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:
|k+1||number of control points|
NOTE Entity adapted from rational_b_spline_curve in ISO 10303-42.
HISTORY New entity in IFC4.
|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.
|Dim||INVALID||The space dimensionality of this abstract class, defined differently for all subtypes, i.e. for IfcLine, IfcConic and IfcBoundedCurve.|
|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||INVALID||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.|
|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.|
|9||WeightsData||LIST [2:?] OF IfcReal||The supplied values of the weights.|
|Weights||INVALID||The array of weights associated with the control points. This is derived from the weights data.|
ENTITY IfcRationalBSplineCurveWithKnots SUBTYPE OF (IfcBSplineCurveWithKnots); WeightsData : LIST [2:?] OF IfcReal; DERIVE Weights : ARRAY [0:UpperIndexOnControlPoints] OF IfcReal := IfcListToArray(WeightsData,0,SELF\IfcBSplineCurve.UpperIndexOnControlPoints); WHERE SameNumOfWeightsAndPoints : SIZEOF(WeightsData) = SIZEOF(SELF\IfcBSplineCurve.ControlPointsList); WeightsGreaterZero : IfcCurveWeightsPositive(SELF); END_ENTITY;