The IfcTriangulatedFaceSet is a tessellated face set with all faces being bound by triangles. The faces are constructed by implicit polylines defined by three Cartesian points. Depending on the value of the attribute Closed the instance of IfcTriangulatedFaceSet represents:

• if TRUE, a boundary represention (or B-rep);
• if FALSE, a face based surface representation.

The coordinates of each point are provided by a one-based index into an ordered list of Cartesian points provided by the two-dimensional list CoordIndex, where

• the first dimension of the two-dimensional list addresses the list of triangular faces;
• the second dimension of the two-dimensional list provides exactly three indices. Each index points to a Cartesian point being a vertex of the triangle.

Depending on the provision of PnIndex the indices point either directly into the IfcCartesianPointList3D referenced by Coordinates defined at the supertype IfcTessellatedFaceSet, or they point into the PnIndex where the integer values at that position provides the location of the coordinate values within the IfcCartesianPointList3D. See Figure 352 for the handling of point index.

Figure 352 — Use of PnIndex

See Figure 353 for using the IfcTriangulatedFaceSet without (as default) and with using the PnIndex.

Figure 353 — Triangulated face set

Optionally normals can be provided. Only normals per vector are supported allowing for visually curved triangles when the three normals of the corresponding three vertices of a triangle are not co-linear to the normal of the triangular face. If they are identical, normals shall be omited.

NOTE  In case of a body representation having triangular faces with curvature, different normals for all three vertices in context of this triangles are exported, describing the curvature for smooth visualisation. In case of triangular faces with no curvature, the three vertex-based normals are identical and could always be computed from the face. Therefore vertices should only be exported to support smooth visualisation by curvature of triangles. In case of faceted representation no vertices should be exported.

Normals are provided by the two-dimensional list Normals, where

• the first dimension of the two-dimensional list addresses the corresponding list of vertex indices of the triangular faces;
• the second dimension of the two-dimensional list provides exactly three real values to determine the direction of the normal at this vertex of the triangle.

The indices in the CoordIndex point into the list of Normals. The indices of CoordIndex are applied to both points, either directly directly, or via the PnIndex and normals. See Figure 354 for handling of normals.

Figure 354 — Triangulated face set with normals

NOTE  Using corresponding lists of points and normals allows to use only a single list of indices into both at the expense of having non-unique collections of vertices.

Figure 355 shows the use of IfcTriangulatedFaceSet without annotation. The diagram of the IfcTriangulatedFaceSet represents the indices and the ordered list into which the indices point. The index starts with 1 (indexed as 1 to N), if the greatest index in CoordIndex in N, then the IfcCartesianPointList shall have N lists of 3:3 coordinates.

	Figure 168 shows an IfcTriangulatedFaceSet represented by CoordIndex: ((1,6,5), (1,2,6), (6,2,7), (7,2,3), (7,8,6), (6,8,5), (5,8,1), (1,8,4), (4,2,1), (2,4,3), (4,8,7), (7,3,4)) IfcCartesianPointList3D: ((0.,0.,0.), (1.,0.,0.), (1.,1.,0.), (0.,1.,0.), (0.,0.,2.), (1.,0.,2.), (1.,1.,2.), (0.,1.,2.))
Figure 355 — Triangulated face set geometry

NOTE  The definition of IfcTriangulatedFaceSet is based on the indexedFaceSet and indexedTriangleSet defined in ISO/IEC 19775-1. The use of PnIndex has been adopted from triangulated_surface_set defined in ISO-10303-42.

HISTORY  New entity in IFC4.

Informal Propositions:

1. If Closed = TRUE, the same topological constraints as imposed by an faceted boundary representation shall be safisfied. In particular:
   • each face shall be unique;
   • each edge shall be used by exactly two boundaries of faces;
   • the topological normal at each point on the boundary of the triangulated face set shall point away from the mater ial of the solid.