8.9.3.68 IfcSurfaceCurve

8.9.3.68.1 Semantic definition

An IfcSurfaceCurve is a 3-dimensional curve that has additional representations provided by one or two pcurves.

NOTE Definition according to ISO/CD 10303-42:1992 A surface curve is a type of curve, that is a curve on a surface. The curve is represented as a curve (curve_3d) in three-dimensional space and possibly as a curve, corresponding to a pcurve, in the two-dimensional parametric space of a surface. The ability of this curve to reference a list of 1 or 2 pcurves enables this entity to define either a curve on a single surface, or an intersection curve which has two distinct surface associations. A `seam' on a closed surface can also be represented by this entity; in this case each associated_geometry will be a pcurve lying on the same surface. Each pcurve shall be parametrised to have the same sense as curve_3d. The surface curve takes its parametrisation directly from either curve_3d or pcurve as indicated by the attribute master representation.

Informal Propositions

Where Curve3D and one or two pcurves exist they shall represent the same mathematical point set. (i.e., they shall coincide geometrically but may differ in parametrisation.)
The Curve3D and any associated pcurves shall agree with respect to their senses.

8.9.3.68.2 Entity inheritance

8.9.3.68.3 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.
IfcCurve (1)
*	Dim	IfcDimensionCount	This attribute is formally derived. `IfcCurveDim(SELF)` The space dimensionality of this abstract class, defined differently for all subtypes, i.e. for IfcLine, IfcConic and IfcBoundedCurve.
Click to show 3 hidden inherited attributes Click to hide 3 inherited attributes
IfcSurfaceCurve (4)
1	Curve3D	IfcCurve	The curve which is the three-dimensional representation of the surface curve.
2	AssociatedGeometry	LIST [1:2] OF IfcPcurve	A list of one or two pcurves which define the surface or surfaces associated with the surface curve. Two elements in this list indicate that the curve has two surface associations which need not be two distinct surfaces. Being a pcurve, it also associates a basis curve in the parameter space of this surface as an alternative representation of the surface curve.
3	MasterRepresentation	IfcPreferredSurfaceCurveRepresentation	The <em<MasterRepresentation defines the curve used to determine the unique parametrisation of the IfcSurfaceCurve. The master_representation takes one of the values Curve3D, PCurve_S1 or PCurve_S2 to indicate a preference for the 3D curve, or the first or second pcurve, in the associated geometry list, respectively. Multiple representations provide the ability to communicate data in more than one form, even though the data is expected to be geometrically identical. NOTE The master representation attribute acknowledges the impracticality of ensuring that multiple forms are indeed identical and allows the indication of a preferred form. This would probably be determined by the creator of the data. All characteristics, such as parametrisation, domain, and results of evaluation, for an entity having multiple representations, are derived from the master representation. Any use of the other representations is a compromise for practical considerations.
*	BasisSurface	SET [1:2] OF IfcSurface	This attribute is formally derived. `IfcGetBasisSurface(SELF)` The surface, or surfaces on which the IfcSurfaceCurve lies. This is determined from the AssociatedGeometry list.

Table 8.9.3.68.A

8.9.3.68.4 Formal propositions

Name	Description
CurveIs3D	The Curve2D shall be defined in three-dimensional space.
`Curve3D.Dim = 3`
CurveIsNotPcurve	The Curve3D shall not be a pcurve.
`NOT ('IFC4X3_DEV_20bdb3d.IFCPCURVE' IN TYPEOF(Curve3D))`

Table 8.9.3.68.B

8.9.3.68.5 Formal representation

ENTITY IfcSurfaceCurve
 SUPERTYPE OF (ONEOF
	(IfcIntersectionCurve
	,IfcSeamCurve))
 SUBTYPE OF (IfcCurve);
	Curve3D : IfcCurve;
	AssociatedGeometry : LIST [1:2] OF IfcPcurve;
	MasterRepresentation : IfcPreferredSurfaceCurveRepresentation;
 DERIVE
	 BasisSurface : SET [1:2] OF IfcSurface := IfcGetBasisSurface(SELF);
 WHERE
	CurveIs3D : Curve3D.Dim = 3;
	CurveIsNotPcurve : NOT ('IFC4X3_DEV_20bdb3d.IFCPCURVE' IN TYPEOF(Curve3D));
END_ENTITY;