IFC 4.3.x development

8.8.3.27 IfcPolygonalFaceSet

Change log

IFC4

added

8.8.3.27.1 Semantic definitions at the entity

The IfcPolygonalFaceSet is a tessellated face set with all faces being bound by polygons. The planar faces are constructed by implicit polylines defined by three or more Cartesian points. Each planar face is defined by an instance of IfcIndexedPolygonalFace, or in case of faces with inner loops by IfcIndexedPolygonalFaceWithVoids.

Depending on the value of the attribute Closed the instance of IfcPolygonalFaceSet represents:

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

The attribute Faces refers to a list of IfcIndexedPolygonalFace's having an one-based CoordIndex that provides three or more indices. If the face has inner loops, the IfcIndexedPolygonalFaceWithVoids has a two-dimensional, one-based list, where:* the first dimension of the two-dimensional list addresses the list of inner loops; * the second dimension of the two-dimensional list provides three or more indices each being a vertex of the planar polygon representing the inner loop.

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 1 for the handling of point index.

"PnIndex"

NOTE  If the IfcPolygonalFaceSet is closed, the face vertices of the IfcIndexedPolygonalFace's shall connect counterclock-wise when seen from the outside of the closed shell to guarantee that the face normal points away from the material of the shell. Inner loops, provided by the IfcIndexedPolygonalFaceWithVoids, shall connect clock-wise.

 

IfcFacetedFaceSet_Example-01

Figure 2 shows an IfcFacetedFaceSet represented by

#1 IfcIndexedPolygonalFace: ((1, 2, 3, 4) #2 IfcIndexedPolygonalFace: ((6, 2, 3, 7) #3 IfcIndexedPolygonalFace: ((7, 3, 4, 8) #4 IfcIndexedPolygonalFace: ((8, 4, 1, 5) #5 IfcIndexedPolygonalFace: ((1, 4, 3, 2) #6 IfcIndexedPolygonalFace: ((6, 7, 8, 5) #7 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 2 — Polygonal face set geometry

 

NOTE  The definition of IfcFacetedFaceSet is based on the indexedFaceSet defined in ISO/IEC 19775-1

HISTORY  New entity in IFC4 Addendum 2.

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 outer or inner boundaries of faces;
    • the topological normal at each point on the boundary of the polygonal face set shall point away from the material of the solid.

8.8.3.27.1 Attributes

# Attribute Type Description
IfcRepresentationItem
LayerAssignmentSET [0:1] OF IfcPresentationLayerAssignment FOR AssignedItemsAssignment of the representation item to a single or multiple layer(s). The LayerAssignments can override a LayerAssignments of the IfcRepresentation it is used within the list of Items.

> IFC2x3 CHANGE  The inverse attribute LayerAssignments has been added.

> IFC4 CHANGE  The inverse attribute LayerAssignment has been restricted to max 1. Upward compatibility for file based exchange is guaranteed.

StyledByItem SET [0:1] OF IfcStyledItem FOR Item Reference to the IfcStyledItem that provides presentation information to the representation, e.g. a curve style, including colour and thickness to a geometric curve.



> IFC2x3 CHANGE  The inverse attribute StyledByItem has been added.
IfcGeometricRepresentationItem
IfcTessellatedItem
IfcTessellatedFaceSet
1 Coordinates IfcCartesianPointList3D An ordered list of Cartesian points used by the coordinate index defined at the subtypes of IfcTessellatedFaceSet.
Dim INVALID The space dimensionality of this geometric representation item, it is always 3.
HasColours SET [0:1] OF IfcIndexedColourMap FOR MappedTo Reference to the indexed colour map providing the corresponding colour RGB values to the faces of the subtypes of IfcTessellatedFaceSet.
HasTextures SET [0:?] OF IfcIndexedTextureMap FOR MappedTo Reference to the indexed texture map providing the corresponding texture coordinates to the vertices bounding the faces of the subtypes of IfcTessellatedFaceSet.
IfcPolygonalFaceSet
2 Closed OPTIONAL IfcBoolean Indication whether the IfcPolygonalFaceSet is a closed shell or not. If omited no such information can be provided.
3 Faces LIST [1:?] OF IfcIndexedPolygonalFace The list of polygonal faces, with or without inner loops, that bound the faceted face set.
4 PnIndex OPTIONAL LIST [1:?] OF IfcPositiveInteger The list of integers defining the locations in the IfcCartesianPointList3D to obtain the point coordinates for the indices at the indexed polygonal faces. If the PnIndex is not provided the indices at the indexed polygonal faces point directly into the IfcCartesianPointList3D.

8.8.3.27.2 Entity inheritance

dot_inheritance IfcPolygonalFaceSet IfcPolygonalFaceSet IfcTessellatedFaceSet IfcTessellatedFaceSet IfcPolygonalFaceSet->IfcTessellatedFaceSet IfcTriangulatedFaceSet IfcTriangulatedFaceSet IfcTriangulatedFaceSet->IfcTessellatedFaceSet IfcTessellatedItem IfcTessellatedItem IfcTessellatedFaceSet->IfcTessellatedItem IfcIndexedPolygonalFace IfcIndexedPolygonalFace IfcIndexedPolygonalFace->IfcTessellatedItem IfcGeometricRepresentationItem IfcGeometricRepresentationItem IfcTessellatedItem->IfcGeometricRepresentationItem IfcGeometricRepresentationItem_children 24 more... IfcGeometricRepresentationItem_children->IfcGeometricRepresentationItem IfcRepresentationItem IfcRepresentationItem IfcGeometricRepresentationItem->IfcRepresentationItem IfcRepresentationItem_children 3 more... IfcRepresentationItem_children->IfcRepresentationItem

8.8.3.27.3 Formal representations

ENTITY IfcPolygonalFaceSet
 SUBTYPE OF (IfcTessellatedFaceSet);
	Closed : OPTIONAL IfcBoolean;
	Faces : LIST [1:?] OF IfcIndexedPolygonalFace;
	PnIndex : OPTIONAL LIST [1:?] OF IfcPositiveInteger;
END_ENTITY;