8.18.3.4 IfcGeometricRepresentationSubContext

8.18.3.4.1 Semantic definition

IfcGeometricRepresentationSubContext defines the context that applies to several shape representations of a product being a sub context, sharing the WorldCoordinateSystem, CoordinateSpaceDimension, Precision and TrueNorth attributes with the parent IfcGeometricRepresentationContext.

The IfcGeometricRepresentationSubContext is used to define semantically distinguished representation types for different information content, dependent on the representation view and the target scale. It can be used to control the level of detail of the shape representation that is most applicable to this geometric representation context. In addition the sub context is used to control the later appearance of the IfcShapeRepresentation within a plot view.

Each IfcProduct can then have several instances of subtypes of IfcRepresentation, each being assigned to a different IfcGeometricRepresentationSubContext). The applicable values for the inherited ContextIdentifier attribute shall be identical to the RepresentationIdentifier attribute defined at IfcShapeRepresentation.

8.18.3.4.2 Entity inheritance

8.18.3.4.3 Attributes

#	Attribute	Type	Description
IfcRepresentationContext (3)
1	ContextIdentifier	OPTIONAL IfcLabel	The optional identifier of the representation context as used within a project.
2	ContextType	OPTIONAL IfcLabel	The description of the type of a representation context. The supported values for context type are to be specified by implementers agreements.
	RepresentationsInContext	SET [0:?] OF IfcRepresentation FOR ContextOfItems	All shape representations that are defined in the same representation context.
IfcGeometricRepresentationContext (6)
3	CoordinateSpaceDimension	IfcDimensionCount	The integer dimension count of the coordinate space modeled in a geometric representation context.
4	Precision	OPTIONAL IfcReal	Value of the model precision for geometric models. It is a double value (REAL), typically in 1E-5 to 1E-8 range, that indicates the tolerance under which two given points are still assumed to be identical. The value can be used e.g. to sets the maximum distance from an edge curve to the underlying face surface in brep models.
5	WorldCoordinateSystem	IfcAxis2Placement	Establishment of the engineering coordinate system (often referred to as the world coordinate system in CAD) for all representation contexts used by the project. NOTE It can be used to provide better numeric stability if the placement of the building(s) is far away from the origin. In most cases however it would be set to origin: (0.,0.,0.) and directions x(1.,0.,0.), y(0.,1.,0.), z(0.,0.,1.). If an geographic placement is provided using IfcMapConversion then the WorldCoordinateSystem atttibute is used to define the offset between the zero point of the local engineering coordinate system and the geographic reference point to which the IfcMapConversion offset relates. In preferred practise both points (also called "project base point" and "survey point") should be coincidental. However it is possible to offset the geographic reference point from the local zero point.
6	TrueNorth	OPTIONAL IfcDirection	Direction of the true north, or geographic northing direction, relative to the underlying project coordinate system. It is given by a 2 dimensional direction within the xy-plane of the project coordinate system. If not present, it defaults to 0. 1., meaning that the positive Y axis of the project coordinate system equals the geographic northing direction. NOTE If a geographic placement is provided using IfcMapConversion then the true north is for information only. In case of inconsistency, the value provided with IfcMapConversion shall take precedence.
	HasSubContexts	SET [0:?] OF IfcGeometricRepresentationSubContext FOR ParentContext	The set of IfcGeometricRepresentationSubContexts that refer to this IfcGeometricRepresentationContext.
	HasCoordinateOperation	SET [0:1] OF IfcCoordinateOperation FOR SourceCRS	Indicates conversion between coordinate systems. In particular it refers to an IfcCoordinateOperation between a Geographic map coordinate reference system, and the engineering coordinate system of this construction project. If there is more then one IfcGeometricRepresentationContext provided to the IfcProject then all contexts shall have an identical instance of IfcCoordinateOperation as HasCoordinateOperation referring to the same instance of IfcCoordinateReferenceSystem.
Click to show 9 hidden inherited attributes Click to hide 9 inherited attributes
IfcGeometricRepresentationSubContext (8)
7	ParentContext	IfcGeometricRepresentationContext	Parent context from which the sub context derives its world coordinate system, precision, space coordinate dimension and true north.
8	TargetScale	OPTIONAL IfcPositiveRatioMeasure	The target plot scale of the representation to which this representation context applies. NOTE Scale indicates the target plot scale for the representation sub context, all annotation styles are given in plot dimensions according to this target plot scale. If multiple instances of IfcGeometricRepresentationSubContext are given having the same TargetView value, the target plot scale applies up to the next smaller scale, or up to unlimited small scale. NOTE Scale 1:100 (given as 0.01 within TargetScale) is bigger then 1:200 (given as 0.005 within TargetScale).
9	TargetView	IfcGeometricProjectionEnum	Target view of the representation to which this representation context applies.
10	UserDefinedTargetView	OPTIONAL IfcLabel	User defined target view, this attribute value shall be given, if the TargetView attribute is set to USERDEFINED.
5*	WorldCoordinateSystem	IfcAxis2Placement	This attribute is formally derived. `ParentContext.WorldCoordinateSystem`
3*	CoordinateSpaceDimension	IfcDimensionCount	This attribute is formally derived. `ParentContext.CoordinateSpaceDimension`
6*	TrueNorth	IfcDirection	This attribute is formally derived. `NVL(ParentContext.TrueNorth, IfcConvertDirectionInto2D(SELF\IfcGeometricRepresentationContext.WorldCoordinateSystem.P[2]))`
4*	Precision	IfcReal	This attribute is formally derived. `NVL(ParentContext.Precision,1.E-5)`

Table 8.18.3.4.A

8.18.3.4.4 Formal propositions

Name	Description
NoCoordOperation	An IfcCoordinateOperation shall not be provided to a geometric representation sub context, only to the parent geometric representation context. NOTE The coordinate operation is used to define the coordinate transformation of the project engineering coordinate system to a geographic coordinate reference system.
`SIZEOF(SELF\IfcGeometricRepresentationContext.HasCoordinateOperation) = 0`
ParentNoSub	The parent context shall not be another geometric representation sub context.
`NOT('IFC4X3.IFCGEOMETRICREPRESENTATIONSUBCONTEXT' IN TYPEOF(ParentContext))`
UserTargetProvided	The attribute UserDefinedTargetView shall be given, if the attribute TargetView is set to USERDEFINED.
`(TargetView <> IfcGeometricProjectionEnum.USERDEFINED) OR ((TargetView = IfcGeometricProjectionEnum.USERDEFINED) AND EXISTS(UserDefinedTargetView))`

Table 8.18.3.4.B

8.18.3.4.5 Formal representation

ENTITY IfcGeometricRepresentationSubContext
 SUBTYPE OF (IfcGeometricRepresentationContext);
	ParentContext : IfcGeometricRepresentationContext;
	TargetScale : OPTIONAL IfcPositiveRatioMeasure;
	TargetView : IfcGeometricProjectionEnum;
	UserDefinedTargetView : OPTIONAL IfcLabel;
 DERIVE
	 SELF\IfcGeometricRepresentationContext.WorldCoordinateSystem : IfcAxis2Placement := ParentContext.WorldCoordinateSystem;
	 SELF\IfcGeometricRepresentationContext.CoordinateSpaceDimension : IfcDimensionCount := ParentContext.CoordinateSpaceDimension;
	 SELF\IfcGeometricRepresentationContext.TrueNorth : IfcDirection := NVL(ParentContext.TrueNorth, IfcConvertDirectionInto2D(SELF\IfcGeometricRepresentationContext.WorldCoordinateSystem.P[2]));
	 SELF\IfcGeometricRepresentationContext.Precision : IfcReal := NVL(ParentContext.Precision,1.E-5);
 WHERE
	NoCoordOperation : SIZEOF(SELF\IfcGeometricRepresentationContext.HasCoordinateOperation) = 0;
	ParentNoSub : NOT('IFC4X3.IFCGEOMETRICREPRESENTATIONSUBCONTEXT' IN TYPEOF(ParentContext));
	UserTargetProvided :  (TargetView <> IfcGeometricProjectionEnum.USERDEFINED) OR 
((TargetView =  IfcGeometricProjectionEnum.USERDEFINED) AND EXISTS(UserDefinedTargetView)) ;
END_ENTITY;

8.18.3.4.6 References

IfcGeometricRepresentationContext