IfcOffsetCurve2D
Definition from ISO/CD 1030342:1992: An offset curve 2d (IfcOffsetCurve2d) is a curve at a constant distance from a basis curve in twodimensional space. This entity defines a simple planeoffset curve by offsetting by distance along the normal to basis curve in the plane of basis curve. The underlying curve shall have a welldefined tangent direction at every point. In the case of a composite curve, the transition code between each segment shall be cont same gradient or cont same gradient same curvature.
NOTE: The offset curve 2d may differ in nature from the basis curve; the offset of a non self intersecting curve can be selfintersecting. Care should be taken to ensure that the offset to a continuous curve does not become discontinuous.
The offset curve 2d takes its parametrisation from the basis curve. The offset curve 2d is parametrised as
where T is the unit tangent vector to the basis curve C(u) at parameter value u, and d is distance. The underlying curve shall be twodimensional.
NOTE Corresponding STEP entity: offset_curve_2d, Please refer to ISO/IS 1030342:1994, p.65 for the final definition of the formal standard.
HISTORY New entity in IFC Release 2.x
EXPRESS specification:

BasisCurve  :  The curve that is being offset. 
Distance  :  The distance of the offset curve from the basis curve. distance may be positive, negative or zero. A positive value of distance defnes an offset in the direction which is normal to the curve in the sense of an anticlockwise rotation through 90 degrees from the tangent vector T at the given point. (This is in the direction of orthogonal complement(T).) 
SelfIntersect  :  An indication of whether the offset curve selfintersects; this is for information only. 
WR1  :  The underlying curve shall be defned in twodimensional space. 
Name  Type  Referred through  ExpressG 
IfcCurve  Entity 

Diagram 5 
