IfcOffsetCurve3D

Definition from ISO/CD 10303-42:1992: An offset curve 3d is a curve at a constant distance from a basis curve in three-dimensional space. 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. The offset curve at any point (parameter) on the basis curve is in the direction V x T where V is the fixed reference direction and T is the unit tangent to the basis curve. For the offset direction to be well defined, T shall not at any point of the curve be in the same, or opposite, direction as V.

> NOTE: The offset curve 3d 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 3d takes its parameterization from the basis curve. The offset curve 3d is parameterized 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 three-dimensional.

> NOTE Corresponding STEP entity: offset_curve_3d, Please refer to ISO/IS 10303-42:1994, p.66 for the final definition of the formal standard.

> HISTORY New entity in IFC Release 2.x

Informal propositions:

1. At no point on the curve shall ref direction be parallel, or opposite to, the direction of the tangent vector.

EXPRESS specification:

ENTITY IfcOffsetCurve3D
SUBTYPE OF ( | IfcCurve);
BasisCurve | : | IfcCurve;
Distance | : | IfcLengthMeasure;
SelfIntersect | : | LOGICAL;
RefDirection | : | IfcDirection;
WHERE
WR1 | : | BasisCurve.Dim = 3;
END_ENTITY;

Attribute definitions:

BasisCurve | : | The curve that is being offset.
Distance | : | The distance of the offset curve from the basis curve. The distance may be positive, negative or zero.
SelfIntersect | : | An indication of whether the offset curve self-intersects, this is for information only.
RefDirection | : | The direction used to define the direction of the offset curve 3d from the basis curve.

Formal Propositions:

WR1 | : | The underlying curve shall be defined in three-dimensional space.

Inheritance graph

ENTITY IfcOffsetCurve3D;
ENTITY IfcRepresentationItem;
INVERSE
LayerAssignments | : | SET OF IfcPresentationLayerAssignment FOR AssignedItems;
StyledByItem | : | SET [0:1] OF IfcStyledItem FOR Item;
ENTITY IfcGeometricRepresentationItem;
ENTITY IfcCurve;
DERIVE
Dim | : | IfcDimensionCount := IfcCurveDim(SELF);
ENTITY IfcOffsetCurve3D;
BasisCurve | : | IfcCurve;
Distance | : | IfcLengthMeasure;
SelfIntersect | : | LOGICAL;
RefDirection | : | IfcDirection;
END_ENTITY;