IFC (IFC4X3_ADD2) under development IfcRationalBSplineCurveWithKnots Semantic definition

A rational B-spline curve with knots is a B-spline curve described in terms of control points and basic functions. It describes weights in addition to the control points defined at the supertype IfcBSplineCurve.

All weights shall be positive and the curve is given by:



Variable Definition
k+1 number of control points
Pi control points
wi weights
d degree
Table Entity inheritance 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.

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.

IfcCurve (1)
* Dim IfcDimensionCount

This attribute is formally derived.


The space dimensionality of this abstract class, defined differently for all subtypes, i.e. for IfcLine, IfcConic and IfcBoundedCurve.

IfcBSplineCurve (7)
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.

* UpperIndexOnControlPoints IfcInteger

This attribute is formally derived.

(SIZEOF(ControlPointsList) - 1)

The upper index on the array of control points; the lower index is 0. This value is derived from the control points list.

* ControlPoints ARRAY [0:UpperIndexOnControlPoints] OF IfcCartesianPoint

This attribute is formally derived.


The array of control points used to define the geometry of the curve. This is derived from the list of control points.

IfcBSplineCurveWithKnots (4)
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 IfcInteger

This attribute is formally derived.


The upper index on the knot arrays; the lower index is 1.

Click to show 14 hidden inherited attributes Click to hide 14 inherited attributes
IfcRationalBSplineCurveWithKnots (2)
9 WeightsData LIST [2:?] OF IfcReal

The supplied values of the weights.

* Weights ARRAY [0:UpperIndexOnControlPoints] OF IfcReal

This attribute is formally derived.


The array of weights associated with the control points. This is derived from the weights data.

Table Formal propositions

Name Description

There shall be the same number of weights as control points.

SIZEOF(WeightsData) = SIZEOF(SELF\IfcBSplineCurve.ControlPointsList)

All the weights shall have values greater than 0.0.

Table Formal representation

ENTITY IfcRationalBSplineCurveWithKnots
 SUBTYPE OF (IfcBSplineCurveWithKnots);
	WeightsData : LIST [2:?] OF IfcReal;
	 Weights : ARRAY [0:UpperIndexOnControlPoints] OF IfcReal := IfcListToArray(WeightsData,0,SELF\IfcBSplineCurve.UpperIndexOnControlPoints);
	SameNumOfWeightsAndPoints : SIZEOF(WeightsData) = SIZEOF(SELF\IfcBSplineCurve.ControlPointsList);
	WeightsGreaterZero : IfcCurveWeightsPositive(SELF);
END_ENTITY; References

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