8.20.3.11 IfcLoop

8.20.3.11.1 Semantic definition

NOTE Definition according to ISO/CD 10303-42:1992 A loop is a topological entity constructed from a single vertex, or by stringing together connected (oriented) edges, or linear segments beginning and ending at the same vertex. It is typically used to bound a face lying on a surface. A loop has dimensionality of 0 or 1. The domain of a 0-dimensional loop is a single point. The domain of a 1-dimensional loop is a connected, oriented curve, but need not to be manifold. As the loop is a circle, the location of its beginning/ending point is arbitrary. The domain of the loop includes its bounds, an 0 ≤ Ξ < ∞. A loop is represented by a single vertex, or by an ordered collection of oriented edges, or by an ordered collection of points. A loop is a graph, so M and the graph genus G^l^ may be determined by the graph traversal algorithm. Since M = 1, the Euler equation (1) reduces in this case to

where V and E~l~ are the number of unique vertices and oriented edges in the loop and G^l^ is the genus of the loop.
Figure 8.20.3.11.A

Informal Propositions

A loop has a finite extent.
A loop describes a closed (topological) curve with coincident start and end vertices.

8.20.3.11.2 Entity inheritance

8.20.3.11.3 Attributes

#	Attribute	Type	Description
IfcRepresentationItem (2)
	LayerAssignment	SET [0:1] OF IfcPresentationLayerAssignment FOR AssignedItems	Assignment 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.

Table 8.20.3.11.B

8.20.3.11.4 Formal representation

ENTITY IfcLoop
 SUPERTYPE OF (ONEOF
	(IfcEdgeLoop
	,IfcPolyLoop
	,IfcVertexLoop))
 SUBTYPE OF (IfcTopologicalRepresentationItem);
END_ENTITY;