| 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;
Coordinates is the shared IfcCartesianPointList3D point pool.Dim is DERIVED and always 3 : a tessellated face set is always 3D.IfcTessellatedFaceSet is abstract : NEVER instantiate it. Use
IfcTriangulatedFaceSet or IfcPolygonalFaceSet.
IfcTriangulatedFaceSet : a triangle mesh
Verified verbatim (IFC4.3) :
ENTITY IfcTriangulatedFaceSet
SUPERTYPE OF (ONEOF
(IfcTriangulatedIrregularNetwork))
SUBTYPE OF (IfcTessellatedFaceSet);
Normals : OPTIONAL LIST [1:?] OF LIST [3:3] OF IfcParameterValue;
Closed : OPTIONAL IfcBoolean;
CoordIndex : LIST [1:?] OF LIST [3:3] OF IfcPositiveInteger;
PnIndex : OPTIONAL LIST [1:?] OF IfcPositiveInteger;
DERIVE
NumberOfTriangles : IfcInteger := SIZEOF(CoordIndex);
END_ENTITY;
CoordIndex is the triangle list : every row is exactly three
IfcPositiveInteger indices. Each index resolves a vertex (directly into
Coordinates, or through PnIndex when that is present).Normals (optional) carries per-vertex normals for smooth shading.Closed : TRUE means the mesh forms a closed solid shell ; FALSE or omitted
means an open surface.PnIndex (optional) is the indirection layer : see below.IfcTriangulatedIrregularNetwork (a terrain TIN subtype) is an IFC4.3
addition : IFC4 IfcTriangulatedFaceSet has no subtype.
IfcPolygonalFaceSet : a polygon mesh
Verified verbatim (IFC4.3) :
ENTITY IfcPolygonalFaceSet
SUBTYPE OF (IfcTessellatedFaceSet);
Closed : OPTIONAL IfcBoolean;
Faces : LIST [1:?] OF UNIQUE IfcIndexedPolygonalFace;
PnIndex : OPTIONAL LIST [1:?] OF IfcPositiveInteger;
END_ENTITY;
Each entry in Faces is an IfcIndexedPolygonalFace, which may have any number of
vertices (3 or more), not just three.
ENTITY IfcIndexedPolygonalFace
SUPERTYPE OF (ONEOF
(IfcIndexedPolygonalFaceWithVoids))
SUBTYPE OF (IfcTessellatedItem);
CoordIndex : LIST [3:?] OF IfcPositiveInteger;
END_ENTITY;
ENTITY IfcIndexedPolygonalFaceWithVoids
SUBTYPE OF (IfcIndexedPolygonalFace);
InnerCoordIndices : LIST [1:?] OF LIST [3:?] OF UNIQUE IfcPositiveInteger;
END_ENTITY;
IfcIndexedPolygonalFace.CoordIndex is the outer loop : 3 or more indices.IfcIndexedPolygonalFaceWithVoids adds InnerCoordIndices : one index loop per
hole. ALWAYS use it for a face with a hole ; NEVER cut a hole by reusing the plain
IfcIndexedPolygonalFace.
PnIndex : the indirection layer
When PnIndex is absent, a CoordIndex value is a direct 1-based index into
Coordinates.CoordList.
When PnIndex is present, a CoordIndex value indexes into PnIndex, and the
PnIndex value then indexes into Coordinates.CoordList :
without PnIndex : CoordIndex value ----------------------> CoordList[value]
with PnIndex : CoordIndex value -> PnIndex[value] ------> CoordList[ PnIndex[value] ]
PnIndex lets several face sets share one coordinate pool, or compresses a model
where many faces reuse a small set of points. ALWAYS apply the indirection exactly
once when PnIndex is present. NEVER apply it when PnIndex is absent.
Mapping to a representation
A tessellated face set is a IfcRepresentationItem placed in an
IfcShapeRepresentation whose RepresentationType is 'Tessellation' and whose
RepresentationIdentifier is normally 'Body'. See
ifc-syntax-geometry-representations.
Version differences (compact)
| Item | IFC2x3 | IFC4 | IFC4.3 |
|---|
| Tessellation entities | absent | present | present |
IfcCartesianPointList3D.TagList | n/a | absent | present |
IfcCartesianPointList2D.TagList | n/a | absent | present |
IfcTriangulatedIrregularNetwork | n/a | absent | present |
CoordIndex / PnIndex / Normals / Closed semantics | n/a | identical | identical |
In IFC2x3 the faceted equivalent is IfcFacetedBrep with IfcPolyLoop faces : see
ifc-syntax-geometry-representations and the Brep skill.
Decision Trees
Triangulated or polygonal face set?
Is every face a triangle?
|
+-- YES --> IfcTriangulatedFaceSet. CoordIndex rows are LIST [3:3] : always 3.
| Cheapest to render ; the Reference View default mesh form.
|
+-- NO, faces have varying vertex counts, or some faces have holes
--> IfcPolygonalFaceSet with IfcIndexedPolygonalFace entries.
A face with a hole --> IfcIndexedPolygonalFaceWithVoids.
Do you need PnIndex?
Does CoordIndex index straight into the point pool?
|
+-- YES, one face set, no shared pool, no compression
| --> omit PnIndex. CoordIndex values index Coordinates.CoordList directly.
|
+-- NO, several face sets share one pool, or you compress repeated points
--> include PnIndex. CoordIndex -> PnIndex -> CoordList. Apply once.
Tessellation or Brep?
Which schema is the file?
|
+-- IFC4 or IFC4.3 --> tessellation (IfcTriangulatedFaceSet / IfcPolygonalFaceSet).
|
+-- IFC2x3 --> tessellation does NOT exist. Use IfcFacetedBrep with IfcPolyLoop.
Patterns
Pattern : Build a triangulated face set
ALWAYS store every vertex once in an IfcCartesianPointList3D and list each triangle
as a three-index row in CoordIndex.
ALWAYS use 1-based indices : index 1 is the first point in CoordList. IFC
index lists are IfcPositiveInteger and start at 1.
NEVER write a CoordIndex row with two or four indices : the type is LIST [3:3],
exactly three per triangle.
Pattern : Build a polygonal face set with a hole
ALWAYS model a multi-sided face as an IfcIndexedPolygonalFace whose CoordIndex
lists the outer loop (3 or more indices).
ALWAYS model a face that has a hole as an IfcIndexedPolygonalFaceWithVoids : the
outer loop in CoordIndex, each hole as one loop in InnerCoordIndices.
NEVER represent a hole by connecting the outer and inner loops with a bridge edge in
a plain IfcIndexedPolygonalFace : that is a Brep workaround, not the tessellation
model.
Pattern : Apply PnIndex correctly
ALWAYS check whether PnIndex is present before resolving a CoordIndex value.
ALWAYS resolve through one indirection when PnIndex is present :
CoordList[ PnIndex[ CoordIndex_value ] ].
NEVER apply PnIndex when it is absent, and NEVER apply it twice : a double
indirection scrambles the mesh.
Pattern : Set Closed honestly
ALWAYS set Closed = .T. only when the mesh is a watertight, closed solid shell.
ALWAYS set Closed = .F. or omit it for an open surface (a single panel, a terrain
patch).
NEVER set Closed = .T. on an open surface : a consumer that trusts the flag will
compute a wrong volume or attempt invalid solid operations.
Pattern : Place tessellation in a representation
ALWAYS put a tessellated face set into an IfcShapeRepresentation with
RepresentationType = 'Tessellation' and, for a solid body,
RepresentationIdentifier = 'Body'.
NEVER label a tessellated representation 'SweptSolid' or 'Brep' : the
RepresentationType MUST match the geometry kind so consumers dispatch correctly.
Pattern : Honour the version differences
ALWAYS target IFC4 or IFC4.3 for tessellation : NEVER write IfcTriangulatedFaceSet
or IfcPolygonalFaceSet in an IFC2x3 file, where the entities do not exist.
ALWAYS write IfcCartesianPointList3D with only CoordList for an IFC4 file ; add
TagList only when the target is IFC4.3.
NEVER write IfcTriangulatedIrregularNetwork in an IFC4 file : that subtype is an
IFC4.3 addition.
Reference Links
references/methods.md : the complete verified entity definitions for the point
lists and every tessellated face set entity, the PnIndex resolution algorithm,
index-base and winding rules, and the per-version difference matrix.references/examples.md : worked STEP scenarios : a triangulated cube, a polygonal
face set, a face with a void, a PnIndex shared pool, and the enclosing
IfcShapeRepresentation.references/anti-patterns.md : the tessellation failure modes this skill prevents,
each with the reason it fails.
Official sources
Related skills
ifc-syntax-geometry-representations : IfcShapeRepresentation,
RepresentationType values, and the representation context.ifc-core-mvd : the Reference View, whose mesh geometry is tessellation-based.
