IfcOffsetCurve3D

IfcOffsetCurve3D是由三维空间中与其基本曲率的偏移量定义的曲线。

注：定义根据ISO/CD 10303-42:1992

偏移曲线3d是三维空间中与基本曲线保持恒定距离的曲线。基础曲线应在每个点处具有明确的切线方向。在复合曲线的情况下，每段之间的过渡代码应为连续相同坡度或相同坡度相同曲率。基准曲线上任何点（参数）的偏移曲线在V x T方向，其中V是固定的参考方向，T是与基准曲线相切的单位。对于要明确定义的偏移方向，T不得在曲线的任何点与V方向相同或相反。

注：偏移曲线3d在性质上可能不同于基准曲线；非自相交曲线的偏移可以是自相交的。应注意确保连续曲线的偏移不会变得不连续。

偏移曲线3d从基础曲线获取其参数化。偏移曲线3d参数化为：

T是参数值u处基本曲线C（u）的单位切线向量，d是距离。基础曲线应为三维曲线。

注：实体改编自ISO 10303-42中定义的偏移曲线_3d

IFC2x中增加的新实体

非正式提议：

在曲线上的任何点上，ref方向不得与切线向量的方向平行或相反。

Formal Propositions

Rule	Description
DimIs2D	The underlying curve shall be defined in three-dimensional space.

 

#	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
IfcOffsetCurve3D
1	BasisCurve	IfcCurve	[1:1]	The curve that is being offset.	X
2	Distance	IfcLengthMeasure	[1:1]	The distance of the offset curve from the basis curve. The distance may be positive, negative or zero.	X
3	SelfIntersect	IfcLogical	[1:1]	An indication of whether the offset curve self-intersects, this is for information only.	X
4	RefDirection	IfcDirection	[1:1]	The direction used to define the direction of the offset curve 3d from the basis curve.	X

EXPRESS Specification

ENTITY IfcOffsetCurve3D
 SUBTYPE OF (IfcCurve);
  BasisCurve : IfcCurve;
  Distance : IfcLengthMeasure;
  SelfIntersect : IfcLogical;
  RefDirection : IfcDirection;
 WHERE
  DimIs2D : BasisCurve.Dim = 3;
END_ENTITY;