IFC 4.3.x development

8.9.3.6 IfcBSplineCurveWithKnots

Change log

IFC4

added

8.9.3.6.1 Semantic definitions at the entity

The IfcBSplineCurveWithKnots is a spline curve parameterized by spline functions for which the knot values are explicitly given.

NOTE  Definition according to ISO/CD 10303-42:1992
This is the type of b-spline curve for which the knot values are explicitly given. This subtype shall be used to represent non-uniform B-spline curves and may be used for other knot types.
Let L denote the number of distinct values amongst the d+k+2 knots in the knot list; L will be referred to as the ‘upper index on knots’. Let m~j~ denote the multiplicity (i.e., number of repetitions) of the _j_th distinct knot. Then:

formula

All knot multiplicities except the first and the last shall be in the range 1,...,d; the first and last may have a maximum value of d + 1. In evaluating the basis functions, a knot u of, e.g., multiplicity 3 is interpreted as a sequence u, u, u,; in the knot array.

NOTE  Entity adapted from b_spline_curve_with_knots defined in ISO10303-42.

HISTORY  New entity in IFC4.

8.9.3.6.1 Attributes

# Attribute Type Description
IfcRepresentationItem
LayerAssignment SET [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
IfcCurve
Dim INVALID The space dimensionality of this abstract class, defined differently for all subtypes, i.e. for IfcLine, IfcConic and IfcBoundedCurve.
IfcBoundedCurve
IfcBSplineCurve
1 Degree IfcInteger The algebraic degree of the basis functions.
2 ControlPointsList LIST [2:?] OF IfcCartesianPoint The list of control points for the curve.
3 CurveForm IfcBSplineCurveForm Used to identify particular types of curve; it is for information only.
4 ClosedCurve IfcLogical Indication of whether the curve is closed; it is for information only.
5 SelfIntersect IfcLogical Indication whether the curve self-intersects or not; it is for information only.
UpperIndexOnControlPointsINVALID The upper index on the array of control points; the lower index is 0.

This value is derived from the control points list.
ControlPoints INVALID The array of control points used to define the geometry of the curve. This is derived from the list of control points.
IfcBSplineCurveWithKnots
6 KnotMultiplicities LIST [2:?] OF IfcInteger The multiplicities of the knots. This list defines the number of times each knot in the knots list is to be repeated in constructing the knot array.
7 Knots LIST [2:?] OF IfcParameterValue The list of distinct knots used to define the B-spline basis functions.
8 KnotSpec IfcKnotType The description of the knot type. This is for information only.
UpperIndexOnKnots INVALID The upper index on the knot arrays; the lower index is 1.

8.9.3.6.2 Entity inheritance

dot_inheritance IfcRationalBSplineCurveWithKnots IfcRationalBSplineCurveWithKnots IfcBSplineCurveWithKnots IfcBSplineCurveWithKnots IfcRationalBSplineCurveWithKnots->IfcBSplineCurveWithKnots IfcBSplineCurve IfcBSplineCurve IfcBSplineCurveWithKnots->IfcBSplineCurve IfcBoundedCurve IfcBoundedCurve IfcBSplineCurve->IfcBoundedCurve IfcBoundedCurve_children 4 more... IfcBoundedCurve_children->IfcBoundedCurve IfcCurve IfcCurve IfcBoundedCurve->IfcCurve IfcCurve_children 7 more... IfcCurve_children->IfcCurve IfcGeometricRepresentationItem IfcGeometricRepresentationItem IfcCurve->IfcGeometricRepresentationItem IfcGeometricRepresentationItem_children 24 more... IfcGeometricRepresentationItem_children->IfcGeometricRepresentationItem IfcRepresentationItem IfcRepresentationItem IfcGeometricRepresentationItem->IfcRepresentationItem IfcRepresentationItem_children 3 more... IfcRepresentationItem_children->IfcRepresentationItem

8.9.3.6.3 Formal representations

ENTITY IfcBSplineCurveWithKnots
 SUPERTYPE OF (ONEOF
	(IfcRationalBSplineCurveWithKnots))
 SUBTYPE OF (IfcBSplineCurve);
	KnotMultiplicities : LIST [2:?] OF IfcInteger;
	Knots : LIST [2:?] OF IfcParameterValue;
	KnotSpec : IfcKnotType;
 DERIVE
	 UpperIndexOnKnots : IfcInteger := SIZEOF(Knots);
 WHERE
	ConsistentBSpline : IfcConstraintsParamBSpline(Degree, UpperIndexOnKnots,
UpperIndexOnControlPoints, KnotMultiplicities, Knots);
	CorrespondingKnotLists : SIZEOF(KnotMultiplicities) = UpperIndexOnKnots;
END_ENTITY;