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Skillsifc-syntax-geometry-tessellation

ifc-syntax-geometry-tessellation

Testing & quality

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Updated 10-15 22:18

About this skill

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.