IfcOffsetCurve2D

IfcOffsetCurve2D是由二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2d是二维空间中与基本曲线保持恒定距离的曲线。此图元通过在基本曲线平面中沿法线到基本曲线的距离偏移来定义简单的平面偏移曲线。基础曲线应在每个点处具有明确的切线方向。对于复合曲线，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。

注：偏移曲线2d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2d从基本曲线获取其参数化。偏移曲线2d参数化为：

式中，T是参数值u处基本曲线C（u）的单位切线向量，d是距离。基本曲线应为二维曲线。

注：实体改编自ISO 10303-42中定义的偏移曲线_2d。

IFC2x中增加的新实体

Formal Propositions

Rule	Description
DimIs2D	The underlying curve shall be defined in two-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
IfcOffsetCurve2D
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. distance may be positive, negative or zero. A positive value of distance defines an offset in the direction which is normal to the curve in the sense of an anti-clockwise rotation through 90 degrees from the tangent vector T at the given point. (This is in the direction of orthogonal complement(T).)	X
3	SelfIntersect	IfcLogical	[1:1]	An indication of whether the offset curve self-intersects; this is for information only.	X

EXPRESS Specification

ENTITY IfcOffsetCurve2D
 SUBTYPE OF (IfcCurve);
  BasisCurve : IfcCurve;
  Distance : IfcLengthMeasure;
  SelfIntersect : IfcLogical;
 WHERE
  DimIs2D : BasisCurve.Dim = 2;
END_ENTITY;