3D      the point pool : every vertex, once
        ^
        | Coordinates
IfcTriangulatedFaceSet       faces as integer-index rows into the pool
   or IfcPolygonalFaceSet
        ^
        | Items
IfcShapeRepresentation       RepresentationType = 'Tessellation'

ALWAYS store every vertex once in the point pool and reference it by index. NEVER repeat coordinate values per face : the index model exists to avoid that.

IfcCartesianPointList3D : the point pool

Verified verbatim (IFC4.3) :

ENTITY IfcCartesianPointList3D
 SUBTYPE OF (IfcCartesianPointList);
	CoordList : LIST [1:?] OF LIST [3:3] OF IfcLengthMeasure;
	TagList : OPTIONAL LIST [1:?] OF IfcLabel;
END_ENTITY;

• CoordList is an ordered list of points ; each point is exactly three IfcLengthMeasure values. Point N is CoordList[N], indexed from 1.
• TagList (optional) carries one IfcLabel per point. It is an IFC4.3 addition : in IFC4 the entity has only CoordList.
• IfcCartesianPointList2D is the 2D analogue (LIST [2:2] points). A tessellated face set always uses the 3D list ; the 2D list serves 2D index geometry such as IfcIndexedPolyCurve.

IfcTessellatedFaceSet : the abstract supertype

Verified verbatim (IFC4.3) :

ENTITY IfcTessellatedFaceSet
 ABSTRACT SUPERTYPE OF (ONEOF
	(IfcPolygonalFaceSet
	,IfcTriangulatedFaceSet))
 SUBTYPE OF (IfcTessellatedItem);
	Coordinates : IfcCartesianPointList3D;
 DERIVE
	Dim : IfcDimensionCount := 3;
 INVERSE
	HasColours : SET [0:1] OF IfcIndexedColourMap FOR MappedTo;
	HasTextures : SET [0:?] OF IfcIndexedTextureMap FOR MappedTo;
END_ENTITY;