An IfcFixedReferenceSweptAreaSolid is a type of swept area solid which is the result of sweeping an area along a Directrix. The swept area is provided by a subtype of IfcProfileDef. The profile is placed by an implicit cartesian transformation operator at the start point of the sweep, where the profile normal agrees to the tangent of the directrix at this point, and the profile''s x-axis agrees to the FixedReference direction. The orientation of the curve during the sweeping operation is controlled by the FixedReference direction.

The SweptArea is swept along the Directrix in such a way that the origin of the local coordinate system used to define the SweptArea is on the Directrix and the local X axis is in the direction of the projection of FixedReference onto the normal plane to the directrix at this point. The resulting solid has the property that the cross section of the surface by the normal plane to the Directrix at any point is a copy of the SweptArea. The resulting swept solid is placed by the Position coordinate system.

The Directrix and the ReferenceSurface are positioned within the object coordinate system. The start of the sweeping operation is at the StartParam, the parameter value is provided based on the curve parameterization. If no StartParam is provided the start defaults to the begin of the directrix. The end of the sweeping operation is at the EndParam, the parameter value is provided based on the curve parameterization. If no EndParam is provided the end defaults to the end of the directrix.

NOTE  The StartParam and the EndParam are not normalized by default, they depend upon the parameterization of the curve. However using the IfcReparametrisedCompositeCurveSegment within an IfcCompositeCurve as the directrix allows to explicitly reparameterize the underlying sweeping curve.

EXAMPLE  The reference surface is any surface (plane, cylindric, composite) situated in 3D space and positioned in the object coordinate system. In many cases, it is a surface of extrusion. The directrix lies on the surface, often defined as a p-curve at this reference surface. At any point of the directrix, a plane can be constructed. The origin of the position coordinate system lies at the directrix. The Axis3 (the z-axis, or normal) of the position coordinate system is identical to the tangent of the directrix at this point, the Axis1 (the x axis, or u) of the position coordinate system is identical to the FixedReference direction. The Axis2 (the y axis, or v) is constructed. In this case the resulting swept solid is not repositioned.

The orientation of the SweptArea as it sweeps along the Directrix is precisely defined by a CartesianTransformationOperator3d with attributes:

* LocalOrigin as point (0; 0; 0),
* Axis1 as the FixedReference.
* Axis3 as the direction of the tangent vector **t** at the point of the Directrix with parameter **u**.

The remaining attributes are defaulted to define a corresponding transformation matrix **T(u)**, which varies with the Directrix parameter **u**.

NOTE  The geometric shape of the solid is not dependent upon the curve parameterization; the volume depends upon the area swept and the length of the Directrix.

NOTE  Entity adapted from **fixed_reference_swept_surface** defined in ISO 10303-42.

HISTORY  New entity in IFC4.

Informal Propositions:

1. The SweptArea shall lie in the plane z = 0.
2. The FixedReference shall not be parallel to a tangent vector to the directrix at any point along this curve.
3. The Directrix curve shall be tangent continuous.