Property Link Model Managing Relationships Between Parts of by Jeronohnson

VIEWS: 4 PAGES: 10

									                                 Property Link Model:
               Managing Relationships Between Parts of Architectural Plans
                     Extending the Standard CAAD Data Structure

                                                                         Kovács András




1. EARLIER APPROACHES ................................................................................................................................................. 2
   1.1. DEVELOPMENT OF COMPUTER AIDED ARCHITECTURAL DESIGN........................................................................................... 2
   1.2. CONSTRAINT-BASED SYSTEMS ........................................................................................................................................... 2
   1.3. EXTENDING THE STANDARD CAAD DATA STRUCTURE ...................................................................................................... 3
2. CONNECTIONS AND INTERACTION BETWEEN FIELDS OF GEOMETRIC DATABASES .............................. 3
   2.1. THE STANDARD CAAD DATA STRUCTURES ....................................................................................................................... 3
   2.2. CONSTRAINTS ................................................................................................................................................................... 3
   2.3. CONNECTION BETWEEN STANDARD CAAD OBJECTS .......................................................................................................... 4
3. THE PROPERTY LINK MODEL ..................................................................................................................................... 4
   3.1. PROPERTY LINKS .............................................................................................................................................................. 4
   3.2. DERIVED PROPERTIES ........................................................................................................................................................ 5
     3.2.1. Alternative data structures ....................................................................................................................................... 6
     3.2.2. Dependencies............................................................................................................................................................ 6
     3.2.3. Setting a new value................................................................................................................................................... 6
     3.2.4. Read-only properties ................................................................................................................................................ 6
     3.2.5. Object coordinate system (OCS) .............................................................................................................................. 7
   3.3. APPLYING CHANGES .......................................................................................................................................................... 7
     3.3.1. Not mutually dependent properties........................................................................................................................... 7
     3.3.2. Returned link sequences ........................................................................................................................................... 8
     3.3.3. Mapping the modification graph .............................................................................................................................. 8
     3.3.4. Processes following the modification cycle .............................................................................................................. 9
     3.3.5. Handling temporary states ....................................................................................................................................... 9
   3.4. BI-DIRECTIONAL CONNECTION TO NUMERIC DATABASES .................................................................................................... 9
4. POTENTIAL APPLICATIONS ......................................................................................................................................... 9

5. REFERENCES ................................................................................................................................................................... 10




                                                                                           1
                                                           CAAD softwares used nowadays can be divided into
This paper discusses the connections and
                                                           two groups depending on how they store and
interactions among CAAD objects focusing on the
                                                           handle geometric relationships. The majority of
development of computer aided architectural design
                                                           CAAD softwares form the first group: they do not
and, especially, constraint-based systems. It
                                                           support this possibility except for some special
presents the Property Link Model, a system that
                                                           cases. Connecting geometric data is carried out
provides an efficient solution to manage
                                                           mostly by relational databases: line and fill types,
relationships between parts of architectural plans
                                                           materials, library parts, blocks, relative heights.
extending the standard CAAD data structure. The
                                                           Occasionally connections between data fields of
model introduces high-level object descriptors, the
                                                           equal ranks can be found as well: dimensioning,
so-called derived properties, which can be joined
                                                           associative hatches (AutoCAD), relative positioning
with the aid of property links.
                                                           of cells (MicroStation); or some data connected with
                                                           the aid of refreshing algorithms e.g., zone contours
Possibilities of PLM cannot be accessed directly by        (ArchiCAD). The other group consists of programs
the user, only by the built-in tools, imported plug-       which aspire to give the general solution of the
ins or geometric descriptive languages. Plug-ins or        problem. Constraint-based and process-based1
object libraries based on PLM can be developed             systems can be highlighted; their common feature is
which support e.g., working with a special                 that the structure of the system is subordinated to
construction system.                                       that issue.


1. EARLIER APPROACHES                                      1.2. Constraint-based systems
                                                           Constraint-based CAD systems can be found in
1.1. Development of computer aided                         academe and on the market as well. MicroStation
architectural design                                       with the Dimension Driven Design plug-in is a
                                                           general constraint-based CAD; the most known
Manual drawing techniques provided solutions to all        special architectural application is Revit. Geometric
the drafting problems occurring in architectural           constraint-based CAD system are Cabri Géomètre
practice when computer aided design appeared.              and SketchPad. An example for experimental
CAD softwares could not compete with this set of           applications is the Interactive 2D Constraint-Based
tools for a long time; they still possessed some           Geometric Construction System.2
important     advantages:     high    accuracy   of
representation, ease of modification, data exchange        Constraint-based systems are already accepted in
without loss of information. In addition to                other special fields (e.g., engineering), but they did
technological development, the development of              not spread in architectural planning. Since a
architectural drafting began in consequence of the         constraint-based system is efficient if the planning
new possibilities of CAD, as well.                         proceeds according to a well-defined rule set, each
                                                           possible variation needs to suit its requirements.
Elements of traditional drawings only include simple       Architectural design is not like this, especially when
geometric data, the interpretation of the plan is          it comes to aesthetic and conceptional planning:
necessary to gain further information which makes          rules are inconceivable and far-reaching. Variations
the drawing an architectural plan. The first CAD           of the plan are often in the used set of rules itself:
softwares did not make it possible to handle any           from the point of view of a constraint-based system
additional information: methods of architectural           there is no common structure among these
drafting remained unchanged. Introducing the 3rd           variations.
dimension meant the first considerable deviation
from traditional drafting: 3D CAD programs                 To use the constraint-based model efficiently it is
possesses data with which lines appearing in               necessary to build up the relations among elements
different views can be handled together, they are          of the plan according to the real considerations of
able to perform operations which require this
information. Extending the CAD database with non-
geometric data made it also possible to apply              1
                                                             Structure of process-based systems fundamentally differ
methods which are not supported by the manual              from manual techniques and softwares discussed above.
drafting.                                                  They provide parametric solutions to minor problems, but
                                                           they are unable to handle complex planning tasks
The CAD software possesses more and more                   efficiently, so complete process-based applications cannot
information about the plan in each step of the             be found, rather they are integrated in standard CAAD
                                                           systems and are used for solving partial tasks (GDL,
development process. The CAD database can be
                                                           AutoLisp).
extended with a new type of data: the software is
capable of handling and storing relationships              2
                                                               Medjdoub, 1999.
between elements of the plan, so it will be able to
automatically perform operations which require
information about geometric or other relationships.


                                                       2
the planning process. The architect is not                    standard spreaded in the construction industry
necessarily aware of these considerations; a                  called IFC4.
significant part of the problem-solving ability of a
skilled designer is heuristic and is based on
experience.3 The number of considerations which
can be consciously taken into account is related to
                                                              2. CONNECTIONS AND
the capacity of the short-term memory (7±2                           INTERACTION BETWEEN
cognitive scheme). So the designer cannot
formulate the reason for his complex decisions as                    FIELDS OF GEOMETRIC
rules, or only with a significant additional mental
effort.                                                              DATABASES

Every step of architectural planning is executed on
the basis of an inconceivable and far-reaching rule
                                                              2.1. The standard CAAD data
set, using the constraint-based model does not                structures
seem to be desirable, return on the work invested is
only expected in special fields such as standard              Standard data structures used nowadays provide no
designs.                                                      general solution to connect geometric data, joining
                                                              objects and following changes is possible only in
                                                              some special cases: e.g., associative dimensions
Constraint-based systems have to solve design
                                                              and hatches follow the movement of CAAD objects
problems of modifying or enlarging the plan by
                                                              which determine their geometry. Similarly joined
relying on known relations. Because of the
                                                              are, for instance, the ArchiCAD zone contours to
undefined problem space and the need for control
                                                              neighbouring walls or drawings inserted in
criteria the system cannot perform this task; in the
                                                              PlotMaker to a story of an ArchiCAD plan file. In
field of the general architectural planning constraint-
                                                              these cases a refreshing algorithm started by the
based systems are not efficient.
                                                              user is also necessary.
They are still viable in fields where planning runs
according to a well-defined rule set and suitable             2.2. Constraints
control criteria are available.
                                                              According to the general terminology, a constraint
                                                              in a constraint-based CAD system means a
1.3. Extending the standard CAAD data                         geometric relationship which has to be created or
structure                                                     preserved, the task of the software is to obtain and
                                                              maintain the state required by these relationships.
The system presented in the following works as an             The constraint solver methods compute the state
extension of the standard CAAD data structure, it             determined by the constraints as an equation with
combines the versatility and reliability of the               many unknowns and generally use iterative
standard CAAD softwares with the “intelligence” of            algorithms.
constraint-based systems.
                                                              In case of well constrained models5, as many
Joining to the standard data structure has several            constraints are needed such that the geometric
advantages. Full compatibility with standard CAAD             problem has one or more (but in any case finite)
systems can be reached. Since systems managing                solution; this approach is suitable to solve geometric
relationships between parts of plans such as                  equations and construction problems. If the model
constraint-based systems are efficient only in special        is under constrained, constraints are responsible for
fields, the user has to use the possibility offered by        required executing of modifications while retaining
the model if they are necessary, but there is no              a certain degree of freedom. When processing an
need to solve all the problems within the new                 architectural plan, a constraint-based system works
system.      Thus partial compatibility and data              with under constrained models.
exchange with other CAD systems can be reached
more easily, and developers do not need to make
up for a backlog of many decades to make the
software marketable.

The extension of the standard CAAD data structure             4
                                                                 Property links presented in the following can be
is developed to be 100% compatible with the                   described with the IfcRelationShip class. References to
                                                              derived properties can be stored in Property Sets
                                                              extending the above class.

3                                                             5
    Rasmussen, 1983.                                              Medjdoub 1999.




                                                          3
In constraint-based systems constraint solvers                    The problems of the direct use of constraint-based
ensure that the state defined by the constraints is               systems in the general architectural planning were
reached, the data structure must be aligned to                    discussed in the first section. The difficulty of
them. Constraint solvers are not efficient with large             formulating the user interface of PLM is the same:
number of CAAD elements and complex                               manual creating and managing of property links
relationships, they are computationally expensive,                would mean entering additional information in the
handling of temporary states and errors is not                    model with more invested work. PLM is worked out
supported.                                                        as a low-level geometric database managing tool:
                                                                  possibilities of PLM cannot be accessed directly by
                                                                  the user, only by the built-in tools or imported plug-
2.3. Connection between standard                                  ins; geometric descriptive languages (e.g.,
CAAD objects                                                      Graphisoft GDL) can be also completed with
                                                                  functions which support creating and managing
The model presented in this paper gives the general               links between CAAD objects.
solution of the mechanism already available in
special cases in current CAAD softwares. The model
is based on the standard CAAD data structure. To                  3.1. Property Links
maintain the determined relationships between
parts of the plan, property links are introduced,                 The properties of the objects can be connected by
which connect certain properties of objects.                      means of property links. Links are directed: all of
Property links can perform simple value copy or                   them have at least one base and one target
complex geometric transformations, too.                           property. The link object can also contain extra data
                                                                  fields needed for modification.
Property links play a role similar to constraints in
constraint-based systems, but their structure and                  Object                              Object
operation differ from the constraints. They are                                     base
standalone parts of the data structure, their task is                Property                            Property
to manage relationships between CAAD objects.
Property links work like references and formulas in                  Property                            Property
spreadsheets: they determine properties of CAAD
plan elements as a function of data coming from                      Property                            Property
other elements. In this system constraint solver
functions are not necessary: applying property links
are initiated by modified CAAD objects, further                    Object                              Object
modifications are made by the links themselves,
with mapping a tree-structure.                                       Property                            Property

                                                                     Property                            Property
3. THE PROPERTY LINK                                                                         target
   MODEL                                                             Property                            Property

The Property Link Model provides connecting                       A link can be direct or indirect depending on
properties of CAAD objects and can be implemented                 whether it modifies the value of the target property
in any CAAD system based on the standard data                     from the value or the change of the base property.
structure6.                                                       In case of using direct links, the new value of the




6
  A sample application can be downloaded from the site:
www.arch.bme.hu/publikaciok. The program is made only
in order to demonstrate the possibilities of PLM, the user
interface serve for presenting the operation of the system.




                                                              4
target property depends only on the value of the                   3.2. Derived properties
base, indirect links consider the old values of
properties to set new values7.                                     The standard CAAD data structure can be described
                                                                   as follows: the plan consists of CAAD objects with
                                                                   geometric and other data fields according to their
                                                                   types.

                                                                   The Property Link Model introduces derived
7                                                                  properties which describe the CAAD object at a
    Property links implemented in the sample application:
Value Copy connects all properties of the same type, one           higher level. A derived property represents a quality
base and one target property can be joined. When applied,          determined by one or more data fields of the object.
it copies the value of the base property to the target. Line       There is a derived property corresponding to each
Connect connects the endpoints of two straight lines while         data field and complex properties determined by
keeping their slopes. Polygon Clip places the endpoints of         more data fields can be used, as well8.
a straight line on the border of the given polygon. Base
properties of Summarize have got numerical values, the
                                                                   From the point of view of the software structure,
link adds them, and the value of the target will be equal to
their sum.                                                         property links are OOP objects with no data
                                                                   members of their own. They have only member
                                                                   functions, with which the value of a property can be
                                                                   computed or set up. Increasing the number of
                                                                   derived properties does not expand the memory
                                                                   used to store the data of a plan and causes no
                                                                   decrease of performance or speed.

                                                                   Derived properties provide an interface to CAAD
                                                                   objects: property links can be joined to them,
                                                                   modifications started by the user must be applied
                                                                   through derived properties as well. Introducing
                                                                   some derived properties and property links makes
                                                                   the system flexible, and many kinds of connections
                                                                   between CAAD objects can be created.

                                                                   The set of derived properties defined for a CAAD
The only indirect link implemented in the sample                   object does not depend on the method of storing
application performs a simple change of base                       the actual drawing data; functions that get and set
transformation. Computations in case of given types (t –           the value of a derived property must be adjusted to
target; b – base; X’ – new value of X):                            the data structure used by the CAAD software. This
                                                                   solution ensures the software-independence of the
                   b'                                              model. On the other hand, standardised property
                        t'                                         sets can make simple data transfers between
                                          B'    B                  different CAD systems possible.
     b
               t

           B
                             B'                          B'
T           b                                       B
                                  b'
      t                 T'
                                  t'
                                                                   8
                                                                     Derived properties of the line segment object in the
                                                                   sample application:
      -   Integer, real: t' = t - b + b'
      -   Point with 3 coordinates: t' = t - b + b'                -   OCS, Origo, Direction (global geometric properties)
      -   Direction (3x3 matrix): T' = B-1 B' T                    -   ID, Color, Layer (not geometric properties)
      -   Coordinate system (a direction and a point):             -   PointA, PointB (Structure #1)
             T' = B-1 B' T                                         -   Slope, LineEndA, LineEndB (Structure #2)
             t' = B-1 B' (t - b) + b'                              -   Length (read-only)




                                                               5
3.2.1. Alternative data structures                            Property “A” must be defined as a property
                                                              depending on “B”, if the change of links connected
                                                              to property “A” is expected when property “B” is
                                                              modified.9

                                                              Dependence of two properties is not necessarily
                                                              mutual. Properties concerning the whole CAAD
                                                              object like OCS10 do not depend on any other
                                                              property, but all properties of the same character
                                                              depend on them. If a property is linked, properties
                                                              depending on it cannot be linked any more, except
                                                              for the not mutually linked ones. If property “A”
  Structure #1        Structure #2                            depends on “B”, but “B” is independent of “A” , both
                                                              links can be applied, thus “B” can be linked in this
                                                              case, too.
  Point A             Straight
  Point B              unity vector with direction AB
                       distance OE
                                                              3.2.3. Setting a new value
                      End of line by point A
                       distance EA                            Values of derived properties can be set in the
                      End of line by point B                  following way. First the system calculates the value
                       distance EB                            of the given property from one or more data field of
                                                              the CAAD object. The data necessary for the inverse
A line segment can be described, for example, with            operation (computing the new values of data fields
two data fields: the two endpoints. To link the               according to the modified property) will be stored as
endpoints to two other existing points, two separate          well. After determining the new value of the
links can be applied, the problem can be solved with          property, the system modifies the data fields with
linking the data fields. However, in an other case            the aid of the stored additional data.11
the following relations may also be required: let the
line be parallel to another one; let the endpoints of
                                                              3.2.4. Read-only properties
the line segment be defined by intersection of the
line with two other line segments. To satisfy the             Read-only properties can be used if the modification
parallel condition, the slope of the line segment             of a property definitely cannot be applied to the
must be modified, to perform this operation, both             CAAD object.
endpoints, both data fields must be accessed. To
perform the modifications unambiguously, one data
can be linked by only one link, so there is no
possibility to apply further links to the endpoints, to
place the endpoints to the intersections is no longer
possible. The problem could be solved if the
                                                              9
geometry of the line segment were stored as a                  Dependencies of derived properties of the line segment
straight and two distances on the line; in this case          object in the sample application:
both links could be applied, but linking the two
endpoints independently would not be possible.                - OCS: Origo, Direction
                                                              - Origo, Direction: OCS
Introducing derived properties can be used to solve
                                                              - ID, Color, Layer: nothing
problems like this: the actual storing of the data and        - PointA: OCS, Origo, Direction, Slope, LineEndA
linking certain properties of an object must be               - PointB: OCS, Origo, Direction, Slope, LineEndB
handled separately.                                           - Slope: OCS, Origo, Direction, PointA, PointB
                                                              - LineEndA, LineEndB: OCS, Origo, Direction, PointA,
                                                              PointB, Slope
3.2.2. Dependencies                                           - Length: OCS, Origo, Direction, PointA, PointB, Slope,
                                                              LineEndA, LineEndB
To determine the value of a derived property more
than one data field of the CAAD object may be                 10
                                                                   See also the section Object Coordinate System
required. If more properties refer to a data field,
these     properties       cannot     be     controlled       11
                                                                   To shift a line segment perpendicularly to itself, the
independently. When formulating the property set              Slope property must be modified. Before the modification
of a CAAD object, the dependencies of derived                 the position of the endpoint on the straight line must be
properties must be exactly specified. Dependencies            stored. According to the new value of the Slope property
play a significant role in applying property links.           and the stored endpoint positions the modified line
                                                              segment can be built up.




                                                          6
Reports (e.g., Area list, Element list) can use read-       be developed according to technical considerations.
only properties (e.g., area or length); these               OCS can be defined e.g. on the basis of the points
properties can be used as geometric or numerical            of an object, in this case the necessary number of
data. Some geometric property such as the centre            points are more than the number of dimensions12.
of gravity can be defined as read-only as well.
                                                            If the position of the OCS is defined for the user or
                                                            the OCS property has an own data field, parts of the
3.2.5. Object coordinate system (OCS)
                                                            OCS (e.g. only the direction of coordinate axes) can
In case of modifying or linking the position of an          be accessed: the direction property can be linked.
object, the Object Coordinate System property must
be used. Changing the OCS property applies an
congruency transformation to the CAAD object.
                                                            3.3. Applying changes
Shifting all points of an object or only the OCS
                                                            If the system receives a message initiating the
property are not equivalent: each geometric
                                                            modification of a property, and this property is the
property depend on the OCS but the OCS does not
                                                            base of a link, applying of the property link begins:
depend on any of them, so if the OCS is the base of
                                                            the targets of the link must follow the changes of
a link, the link will not be applied when the points
                                                            the base properties:
are shifted one by one.
                                                                 1.   If the link is indirect, it stores data derived
Since the values of derived properties will be
                                                                      from original values of base properties.
calculated before all changes, the result of a
congruency transformation applied to a CAAD object
                                                                 2.   After that it gives back the control to the
does not depend on the interpretation (way of
                                                                      derived property, which will be changed.
computing from the data fields) of the OCS.
                                                                 3.   Finally the property link determines the
If a polygon does not have a distinguished point,
                                                                      new values of target properties and starts
the origin of the OCS can be attached to corner No.
                                                                      the modification of target properties. In
1. This information will not be accessed by the user,
                                                                      case of using direct links, the new value of
only by the software. In case of rotating the object,
                                                                      the target property depends only on the
the model performs the following steps: 1 - the
                                                                      value of the base; indirect links consider
value of the OCS property is computed according to
                                                                      the old values of properties to set new
the previous correspondence, 2 – the OCS-relative
                                                                      values.
coordinates of each corner are calculated, 3 -
applies the transformation to the OCS property, 4 –
based on the data computed in the step 2 the                During the modification process all properties
rotated polygon will be built in the new OCS.               depending on the originally changed property are
                                                            qualified as modified, too, and property links
                                                            attached to them are applied.
                            1.
                                                            3.3.1. Not mutually dependent properties
              2.                                            The properties of a polygon object are A (a corner)
                                                            and OCS (the Object Coordinate System), property
                                                            A depends on property OCS, but OCS does not
3.                                                          depend on A. Either property can be the target of a
                                                            link. When applying the link joined to A, only A will
                                                            be modified, OCS remains unchanged. When
                                                            applying the link joined to OCS, however, the link
                                     4.

                                                            12
                                                               In the sample applications the OCS properties of some
                                                            objects are under-defined: the line segment and the
                                                            objects derived from it define the OCS on the basis of the
                                                            two endpoints, these objects do not respond properly to
If the OCS were fixed to the second corner, the             some transformations e.g., the mirroring to the plane
                                                            perpendicular to the XY plane. The problem can be solved
result would be the same: there is no need to
                                                            only with adding a new data field, however to do this, it is
reserve an own data field for the OCS property, the         necessary to modify the data structure of the frame CAAD
method which calculates the value of the OCS can            system.




                                                        7
joined to OCS modifies both A and OCS properties,              1.
according to the algorithm outlined above. The link                         M       Link
connected to A remains ineffective, although the                 Property                              Property
model is expected to change property A according                                    Link
to the link joined to it.
                                                               2.
1.                            2.                                            M       Link                           M
           A                            A                        Property                              Property
                                                                                    Link

                                                               3.
                                                                            M       Link                           M
                                                                 Property                              Property
                                                                                    Link

                                                               3.3.3. Mapping the modification graph
To ensure the appropriate behaviour of not mutually
dependent properties, the above link application               In case of property links with more base properties
algorithm must be augmented with the following:                it can happen that two base properties change in a
                                                               modification cycle. The algorithm which handles
     1.   If there are links joined to the above A             returning link sequences marks the target property
          property, the original values of the base            when a first applying request is performed, it cannot
          properties must be stored.                           be modified by the second request any more.

     2.   After changing the value of the OCS, a new           To get rid of these error possibilities, the current
          value must also be assigned to A according           modification graph is mapped before all modification
          to the link joined to it. Links which are            cycles. Mapping means that the properties send
          connected to the properties modified                 application requests to the links, while the links
          during the above operation can be applied            send modification request to properties as during
          only after this.                                     the real modification cycle, but the values of the
                                                               target properties will not be set, each property link
                                                               stores the number of incoming modification
3.3.2. Returned link sequences                                 requests instead.

If the sequence of links forms a returning loop, the           After mapping the modification graph the real
recursive link application algorithm arrives at infinite       modification cycle begins. Knowing the number of
cycle. This case can occur both due to link errors             application requests, property links can wait for the
and when representing existent geometric                       last one, and modify the target properties after that.
relationships; the efficiency of the system would be
reduced if the creation of returning link sequences
                                                               Indirect links store the values of the base properties
were prohibited.
                                                               when the first application request came in.
One of the simplest cases is when two properties
are linked to each other. The system is expected to              Property                              Property
modify either property according to the change of
                                                                       1.                                2.
the other.                                                                          Link
The system provides the following solution: in a                                    1.      2.
                                                                                                 M
modification cycle the link application algorithm                                   Property
marks the properties already changed, so if such
                                                                mapping
property is found later as the target of another
property link, the property will not be changed.
                                                                 Property                              Property
                                                                       1.                                2.
                                                                                    Link
                                                                                            2.
                                                                modification        Property




                                                           8
3.3.4. Processes following the modification                         separately: stores them on a custom layer, draws
cycle                                                               them with a different line type, etc. The invalid
                                                                    status exists as long as the geometric structure
During the performing of the modification cycle the                 remains faulty (the invalidating message is not
relationships of objects, properties and links must                 cancelled by the sender).
be equal to the structure previously mapped, hence
changes which modify this structure are not
allowed. After a modification cycle, it is possible to              3.4. Bi-directional connection to
create or delete links or to start a new modification
                                                                    numeric databases
cycle, during the performing of the cycle the system
only notes the necessary modifications13.                           With the aid of PLM direct connection can be
                                                                    created and managed between the geometric model
3.3.5. Handling temporary states                                    and external numeric databases, data can be both
                                                                    exported e.g., for reports and imported e.g., from
In the modification cycle, error checking is                        expert systems. The interface is a special CAAD
performed before a derived property will be                         object which represents the external database,
changed. If the new value of the property is not                    derived properties of this object represent its
compatible with a valid geometric (or other)                        records. With property links attached to it, any
situation, the property will not be changed, links                  CAAD object can be influenced.
which have this property as a base will not be
applied, but the connection between properties and
link remains. After a new transformation the
property may get an acceptable value, in such a
case the property will be modified and the links
attached to it will be applied again.

If an error occurs in modifying a property, there is
an opportunity to invalidate the object. A property
link or another object, depending on where the
error happened, can send an invalidating
message14. The system handles invalid objects



13
   If the Multiply object of the sample application is              4. POTENTIAL APPLICATIONS
expanded and a new handle comes into existence, the
Multiply object creates a new object by copying the                 The more information a virtual model contains, the
existent object attached to the first or last handle and            more work is needed to create it. The model
connects the new object to the new handle with the aid of
                                                                    created with a lot of work and professional expertise
a new Simple link. During the operation new objects and
property links come into being, these modifications cannot          must be used by enough people in the potential
be carried out in the course of the modification cycle, the         target fields to recover the investment. The world of
Multiply object only indicates that a task remains to be            construction systems can be such a special field: the
performed. After the modification cycle the object gets the         construction of structures is processed according to
control back; it creates the objects and the links and starts
a new modification cycle, if necessary.
                                                                    handles. Invalidating message can be sent by the Polgon
                                                                    Clip link, as well, if the line segment attached to it does
                                                                    not intersect the base polygon.




14
  E.g., invalidating message can be sent by the Multiply
object to the object attached to its handle if it can not
place the object because of the decrease in the number of




                                                                9
a well defined and mathematically describable rule                  ISBN 1-880250-06-3] Cincinatti, Ohio
set.                                                                (USA) 3-5 October 1997, pp. 251-264

To adopt a complete construction system, product               4.   Medjdoub, Benachir: Interactive 2D
catalogues stored in electronic format have no more                 Constraint-Based                Geometric
advantage than a database of independent structure                  Construction System – 1999 –
parts. To help the user of a CAAD system with                       Proceedings of the Eighth International
adopting a special construction system, the data of                 Conference         on       ComputerAided
individual elements and construction methods,                       Architectural Design Futures [ISBN 0-7923-
which are also important parts of the system, must                  8536-5] Atlanta, 7-8 June 1999, pp. 197-
be integrated. In order to utilise the possibilities of             212
PLM, efficient plug-ins and object libraries can be
created for special construction systems.                      5.   Tobin, Kenneth L. : Constraint-Based
                                                                    Three-Dimensional Modeling as a
The suggested structure of a CAAD plug-in                           Design Tool – 1991 – Reality and Virtual
supporting the construction systems is hierarchical.                Reality [ACADIA Conference Proceedings /
From the large units towards the smallest more                      ISBN      1-880250-00-4]   Los    Angeles
detailed levels can be distinguished15, this                        (California - USA) October 1991, pp. 193-
corresponds to processing architectural plans. An                   209
element of a construction system includes
geometric, technological, structural data and the              6.   Rassmussen, J. 1983, Skills, Rules,
constructing methods as well, with which the                        and Knowledge; Signals, Signs, and
objects of the next level can be created and placed.                Symbols, and Other Distinctions in Human
Elements are attached to the objects on the                         Performance Models. IEEE Transactions on
previous level with property links, so the user can                 Systems, Man, and Cybernetics, Vol. Smc-
intervene in any level to reach the final state. The                13, No. 3, May/June.
system can produce plans detailed at any level. The
above mechanism can be completed with a 3D
database which stores the space reservations of
objects, so the construction methods integrated in
the elements of the construction systems can take
holes, intersections, and available space into
consideration.


5. REFERENCES
       1.   Oksala , T.: Logical Models for Rule-
            based CAAD – 1988 – CAAD futures ‘87
            [Conference Proceedings / ISBN 0-444-
            42916-6] Eindhoven (The Netherlands),
            20-22 May 1987, pp. 107-116

       2.   Gougoudilis, Vasileios: Hyperwalls or
            an Application of a Non-deterministic
            Rule-based System in Interactive
            Architectural Modelling – 1995 – Sixth
            International Conference on Computer-
            Aided Architectural Design Futures [ISBN
            9971-62-423-0]      Singapore,     24-26
            September 1995, pp. 173-179

       3.   Seebohm, Thomas and Wallace,
            William: Rule - Based Representation
            Of Design In Architectural Practice –
            1997 – Design and Representation
            [ACADIA ‘97 Conference Proceedings /



15
     e.g. slab beams




                                                          10

								
To top