IfcOffsetCurve2D

Definition from ISO/CD 10303-42:1992: An offset curve 2d (IfcOffsetCurve2d) is a curve at a constant distance from a basis curve in two-dimensional space. This entity defines a simple plane-offset curve by offsetting by distance along the normal to basis curve in the plane of basis curve. The underlying curve shall have a well-defined 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 self-intersecting. 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 two-dimensional.

NOTE Corresponding STEP entity: offset_curve_2d, Please refer to ISO/IS 10303-42:1994, p.65 for the final definition of the formal standard.

HISTORY New entity in IFC Release 2.x

EXPRESS specification:

 ENTITY IfcOffsetCurve2D
 SUBTYPE OF ( IfcCurve);
 BasisCurve : IfcCurve;
 Distance : IfcLengthMeasure;
 SelfIntersect : LOGICAL;
 WHERE
 WR1 : BasisCurve.Dim = 2;
 END_ENTITY;

Attribute definitions:

 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 anti-clockwise 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 self-intersects; this is for information only.

Formal Propositions:

 WR1 : The underlying curve shall be defned in two-dimensional space.

References (1):

Name Type Referred through Express-G
IfcCurve Entity
 Subtype
Diagram 5

Inheritance graph

 ENTITY IfcOffsetCurve2D;
 ENTITY IfcRepresentationItem;
 ENTITY IfcGeometricRepresentationItem;
 ENTITY IfcCurve;
 DERIVE
 Dim : IfcDimensionCount :=  IfcCurveDim(SELF);
 ENTITY IfcOffsetCurve2D;
 BasisCurve : IfcCurve;
 Distance : IfcLengthMeasure;
 SelfIntersect : LOGICAL;
 END_ENTITY;