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 IfcGeometryResource refers to the clause 4,
"Geometry" of the standard. The reference is ISO/IS 10303-42:1994, p. 11-121.
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 pcurves, use of simple 2D curves for the generation of
swept surfaces
- omission of the name attribute at the representation item
The following entities, defined in the Integrated Resources, part 43
"Integrated generic resources: Representation structures" have been
incorporated in this resource schema:
- representation_item as IfcRepresentationItem
- representation_map as IfcRepresentationMap
- mapped_item as IfcMappedItem
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 representation in the
current version of the geometry resource:
- definition of points directly by their coordinate values
- definition of directions, vectors, and axis placements
- definition of transformation operators
- definition of parametric curves (subset of)
- definition of conic curves and elementary surfaces (subset of)
- definition of curves defined on a parametric surface (adapted subset
of)
- definition of swept surfaces (adapted subset of)
- definition of offset curves
Definitions
NOTE: The following definitions are taken
from ISO/CD 10303-42:1992. Please refer to ISO/IS 10303-42:1994, p. 3-7 for the
final definition of the formal standard.
placement coordinate system:
a rectangular Cartesian
coordinate system associated with the placement of a geometric entity in space,
used to describe the interpretation of the attributes and to associate a unique
parametrisation with curve and surface entities."
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. 12-14 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
IfcGeometryResource) are quoted.
Space Dimensionality
All geometry shall be defined in a
right-handed rectangular Cartesian coordinate system with the same units on
each axis. A common scheme has been used for the definition of both
two-dimensional and three-dimensional geometry. Points and directions exists in
both a two-dimensional and a three-dimensional form, these forms are
distinguished solely by the presence, or absence, of a third coordinate value.
Complex geometric entities are all defined using points and directions from
which their space dimensionality can be deduced.
Parameterisation of analytic curves and surfaces
Each curve on
surface specified here has a defined parametrisation. In some instances the
definitions are in parametric terms. In others, the conic curves and elementary
surfaces, the definitions are in geometric terms. In this latter case a
placement coordinate system is used to define the parameterisation. The
geometric definitions contain some, but not all, of the data required for this.
The relevant data to define this placement coordinate system is contained in
the axis2_placement (IfcAxis2Placement) associated with the individual curve
and surface entities.
Curves
The curve entities include lines, some elementary
conics, and some referentially or procedurally defined curves. All the curves
have a well defined parameterization which makes it possible to trim a curve or
identify points on the curve by parameter value. For the conic curves a method
of representation is used which separates their geometric form from their
orientation and position in space. In each case, the position and orientation
information is conveye by an axis2_placement (IfcAxis2Placement). A composite
curve entity, which includes the facility to communicate continuity information
at the curve-to-curve transition points, is provided for the construction of
more complex curves. The offset curve type is a curve defined with reference to
other geometry. Separate offset curves entities exist for 2D and 3D
applications.
Surfaces
The simple surfaces are the planar surface, a surface
of revolution and a surface of linear extrusion. As with curves, all surfaces
have an associated standard parameterization. In many cases the surfaces, as
defined, are unbounded; it is assumed that they will be bounded either
explicitly or implicitly. Explicit bounding is achieved with the bounded
surface (here: plane); implicit bounding requires the association of additional
topological information to define a face.