8.8.3.26 IfcPolygonalBoundedHalfSpace

8.8.3.26.1 Semantic definition

The polygonal bounded half space is a special subtype of a half space solid, where the material of the half space used in Boolean expressions is bounded by a two-dimensional boundary. The base surface of the half space is positioned by its normal relative to the object coordinate system (as defined at the supertype IfcHalfSpaceSolid), and its boundary (with straight and arc segments) is defined in the XY plane of the position coordinate system established by the Position attribute, the subtraction body is extruded perpendicular to the XY plane of the position coordinate system, that is, into the direction of the positive Z axis defined by the Position attribute.

The boundary is defined by a 2 dimensional polyline (or 2 dimensional composite curve, consisting of straight segments and circular arc segments, or indexed poly curve) within the XY plane of the position coordinate system. The side of the surface which is in the half space is determined by the surface normal and the agreement flag. If the agreement flag is TRUE, then the subset is the one the normal points away from. If the agreement flag is FALSE, then the subset is the one the normal points into.

Figure 8.8.3.26.A illustrates a polygonal bounded half space.

Black coordinates indicate the object coordinate system (usually provided by IfcLocalPlacement).
Green coordinates indicate the position coordinate system; the PolygonalBoundary is given within this coordinate system. It is provided by IfcPolygonalBoundedHalfSpace.Position. This coordinate system is relative to the object coordinate system. The extrusion direction of the subtraction body is the positive Z axis.
Red coordinates indicate the normal of the plane. It is provided by the BaseSurface (IfcSurface.Position). This normal is also relative to the object coordinate system.

The polygonal bounded half space is used to limit the volume of the half space in Boolean difference expressions. Only the part that is defined by a theoretical intersection between the half space solid and an extruded area solid, defined by extruding the polygonal boundary, is used for Boolean expressions.

The PolygonalBoundary defines the 2D polyline which bounds the effectiveness of the half space in Boolean expressions. The BaseSurface is defined by a plane, and the normal of the plane together with the AgreementFlag defines the side of the material of the half space.

Informal Propositions:

The IfcPolyline or the IfcCompositeCurve providing the PolygonalBoundary shall be closed.
If the PolygonalBoundary is given by an IfcCompositeCurve, it shall only have IfcCompositeCurveSegment's of type IfcPolyline, or IfcTrimmedCurve (having a BasisCurve of type IfcLine, or IfcCircle)
The BaseSurface defined at supertype IfcHalfSpaceSolid shall be of type IfcPlane
The normal of the plane, being the BaseSurface, shall not be perpendicular to the z-axis of the position coordinate system

8.8.3.26.2 Entity inheritance

8.8.3.26.3 Attributes

#	Attribute	Type	Description
IfcRepresentationItem (2)
	LayerAssignment	SET [0:1] OF IfcPresentationLayerAssignment FOR AssignedItems	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. IFC2x3-CHANGE The inverse attribute LayerAssignments has been added. IFC4-CHANGE The inverse attribute LayerAssignment has been restricted to max 1. Upward compatibility for file based exchange is guaranteed.
	StyledByItem	SET [0:1] OF IfcStyledItem FOR Item	Reference to the IfcStyledItem that provides presentation information to the representation, e.g. a curve style, including colour and thickness to a geometric curve. IFC2x3-CHANGE The inverse attribute StyledByItem has been added.
IfcHalfSpaceSolid (3)
1	BaseSurface	IfcSurface	Surface defining side of half space.
2	AgreementFlag	IfcBoolean	The agreement flag is TRUE if the normal to the BaseSurface points away from the material of the IfcHalfSpaceSolid. Otherwise it is FALSE.
*	Dim	IfcDimensionCount	This attribute is formally derived. `3` The space dimensionality of this class, it is always 3
Click to show 5 hidden inherited attributes Click to hide 5 inherited attributes
IfcPolygonalBoundedHalfSpace (2)
3	Position	IfcAxis2Placement3D	Definition of the position coordinate system for the two-dimensional boundary.
4	PolygonalBoundary	IfcBoundedCurve	Two-dimensional bounded curve, defined in the xy plane of the position coordinate system. IFC2x3-CHANGE The attribute type has been changed from IfcPolyline to its supertype IfcBoundedCurve with upward compatibility for file based exchange.

Table 8.8.3.26.B

8.8.3.26.4 Formal propositions

Name	Description
BoundaryDim	The bounding polyline should have the dimensionality of 2.
`PolygonalBoundary.Dim = 2`
BoundaryType	Only bounded curves of type IfcCompositeCurve, IfcPolyline, and IfcIndexedPolyCurve are valid boundary curves.
`SIZEOF(TYPEOF(PolygonalBoundary) * [ 'IFC4X3.IfcPolyline', 'IFC4X3.IfcCompositeCurve', 'IFC4X3.IfcIndexedPolyCurve' ]) = 1`

Table 8.8.3.26.C

8.8.3.26.5 Formal representation

ENTITY IfcPolygonalBoundedHalfSpace
 SUBTYPE OF (IfcHalfSpaceSolid);
	Position : IfcAxis2Placement3D;
	PolygonalBoundary : IfcBoundedCurve;
 WHERE
	BoundaryDim : PolygonalBoundary.Dim = 2;
	BoundaryType : SIZEOF(TYPEOF(PolygonalBoundary) * [
    'IFC4X3.IfcPolyline',
    'IFC4X3.IfcCompositeCurve',
    'IFC4X3.IfcIndexedPolyCurve'
  ]) = 1;
END_ENTITY;

8.8.3.26.6 References

IfcHalfSpaceSolid