IfcRationalBSplineCurveWithKnots

带节有理B样条曲线是用控制点和基本函数描述的B样条曲线。它描述了除了在超类型ifcbsprinecurve中定义的控制点之外的权重。

所有权重应为正，曲线由以下公式给出：

 

where

k+1	number of control points
Pi	control points
wi	weights
d	degree

 

 

 

 

 

注：实体改编自ISO 10303-42中的有理曲线。

IFC4中增加的新实体。

Formal Propositions

Rule	Description
SameNumOfWeightsAndPoints	There shall be the same number of weights as control points.
WeightsGreaterZero	All the weights shall have values greater than 0.0.

 

 

 

 

#	Attribute	Type	Cardinality	Description	C
IfcRepresentationItem
	LayerAssignment	IfcPresentationLayerAssignment @AssignedItems	S[0:1]	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.	X
	StyledByItem	IfcStyledItem @Item	S[0:1]	Reference to the IfcStyledItem that provides presentation information to the representation, e.g. a curve style, including colour and thickness to a geometric curve.	X
IfcGeometricRepresentationItem
IfcCurve
	Dim :=IfcCurveDim(SELF)	IfcDimensionCount	[1:1]	The space dimensionality of this abstract class, defined differently for all subtypes, i.e. for IfcLine, IfcConic and IfcBoundedCurve.	X
IfcBoundedCurve
IfcBSplineCurve
1	Degree	IfcInteger	[1:1]	The algebraic degree of the basis functions.	X
2	ControlPointsList	IfcCartesianPoint	L[2:?]	The list of control points for the curve.	X
3	CurveForm	IfcBSplineCurveForm	[1:1]	Used to identify particular types of curve; it is for information only.	X
4	ClosedCurve	IfcLogical	[1:1]	Indication of whether the curve is closed; it is for information only.	X
5	SelfIntersect	IfcLogical	[1:1]	Indication whether the curve self-intersects or not; it is for information only.	X
	UpperIndexOnControlPoints :=(SIZEOF(ControlPointsList) - 1)	IfcInteger	[1:1]	The upper index on the array of control points; the lower index is 0. This value is derived from the control points list.	X
	ControlPoints :=IfcListToArray(ControlPointsList,0,UpperIndexOnControlPoints)	IfcCartesianPoint	A[0:UpperIndexOnControlPoints]	The array of control points used to define the geometry of the curve. This is derived from the list of control points.	X
IfcBSplineCurveWithKnots
6	KnotMultiplicities	IfcInteger	L[2:?]	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.	X
7	Knots	IfcParameterValue	L[2:?]	The list of distinct knots used to define the B-spline basis functions.	X
8	KnotSpec	IfcKnotType	[1:1]	The description of the knot type. This is for information only.	X
	UpperIndexOnKnots :=SIZEOF(Knots)	IfcInteger	[1:1]	The upper index on the knot arrays; the lower index is 1.	X
IfcRationalBSplineCurveWithKnots
9	WeightsData	IfcReal	L[2:?]	The supplied values of the weights.	X
	Weights :=IfcListToArray(WeightsData,0,SELF\IfcBSplineCurve.UpperIndexOnControlPoints)	IfcReal	A[0:UpperIndexOnControlPoints]	The array of weights associated with the control points. This is derived from the weights data.	X

EXPRESS Specification

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;