NOTE: The definitions of this resource of
the IFC model have been taken from the Integrated Resource, part 42 "Integrated
generic resources: Geometric and topological representations" of the ISO
standard 10303: "Industrial automation systems and integration - Product data
representation and exchange". The IfcGeometricModelResource refers to the
clause 6, "Geometric Model" of the standard. The reference is ISO/IS
10303-42:1994, pp. 166. The improved definitions of the second edition, ISO/DIS
10303-42:1999 have been used, when applicable.
The definitions taken from ISO/IS 10303-42:1994 have undergone a
adaptation process, characterized by:
- adaptation of the IFC naming convention (inner majuscules and Ifc
prefix)
- adaptation of the STEP entities, where multiple inheritance or
non-exclusive inheritance (i.e. AND or ANDOR subtype constraints) are used
- selection of a subset of the IR, using subtype and select
pruning
- dimensionality of geometric representation items defined at each item
(not through the representation context)
- omission of the name attribute at the representation item
- using profile definitions instead of curve bounded surface for the
swept surface of swept area solid
- restriction of the manifold solid brep to support faceted breps
only
- additional subtype of half space solid added
- additional constraining subtype of Boolean result added
The geometric representation of the shape is defined following the
adaptation of the ISO/CD 10303-42:1992, Industrial Automation Systems
and Integration: Product Data Representation and Exchange - Part 42: Integrated
Generic Resources. Geometric and Topological Representation. The type,
class, and function semantic definition sections follow the adapted wording of
the working draft, which is clearly indicated and quoted at each reference. The
definitions on geometric and topological representation (when taken from ISO/CD
10303-42:1992) are explicitly excluded from the copyright of buildingSMART International Limited.
For more information on the definitions
as defined in the formal ISO standard please refer to: ISO/IS 10303-42:1994,
Industrial Automation Systems and Integration: Product Data Representation and
Exchange - Part 42: Integrated Generic Resources. Geometric and Topological
Representation. The formal standard can be obtained through the local
publishers of standards in each individual country.
The following is within the scope of the geometric model representation
in the current version of the geometric model resource:
- data describing the precise geometric form of three-dimensional solid
objects;
- constructive solid geometry (CSG) models;
- definition of half-spaces ( CSG primitives are not in scope);
- creation of solid models by sweeping operations (swept area solids
only);
- manifold boundary representation (B-rep) models (restricted to
faceted brep);
- surface models;
- geometric sets.
Fundamental Concepts and Assumptions
NOTE: The following fundamental concepts
and assumptions are taken from ISO/CD 10303-42:1992. Please refer to ISO/IS
10303-42:1994, p. 166-167 for the final definition of the formal
standard.
NOTE: Only the parts relevant to the
subset of ISO 10303-42 (which had been incorporated into the
IfcGeometricModelResource) are quoted.
The constructive solid geometry models are represented by teir component
primitives and the sequence of Boolean operations (union, intersection, or
difference) used in their construction. The entity which communicates the
logical sequence of Boolean operations is the boolean_result
(IfcBooleanResult) which identifies an operator and two operands. Since
the operands can themselves be Boolean results thus enabling nested operations.
Swept solids and half-space solids are permissable Boolean operands. The swept
solids are the solid of revolution and the solid of linear extrusion. The swept
solids are obtained by extruding or sweeping a planar face which may contain
holes. The half space solid is essentially defined as a semi-infinite solid on
one side of a surface; it may be limited by a box domain.
B-rep models are represented by the set of shells defining the exterior
or interior boundaries. The facetted B-rep is restricted to represent B-reps in
which all faces are planar and every loop is a poly loop. For such a solid this
entity provides a more efficient form of representation. The shell based
surface model, the face based surface model and the geometric set entities do
not enforce the integrity checks of the manifold solid brep and can be used for
the communication of incomplete models (including two-dimensional
models).