8.9.3.21 IfcCompositeCurveOnSurface

8.9.3.21.1 Semantic definition

The IfcCompositeCurveOnSurface is a collection of segments, based on p-curves. i.e. a curve which lies on the basis of a surface and is defined in the parameter space of that surface. The p-curve segment is a special type of a composite curve segment and shall only be used to bound a surface.

There shall be at least positional continuity between adjacent segments. The parameterization of the composite curve is obtained from the accumulation of the parametric ranges of the segments. The first segment is parameterized from 0 to l~1~, and, for i ≥ 2, the i^th^ segment is parameterized from

where l~k~ is the parametric length (that is, the difference between maximum and minimum parameter values) of the k^th^ curve segment.
Figure 8.9.3.21.A

Entity adapted from composite_curve_on_surface defined in ISO 10303-42.

8.9.3.21.2 Entity inheritance

8.9.3.21.3 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.
IfcCompositeCurve (4)
1	Segments	LIST [1:?] OF IfcSegment	The component bounded curves, their transitions and senses. The transition attribute for the last segment defines the transition between the end of the last segment and the start of the first; this transition attribute may take the value discontinuous, which indicates an open curve. NOTE&nbsp:NOTE&nbsp: For IfcGradientCurve and IfcSegmentedReferenceCurve the segments define the interpolation between segment start and and segment end.
2	SelfIntersect	IfcLogical	Indication of whether the curve intersects itself or not; this is for information only.
*	NSegments	IfcInteger	This attribute is formally derived. `SIZEOF(Segments)` The number of component curves.
*	ClosedCurve	IfcLogical	This attribute is formally derived. `Segments[NSegments].Transition <> Discontinuous` Indication whether the curve is closed or not; this is derived from the transition code of the last segment.
Click to show 7 hidden inherited attributes Click to hide 7 inherited attributes
IfcCompositeCurveOnSurface (1)
*	BasisSurface	SET [0:1] OF IfcSurface	This attribute is formally derived. `IfcGetBasisSurface(SELF)` The surface on which the composite curve is defined.

Table 8.9.3.21.B

8.9.3.21.4 Formal propositions

Name	Description
SameSurface	The BasisSurface shall contain at least one surface (and exactly one surface). This ensures that all segments reference curves on the same surface.
`SIZEOF(BasisSurface) > 0`

Table 8.9.3.21.C

8.9.3.21.5 Formal representation

ENTITY IfcCompositeCurveOnSurface
 SUPERTYPE OF (ONEOF
	(IfcBoundaryCurve))
 SUBTYPE OF (IfcCompositeCurve);
 DERIVE
	 BasisSurface : SET [0:1] OF IfcSurface := IfcGetBasisSurface(SELF);
 WHERE
	SameSurface : SIZEOF(BasisSurface) > 0;
END_ENTITY;