- being a Cartesian point or a direction
- referencing directly a Cartesian point or direction
- referencing indirectly a Cartesian point or direction
An indirect reference to a Cartesian point or direction means that a
given geometric item references the Cartesian point or direction through one or
more intervening geometry or topology items.
EXAMPLE: Consider a circle. It gains its geometric
position and orientation by virtue of a reference to axis2_placement
(IfcAxis2Placement) that is turn references a cartesian_point
(IfcCartesianPoint) and several directions (IfcDirection).
EXAMPLE:
Consider a manifold brep. A manifold_solid_brep (IfcManifoldSolidBrep) is a
geometric_representation_item (IfcGeometricRepresentationItem) that through
several layers of topological_representation_item's
(IfcTopologicalRepresentationItem) references poly loops (IfcPolyLoop). Through
additional intervening entities poly loops reference cartesian_point's
(IfcCartesianPoint).
Definition from IAI: The derivation of the dimensionality of the
IfcGeometricRepresentationItem is different to STEP, there is a specific
derived attribute at each class that defines the dimensionality, whereas STEP
does it for the representation_context and requires all
geometric_representation_item's to have the same dimensionality therein.
The definition of swept area solids as goemteric representation items is
different to STEP, it is based on a set of predefined profiles (or cross
sections), i.e. a set of parameterized geometric primitives widely supported in
the industry. Those profiles are used to create volumes through extrusion,
revolution and cross section based sweep operations. This method was called
attribute driven geometric representation and it was formerly known as implicit
geometry in IFC.
NOTE Corresponding STEP entity:
geometric_representation_item. Please refer to ISO/IS 10303-42:1994, p. 22 for
the final definition of the formal standard. The following changes have been
made: It does not inherit from ISO/IS 10303-43:1994 entity representation_item.
The derived attribute Dim is demoted to the appropriate subtypes. The WR1 has
not been incorporated. Not all subtypes that are in ISO/IS 10303-42:1994 have
been added to the current IFC Release .
HISTORY New class in IFC Release 1.5
ISSUE: See issue log for changes made in IFC Release 1.5.