ISO 10303-55:2005
(Main)Industrial automation systems and integration — Product data representation and exchange — Part 55: Integrated generic resource: Procedural and hybrid representation
Industrial automation systems and integration — Product data representation and exchange — Part 55: Integrated generic resource: Procedural and hybrid representation
ISO 10303-55:2005 specifies resource constructs for the representation of models of the procedural or construction history type, defined in terms of the sequence of constructional operations used to build them. Representations of the operations themselves are not specified here; the mechanisms provided in this document allow the use of entity data types defined in other parts of ISO 10303 for that purpose.The following are within the scope of ISO 10303-55:2005: the specification of sequences of constructional operations for the generation of any kind of explicit representation or model; the hierarchical structuring of constructional sequences; the embedding of explicitly defined elements in constructional sequences for the representation of hybrid models; the use of representation_item definitions from other parts of ISO 10303 to represent constructional operations for instances of those representation_items; the definition of a dual representation by association of a procedural model with an explicit "current result" model, the latter acting as a representative example of the parametric family of models defined by the former; the association of design rationale information with a procedural model; the identification, in a procedural model, of explicit elements selected by interactive picking from the visual display of the model in the sending system; the identification, in a procedural model, of constructional operations that can be suppressed for purposes of model simplification; specialization of the foregoing capabilities for the procedural representation of shape models.
Systèmes d'automatisation industrielle et intégration — Représentation et échange de données de produits — Partie 55: Ressources génériques intégrées: Représentation procédurale et hybride
General Information
Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 10303-55
First edition
2005-02-01
Industrial automation systems — Product
data representation and exchange —
Part 55:
Integrated generic resource: Procedural
and hybrid representation
Systèmes d'automatisation industrielle — Représentation et échange
de données de produits —
Partie 55: Ressources génériques intégrées — Représentation
procédurale et hybride
Reference number
©
ISO 2005
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Contents Page
1 Scope . . . . . . . . . 1
2 Normative references . . . . . . . . . 2
3 Terms, definitions and abbreviations . . . . . . 2
3.1 Terms defined in ISO 10303-1 . . . . . . 2
3.2 Terms defined in ISO 10303-11 . . . . . . . . 3
3.3 Terms defined in ISO 10303-42 . . . . . . . . 3
3.4 Terms defined in ISO 10303-43 . . . . . . . . 3
3.5 Terms defined in ISO 10303-108 . . . . . . . . 4
3.6 Other terms and definitions . . . . . . 5
3.7 Abbreviations . . . . . . . . . 5
4 Procedural model . . . . . . . . 6
4.1 Introduction . . . . . . . . . . 6
4.2 Fundamental concepts and assumptions . . . . . 6
4.2.1 Procedural models . . . . . . . 7
4.2.2 Hybrid models . . . . . . . . . 8
4.2.3 Explicit selected elements . . . . . . . . 8
4.2.4 Dual models . . . . . . . 9
4.2.5 Representation of constructional operations in procedural models . . 10
4.2.6 Implicit and explicit constraints . . . . . 11
4.2.7 Suppression of constructional operations . . . . 12
4.2.8 Exchange of procedural and hybrid models . . . . 12
4.2.9 Variational cases of procedural and hybrid models . . . . 12
4.3 Procedural model entity definitions . . . . . . 13
4.3.1 explicit procedural representation relationship . . . . . . 13
4.3.2 explicit procedural representation item relationship . . . 14
4.3.3 procedural representation . . . . . . . . 15
4.3.4 procedural representation sequence . . . . . . . 16
4.3.5 user selected elements . . . . . . 17
4.3.6 indirectly selected elements . . . . . . . 18
5 Procedural shape model . . . . . . . . 20
5.1 Introduction . . . . . . . . . . 20
5.2 Fundamental concepts and assumptions . . . . . 20
5.2.1 Procedural shape models . . . . . . 21
5.2.2 Hybrid shape models . . . . . . 22
5.2.3 Explicit selected elements in a shape model . . . . 22
5.2.4 Dual shape representations . . . . . . . . 22
5.2.5 Design rationale for shape models . . . . . 22
5.3 Procedural shape model type definitions . . . . . 23
5.3.1 shape representation item . . . . . . . . 23
5.4 Procedural shape model entity definitions . . . . . . . 23
5.4.1 explicit procedural shape representation relationship. . . 23
5.4.2 explicit procedural geometric representation item relationship . . 24
5.4.3 procedural shape representation . . . . . 25
5.4.4 procedural shape representation sequence . . . . 25
5.4.5 procedural solid representation sequence . . . . 26
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5.4.6 procedural surface representation sequence . . . . 27
5.4.7 procedural wireframe representation sequence . . . . . . 28
5.4.8 user selected shape elements . . . . . . 28
5.4.9 indirectly selected shape elements . . . . . . . 29
Annex A (normative) Short names of entities. . . . . 31
Annex B (normative) Information object registration . . . . 32
B.1 Document identification . . . . . . . 32
B.2 Schema identification . . . . . . . . . 32
B.2.1 procedural model schema identification . . . . . 32
B.2.2 procedural shape model schema identification . . . . . . 32
Annex C (informative) Computer interpretable listings . . . . 33
Annex D (informative) EXPRESS-G diagrams . . . . . 34
Annex E (informative) Examples of the use of this part of ISO 10303 . . . 38
E.1 Example of non-geometric application of procedural modelling . . . . . 38
E.2 Example of intended usage of the procedural shape model schema . . 38
E.3 Example of the use of variational (parameterization and constraint) information with a
procedural model . . . . . . . 40
E.4 Example of the embedding of operation sequences and the recording of design rationale 45
Bibliography . . . . . . . . . . 47
Index . . . . . . . . . . 48
Figures
Figure 1 Schema level diagram of relationships among ISO 10303-55 schemas (inside the
box) and other resource schemas . . . . . . viii
Figure D.1 procedural model schema – EXPRESS-G diagram 1 of 1 . . . 35
Figure D.2 procedural shape model schema – EXPRESS-G diagram 1 of 2 . . . . 36
Figure D.3 procedural shape model schema – EXPRESS-G diagram 2 of 2 . . . . 37
Figure E.1 Relationships between instances of procedural, variational and explicit models for
the cases of (a) no variational model, (b) no procedural model, and (c) all three models. . . 42
Table
A.1 Short names of entities . . . . . . . . . 31
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Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical com-
mittee has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates
closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical stan-
dardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Stan-
dards adopted by the technical committees are circulated to the member bodies for voting. Publication
as an International Standard requires approval by at least 75% of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this part of ISO 10303 may be the
subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 10303-55 was prepared by Technical Committee ISO/TC 184/SC 4, Industrial automation systems
and integration, Subcommittee SC 4, Industrial data.
ISO 10303 consists of a series of parts, under the general title Industrial automation systems and
integration — Product data representation and exchange. The structure of ISO 10303 is described in
ISO 10303-1.
Each part of ISO 10303 is a member of one of the following series: description methods, implementation
methods, conformance testing methodology and framework, integrated generic
resources, integrated application resources, application protocols, abstract test suites, application
interpreted constructs, and application modules. This part is a member of the integrated generic
resources series. The integrated generic resources and the integrated application resources specify a
single conceptual product data model.
A complete list of parts of ISO 10303 is available from the Internet:
Should further parts of ISO 10303 be published, they will follow the same numbering pattern.
cISO 2005 — All rights reserved v
Introduction
ISO 10303 is an International Standard for the computer-interpretable representation of product infor-
mation and for the exchange of product data. The objective is to provide a neutral mechanism capable of
describing products throughout their life cycle. This mechanism is suitable not only for neutral file ex-
change, but also as a basis for implementing and sharing product databases, and as a basis for archiving.
This part of ISO 10303 is a member of the integrated resources series. Major subdivisions of this part of
ISO 10303 are:
— Procedural model schema;
— Procedural shape model schema.
This part of ISO 10303 provides general mechanisms for the representation of models defined in terms
of the operations used to construct them. The constructional operations themselves are represented by
entity data types defined in other parts of ISO 10303, interpreted as constructors. Procedural models
have the advantage of being easy to edit, simply by changing values of parameters used as arguments of
their constructional operations. Such models are said to embody design intent information, in the sense
that modifications to them conform to the scheme of parameterization imposed by their original creator,
and also comply with any constraints implied by the particular constructional operations used. Thus the
transfer of a procedural model into a receiving system carries with it information as to how the model
will behave when edited following the transfer.
However, procedural models also have the disadvantage of containing (in their purest form) little or no
explicit information about the result of actually performing the sequence of operations. This fact makes
them unsuitable as a basis for the automation of many engineering processes that depend on the use of
explicit geometric information, for example numerically controlled machining or inspection.
Systems for engineering purposes commonly achieve the advantages of both modelling approaches
through the use of a dual representation, comprising a primary representation of the procedural or con-
struction history type together with a secondary explicit representation. Other ISO 10303 resources pro-
vide the elements needed for explicit representions. This part of the standard not only specifies resources
for procedural representations but also provides a dual model capability by enabling the association of
such a model with its corresponding explicit counterpart.
The initial focus of this part of ISO 10303 was to allow the capture and exchange of CAD shape rep-
resentations of the procedural and hybrid types (a hybrid representation is basically procedural but also
contains some explicit elements). However, the capabilities provided also have general applicability
for the transfer of any type of procedurally represented or hybrid model, whether geometric or non-
geometric. In the case of shape models, ISO 10303-42 is the primary resource for the corresponding
explicit representations.
Because procedural representations are inherently parametric, they can be edited by changing the values
of input arguments of constructional procedures. However, this requires that the system operator has
an appropriate level of understanding of the rationale underlying the original constructional method. At
the time of writing, no method is known for capturing design rationale information automatically during
model construction, and provision is therefore made in this part of ISO 10303 for its representation as
descriptive text, assumed to be supplied by the original designer.
It is useful to emphasize the distinction between design intent and design rationale. Design intent is cap-
tured in the schemes of parameterization and constraints imposed upon models during their construction.
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It therefore governs the ways in which a model may be edited. Design rationale, on the other hand, is
concerned with the reasons why a particular configuration or constructional process was adopted, and
therefore with the logic underlying the design intent.
The industry motivation for the exchange of procedural, hybrid and dual representations arises from the
difficulties that have been encountered in the editing of ISO 10303 explicit models in a receiving system,
following a model transfer. If only an explicit model is transferred, as in the past, the design intent
embodied in the procedural component of the dual model in the sending system is lost in the transfer.
The consequences are that received model is incomplete in vital respects, and that editing it is difficult
or impossible.
Three books and a conference paper providing further background on the topics covered by this part of
ISO 10303 are given in the Bibliography [6 – 9].
The contents of the two schemas making up this part of ISO 10303 are as follows:
procedural model schema: Fundamental mechanisms for the representation of procedural and
hybrid models, and for the capture of design rationale.
procedural shape model schema: Specialization of the foregoing schema for the specific case of
geometric models.
The relationships of the schemas in this part of ISO 10303 to other schemas that define the integrated
resources of ISO 10303 are illustrated in Figure 1 using the EXPRESS-G notation. EXPRESS-G is
defined in annex D of ISO 10303-11. The schemas occurring in Figure 1 are components of ISO 10303
integrated resources, and they are specified in the following resource parts:
product property representationschema ISO 10303-41
support resource schema ISO 10303-41
geometric model schema ISO 10303-42
geometry schema ISO 10303-42
topology schema ISO 10303-42
representationschema ISO 10303-43
variational representationschema ISO 10303-108
NOTE 1 A procedural model is a representation of a constructional process, and it may therefore be envisaged
that ISO 10303-49 (‘Process structure and properties’) [1] would be a suitable underlying resource for this part of
ISO 10303. However, the definition of ‘process’ as given in ISO 10303-49 is a narrow one:
process: a particular procedure for doing something involving one or more steps or operations. The process may
produce a product, a property of a product, or an aspect of a product.
Thus the ISO 10303-49 view of a process is one that is concerned with the generation of a physical object or
some characteristic of it. The purpose of this part of ISO 10303, by contrast, is to provide the means for capturing
and transferring constructional processes for representations or models of general objects, which only exist as
abstractions in a computer or database. For this reason, and also because advantage can be taken of the very close
relationship between procedural modelling operations and existing entities defined in other ISO 10303 integrated
resources, ISO 10303-49 has not been used as the basis for the present part of ISO 10303.
NOTE 2 In the diagram on the following page, the schemas occurring in this part of ISO 10303 are enclosed in
a heavy rectangular box. The specific entities interfaced are not indicated.
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variational_repres-
entation_schema
procedural_model_ support_resource_
schema schema
representation_
schema
product_property_
representation_schema
geometry_schema
procedural_shape_
model_schema
topology_schema
geometric_model_
schema
Figure 1 – Schema level diagram of relationships among ISO 10303-55
schemas (inside the box) and other resource schemas
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INTERNATIONAL STANDARD ISO 10303-55:2005(E)
Industrial automation systems and integration —
Product data representation and exchange —
Part 55:
Integrated generic resource: Procedural and hybrid represen-
tation
1 Scope
This part of ISO 10303 specifies resource constructs for the representation of models of the procedural
or construction history type, defined in terms of the sequence of constructional operations used to build
them. Representations of the operations themselves are not specified here; the mechanisms provided in
this document allow the use of entity data types defined in other parts of ISO 10303 for that purpose (see
clause 4.2.5).
The following are within the scope of this part of ISO 10303:
— The specification of sequences of constructional operations for the generation of any kind of explicit
representation or model;
— The hierarchical structuring of constructional sequences;
— The embedding of explicitly defined elements in constructional sequences for the representation of
hybrid models;
— The use of representation item definitions from other parts of ISO 10303 to represent construc-
tional operations for instances of those representation items;
— The definition of a dual representation by association of a procedural model with an explicit ‘cur-
rent result’ model, the latter acting as a representative example of the parametric family of models
defined by the former;
— The association of design rationale information with a procedural model;
— The identification, in a procedural model, of explicit elements selected by interactive picking from
the visual display of the model in the sending system;
— The identification, in a procedural model, of constructional operations that can be suppressed for
purposes of model simplification;
— Specialization of the foregoing capabilities for the procedural representation of shape models.
The following are outside the scope of this part of ISO 10303:
— Any mechanism for the ‘persistent naming’ of elements of an explicit model based on details of the
procedural sequence used to create them;
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— ‘Macro’ capabilities requiring the use of control structures such as IF. THEN. ELSE or REPEAT.
UNTIL. Such structures are defined in ISO 10303-11 for use in local and global rules, but no
analogous facilities are provided in this document to allow conditional operations in procedural
models.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO/IEC 8824-1, Information technology — Abstract Syntax Notation One (ASN.1): Specification of
basic notation.
ISO 10303-1, Industrial automation systems and integration — Product data representation and
exchange — Part 1: Overview and fundamental principles.
ISO 10303-11, Industrial automation systems and integration — Product data representation and
exchange — Part 11: Description methods: The EXPRESS language reference manual.
ISO 10303-41, Industrial automation systems and integration — Product data representation and
exchange — Part 41: Integrated generic resource: Fundamentals of product description and support.
ISO 10303-42, Industrial automation systems and integration — Product data representation and
exchange — Part 42: Integrated generic resource: Geometric and topological representation.
ISO 10303-43, Industrial automation systems and integration — Product data representation and
exchange — Part 43: Integrated generic resource: Representation structures.
ISO 10303-108, Industrial automation systems and integration — Product data representation and
exchange — Part 108: Integrated application resource: Parameterization and constraints for explicit
geometric product models.
3 Terms, definitions and abbreviations
3.1 Terms defined in ISO 10303-1
For the purposes of this part of ISO 10303, the following terms defined in ISO 10303-1 apply.
— application;
— application context;
— application protocol (AP);
— assembly;
— component;
— data exchange;
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— exchange structure;
— implementation method;
— integrated resource (IR);
— product;
— product data;
— structure.
3.2 Terms defined in ISO 10303-11
For the purposes of this part of ISO 10303, the following terms defined in ISO 10303-11 apply.
— entity;
— entity data type;
— entity (data type) instance;
— instance;
— value.
3.3 Terms defined in ISO 10303-42
For the purposes of this part of ISO 10303, the following terms defined in ISO 10303-42 apply.
— boundary representation solid model (B-rep);
— constructive solid geometry (CSG);
— coordinate space;
— dimensionality;
— model space.
3.4 Terms defined in ISO 10303-43
For the purposes of this part of ISO 10303, the following terms defined in ISO 10303-43 apply.
— context of representation;
— element of representation;
— founded;
— representation.
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3.5 Terms defined in ISO 10303-108
For the purposes of this part of ISO 10303, the following terms defined in ISO 10303-108 apply.
NOTE The capabilities specified in ISO 10303-108 are very closely related to those specified in this part of ISO
10303. Consequently, acquaintance with these terms and their definitions is crucial for understanding the present
document.
— constraint;
— constraint solution;
— current result;
— current value;
— declarative constraint;
— declarative model;
— design intent;
— element;
— evaluated model;
— explicit constraint;
— explicit model;
— feature;
— generative model;
— history-based model;
— hybrid model;
— implicit constraint;
— model parameter;
— procedural constraint;
— procedural model;
— sketch;
— unevaluated model;
— variational.
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3.6 Other terms and definitions
For the purposes of this part of ISO 10303, the following definitions apply.
3.6.1
design rationale
logic underlying the methodology used in constructing the design
3.6.2
dual model
combination of a procedural or hybrid representation with an explicit representation, the second of which
represents an example of the parametric class of models defined by the first
3.7 Abbreviations
For the purposes of this part of ISO 10303 the following abbreviations apply:
AP application protocol (of ISO 10303)
B-rep boundary representation
CAD computer aided design
CSG constructive solid geometry
IR integrated resource (of ISO 10303)
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4 Procedural model
The following EXPRESS declaration begins the procedural model schema and identifies the necessary
external references.
EXPRESS specification:
*)
SCHEMA procedural_model_schema;
REFERENCE FROM support_resource_schema -- ISO 10303-41
(text);
REFERENCE FROM representation_schema -- ISO 10303-43
(item_in_context,
representation,
representation_item,
representation_item_relationship,
representation_relationship,
using_representations);
REFERENCE FROM variational_representation_schema -- ISO 10303-108
(variational_representation);
(*
NOTE 1 The schemas referenced above can be found in the following parts of ISO 10303:
support resource schema ISO 10303-41
representation schema ISO 10303-43
variational representation schema ISO 10303-108
NOTE 2 See annex D, Figure D.1, for a graphical presentation of this schema.
4.1 Introduction
The subject of the procedural representation schema is representation or modelling in terms of con-
structional operations. This may be contrasted with representation or modelling in terms of elements that
are explicitly created as the result of performing those operations.
EXAMPLE ISO 10303-42 defines the entity data type manifold solid brep. This is a representation of a solid
shape in terms of the faces, edges and vertices occurring in the boundary separating the interior from the exterior
of the solid. Such a representation contains no information as to how the shape was actually created, though
whatever constructional operations were used clearly had the effect of generating all the low-level geometrical and
topological elements involved in the manifold solid brep. This part of ISO 10303 provides an alternative method
of representing such a shape, in terms of the manner of its generation.
4.2 Fundamental concepts and assumptions
This schema provides representation methods for the following:
— The specification of sequences of constructional operations for the generation of models or repre-
sentations of any type;
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— The hierarchical structuring of constructional sequences;
— The embedding of explicitly defined elements in constructional sequences for the representation of
hybrid models;
— The use of representation item definitions from other parts of ISO 10303 to represent construc-
tional operations for instances of representation item in procedural and hybrid models;
— The definition of a dual representation by association of a procedural model with an explicit ‘cur-
rent result’ model, the latter acting as a representative example of the parametric family of models
defined by the former;
— The association of design rationale information with procedural models;
— The identification, in a procedural model, of explicit elements selected by interactive picking from
the visual display of the model in the sending system;
— The identification, in a procedural model, of constructional operations that can be suppressed for
purposes of model simplification.
The primary initial aim of this part of ISO 10303 is to provide the means for representing procedural and
hybrid models of geometric shapes as generated by CAD systems. For this reason, many of the examples
given in the descriptive text of this schema are concerned with aspects of CAD modelling. However,
the resource constructs provided in the schema are of general utility in the representation, exchange and
sharing of procedurally defined and hybrid models for any application. An example of a non-geometric
application of procedural modelling is given in clause E.1 of annex E.
4.2.1 Procedural models
A procedural model is represented in terms of the operations used in its creation. For this reason it is
also frequently known as a construction history model. A pure procedural model is defined exclusively
in terms of operations, and it is therefore impossible to refer in such a model to most specific constituents
of the explicit model that is generated when the operations are performed.
EXAMPLE A shape model of a cylindrical solid with radiusand heightmay be generated from a procedural
model containing just two operations:
a) Create a circular area with radius;
b) Sweep the circular area through a distancenormal to its plane.
The cylinder resulting from the performance of these operations has two circular edges. One of them will corre-
spond to the boundary of the circular area created by the first operation, but even this will not exist explicitly until
that first operation has been carried out. The second edge will only be called into existence by the performance of
the second operation. The two operations represent the shape of the cylinder, but by themselves provide no means
of referencing its individual geometrical or topological elements.
In a data exchange context, a transferred procedural model will specify only operations, and the genera-
tion of an explicit model from them will occur after the transfer, in the receiving system. This process is
called evaluation; its input is the unevaluated procedural model, and its output is the evaluated explicit
model.
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A procedural model is inherently parametric. The arguments of many of its constructional operations
will include numerical values. Parametric variations in the model may be made simply by changing the
values of these parameters. To see the effect of these changes in the corresponding explicit model it is
necessary to re-evaluate on the basis of the modified construction history.
The basic element of a procedural model, as defined in this part of ISO 10303, is the entity data type
procedural representation sequence (see clause 4.3.4).
4.2.2 Hybrid models
A hybrid model contains a combination of explicit elements and constructional operations.
EXAMPLE A common constructional procedure in CAD systems makes use of an explicitly defined planar
sketch or profile. This is composed of geometrical and topological elements (curves, points, edges, vertices),
possibly subject to geometric constraints such as parallelism, perpendicularity or tangency between curve elements.
The constructional procedure is generically known as a sweep operation. The sketch is moved through space to
sweep out a volume, and that volume is hence defined in terms of an operation upon an explicit element. Its
representation is therefore of the hybrid type. Various classes of swept surfaces and volumes are defined in ISO
10303-42.
In the exchange of a hybrid shape model, the explicit element transferred is usually a data structure built
from lower-level elements which must be transmitted with the model in order to specify it completely.
These may include variational elements such as model parameters and constraints (see ISO 10303-108).
Explicitly transferred elements are distinguished from procedurally defined elements simply by the fact
that they do not participate in instances of procedural representation sequence.
4.2.3 Explicit selected elements
In the exchange of procedural or hybrid models, it is convenient to use explicit model elements for an-
other important purpose, the identification of elements picked from an evaluated explicit model displayed
on the screen of the sending system.
EXAMPLE Suppose that the system is a CAD system, and consider the linear sweeping (or extrusion)of an
explicitly represented closed two-dimensional sketch composed of straight line segments enclosing an L-shaped
area. The operation will generate a block volume with a step feature defined on it. This object has one concave
edge, the inner edge of the step. Suppose now that it is required to generate a fillet to round off that concave edge.
The following is a possible sequence of operations for generating the desired shape:
a) Sweep the sketch linearly (construction operation);
b) Select the edge to be filleted from the screen display (selection operation);
c) Fillet the edge (construction operation).
The problem here is how to represent the selection operation in the hybrid model defined by the above operation
sequence. The selected edge is not present in that model; it will not arise until the model is evaluated, when it
will be swept out by the motion of one of the vertices of the initial sketch. The solution is to transmit that edge
explicitly from the sending system (it will have been selected from the secondary, evaluated, model in that system).
Then, when the model is being regenerated in the receiving system, the explicit edge information can be used to
determine which edge is to be filleted. The actions in that system will be
a) Perform the sweep operation to generate the L-block. The edge to be filleted will be created in the explicit
model in the receiving system by this operation;
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b) Compare the explicitly transmitted selected edge with all the edges of the newly created L-block until a match
is found. The matching edge is the edge to be filleted;
c) Fillet the identified edge.
There is an important semantic distinction between the roles of an explicit model element in a hybrid
model, as described in the previous clause, and an explicit selected element of the type discussed here:
explicit model element: (see clause 4.2.2). This is used for the direct transfer of an element from
the sending system into the receiving system, where it did not previously exist;
explicit selected element: (as explained in the present clause). This is used for the identification,
in the receiving system, of an element that has already been generated in that system, and that
corresponds to a specified explicit element in the sending system.
This schema provides an entity data type user selected elements (see clause 4.3.5) that is used to distin-
guish explicit selected elements from other elements of a procedural or hybrid model. This has a subtype
indirectly selected elements (see clause 4.3.6) for use in cases where the directly selected element is
representative of some other element or elements. Examples of the use of indirect selection in a shape
modelling context are given in clause 4.3.6.
4.2.4 Dual models
As explained in the Introduction, most CAD systems generate not only a procedural model but also
a secondary explicit model, which plays a vital role in the interaction of the user with the modelling
system. The explicit model is displayed on the screen, providing visual feedback to verify the results of
modelling operations. It also allows the selection, from the screen, of modelling elements to be used as
the basis for further modelling procedures.
NOTE 1 In a CAD system, the secondary explicit model is usually a manifold boundary representation solid
model, as defined in ISO 10303-42 (the relevant entity data type is manifold solid brep). Earlier parts of ISO
10303 have made provision for the representation and exchange of explicit models of this and related types. This
part of the standard adds the basic mechanisms needed for the representation and exchange of procedural repre-
sentations of CAD and other kinds of models.
NOTE 2 In most CAD systems the secondary model is ephemeral, in the sense that it is discarded whenever a
change is made to the primary model, and a new explicit model is then generated that takes the change into account.
Additionally, the explicit form of the model may be used for computational purposes.
EXAMPLE 1 If the model is a geometric model, the explicit form is necessary for the determination of such
things as an edge curve defined by the intersection of two surfaces. As already explained, the primary (procedural)
model does not contain the explicit elements needed to enable calculations of this type.
The importance of the interplay between primary and secondary models demands a representation for a
dual model, a relationship between two representations, one procedural or hybrid, and the other explicit.
The entity data type explicit procedural representation relationship is defined in this part of ISO
10303 for that purpose (see clause 4.3.1). It is assumed that the two components of a dual model are
always consistent alternative models or representations of the same physical entity.
There are also circumstances where it is possible, and may be desirable, to associate procedural and
explicit model elements at the representation item level. For this purpose the additional entity data
type explicit procedural representation item relationship is provided (see clause 4.3.2).
cISO 2005 — All rights reserved 9
This part of ISO 10303 is based on the further assumption that a procedural model exchanged between
systems may be accompanied by an explicit model, in which case what is exchanged is a dual model.
The primary purpose of this is to enable comparison, in the receiving system, of the explicit model
reconstructed there by evaluation of the transmitted procedural model with the explicit model as it existed
in the sending system. This will enable the detection of any gross errors occurring in the exchange. Also,
in certain cases ambiguities can arise in the reconstruction of a hybrid model, and it may be possible to
resolve these in the receiving system by reference to the explicit component of the dual model, which
exhibits the choices made by the creator of the model in the sending system.
EXAMPLE 2 A representation of a CAD model may include a set of nonlinear constraints whose corresponding
equations have multiple solutions. In this case the explicit model exhibits the choice of solution made by the
original designer.
As explained in clause 4.2.1, a procedural or hybrid model is parametric in nature, and the explicit com-
ponent of a dual model is an example from the represented parametric family. Specifically, the explicit
component is the example corresponding to the current values of the parameters in the procedural model
at the time of model transfer. Ideally, therefore, evaluation of the procedural model with those parameter
values in the receiving system should give an explicit model that is identical with the explicit component
of the dual model in the sending system. However, differences between the internal representations of
modelling systems will prevent the achievement of this ideal.
EXAMPLE 3 Two hypothetical CAD systems have the following characteristics:
System A: — Internal numerical tolerance for coincidence of points isunits;
Æ Æ
— Acircular cylindrical face is subdivided into three subfaces each subtending aangle.
System B: — Internal numerical tolerance for coincidence of points isunits;
Æ
— A circular cylindrical face is represented as single face, two of whose edges coincide along a seam
curve lying along a generator of the cylinder.
Such differences ensure that the precise details of the transmitted and regenerated explicit models will rarely be in
total agreement, though they will normally be close enough for practical purposes.
4.2.5 Representation of constructional operations in procedural models
An ISO 10303 model or representation is composed of instances of representation item (see ISO
10303-43). The occurrence of such an instance in an exchange file or shared database was originally in-
tended to be declarative, that is, to indicate the actual presence of the item in the model being transferred.
However, ISO 10303-11 specifies (in clause 9.2.5) that ‘When an entity is declared [in a schema], a con-
structor is also implicitly declared. The constructor identifier is the same as the entity identifier.The
constructor, when invoked, shall return a partial complex entity value for that entity data type to the point
of invocation.’. This capability was intended primarily for use in local or global rules in schemas, but
it is used in this part of ISO 10303 for the representation of constructional operations for instances of the
entity data types concerned to be performed in the receiving system following a transfer. Thus, whereas
the ‘point of invocation’ of the constructor was originally envisaged to be during rule checking by an
ISO 10303 translator, in the transfer of construction history models it will be during model regeneration
in the receiving system. The treatment of attribute values as parameters passed to constructors is spelled
out in detail in the cited clause of ISO 10303-11.
EXAMPLE Consider the following instance in an ISO 10303-21 [2] exchange file:
#210 = CIRCLE(’C1’, #150, 6.0);
c
10ISO 2005 — All rights reserved
here the attribute values represent, respectively, the name of the circle, a reference to its axis placement defined
elsewhere in the file, and its radius. If this instance is transferred as an element of an explicit model, the receiving
system will be expected to rewrite the definition of the circle in its native internal format and to build it into the data
structure of an explicit model of the boundary representation or similar type. By contrast, if the instance is specified
as part of an operation sequence in a construction history, it will invoke in the receiving system a procedure for the
ab initio creation of the specified circle. Thus in the first case the process is conceptually one of translation, and in
the second it is one of generation.
The quotation from ISO 10303-11 given above implies that procedural representations may be generated
in terms of instances of any subtypes of representation item for which EXPRESS definitions exist. A
wide selection of such entity data types is therefore immediately available for use with this part of ISO
10303.
Two consequences of the use of entity data type definitions as construction operations are the following:
a) It is necessary to distinguish between instances that are used as constructors and instances that
represent explicitly transferred elements forming part of a hybrid model;
b) It is also vital to capture the appropriate sequencing of instances used as constructors.
NOTE ISO 10303-21 [2] states that the sequence of instances in an ISO 10303-21 exchange file has no signifi-
cance.
The entity data type procedural representation sequence has been defined in clause 4.3.4 of this part
of ISO 10303 to capture the ordering of instances representing constructional operations in a procedural
model. It also serves to distinguish those instances that are to be interpreted as operations from those that
are not, which are excluded from such sequences.
A further aspect of the use of entity data type instances to represent operations is that local and global
rules applying to such instances are interpreted as constraints on the generated elements in the receiving
system, and not as validity conditions requiring to be checked as they are in the exchange of explicit
models using ISO 10303.
4.2.6 Implicit and explicit constraints
A hybrid model may contain both implicit and explicit constraints. Implicit constraints are constraints
in a model that arise automatically from the operation of a constructional procedure. Explicit constraints
are explicitly represented relationships between elements of an evaluated model.
EXAMPLE ISO 10303-42 defines an entity data type block, representing a rectangular parallelepiped. If block
is used as a constructional operation in a construction history model, the created block will have three pairs of
parallel faces, because this is inherent in the definition of the shape concerned. For this case the parallelism
constraints are implicit. If the dimensional parameters of the block are changed, the regenerated block will always
have three pairs of parallel faces.
On the other hand, consider a pure boundary representation of a block of the same type. This will contain six
faces, twelve edges and eight vertices. The parallelism of opposite faces results from the specific combinati
...
INTERNATIONAL ISO
STANDARD 10303-55
First edition
2005-02-01
Industrial automation systems — Product
data representation and exchange —
Part 55:
Integrated generic resource: Procedural
and hybrid representation
Systèmes d'automatisation industrielle — Représentation et échange
de données de produits —
Partie 55: Ressources génériques intégrées — Représentation
procédurale et hybride
Reference number
©
ISO 2005
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