A Survey on 3D Data Models in Architecture and

A Survey on 3D Data Models in Architecture and Civil Engineering Beatrix de-Cambray*, Thomas Delpy**, Karine Zeitouni* * PRiSM Laboratory - UVSQ 45 Av. des Etats Unis - 78000 Versailles ** EDF/DER - 1, avenue du général de Gaulle - 92140 Clamart, France Email : ABSTRACT This paper gives a brief survey on the areas which have contributed to the 3D modelling in architecture or civil engineering applications. Such models deal with three aspects. First, the usual 3D modelling (CAD models), second, free space representation, and, then, architectural systems. An application in Virtual Reality shows a use of such models. KEY WORDS: 3D data modelling, CAD, Free space, Topology, Virtual Reality

1.

Introduction

Data modelling for architectural applications is one of the domains where the convergence is necessary. These applications lead to be based on 3D spatial databases which constitute a "3D cartography" of a space fitted by and for man. As in the case of GIS (Geographical Information Systems), modelling these maps affects their exploitation and their exchange between systems. This modelling was first based on works on Computer Assisted Design (CAD). Then, it was enhanced to include either knowledge as in expert systems for architecture, or an abstract representation of the architectural entities in view of data exchange between softwares. The aim of the paper is to make a survey on the subject. In the first part, we will remind some notions about 3D modelling (mainly from CAD domain), then we will see the principles of free space representations (used in robotics), in the third part, we will detail some works on architectural modelling, finally we will highlight such modelling in a Virtual Reality application.

2.

3D CAD Models

These models represent the object shape in 3D. Among CAD models, solid models can be sorted in two main groups: surface models and volume models.

2.1 Surface models The boundary representation (BRep) (Requicha, 80) is one of the most spread model in solid modelling. It is based on the description of faces constituting the bounding surface of the object (figure 1). This model is well adapted to the objects that do not comprise regular shapes. On the contrary, it is unsuitable to the representation of objects as a sphere. In this case, it lacks concision and efficiency. Finally, it is generally the descriptive model used for the 3D visualization.

Obj

v5 f1

v4

e2 e3

…

e1

v1

f3 faces

f2

e3 v3 v1 v2 v3 f1 e1 v2 e2 x1,y1,z1 x2,y2,z2 x3,y3,z3

edges vertices coordinates

Figure 1 : The Boundary Representation Model Generalized maps (Lienhardt, 91) belong to this category. However, they offer a constructive process by "sewing". They are defined by using a single type of elements - called darts or half-edges - and specific functions (ai) on these darts called sewing operations -. An edge is formed by putting together two distinct darts by a first operation of sewing (a0), a face will be obtained by another type of sewing (a1), a volume is seen as a sewing (a2) of faces. Finally, several volumes can be tied together, by a3, to form a volume subdivision (figure 2). Sewing faces operation Sewing volumes operation

Figure 2 : Generalized map of dimension 3 By its constructive process, this method is useful in the design phase - its combination with methods of deformations applied to edges and to faces widens its field of application -. Contrarily to the BRep, it is defined for different dimensions and perfectly represents an assemblage of volumes. It also allows the control of the consistency and the calculation of orientability. 2.2 Volume Models

These models describe an object as a set of volume primitives that have a predefined regular shape like a cube, a sphere, or a cylinder. This is the case of Primitive Instancing, Constructive Solid Geometry (CSG), or the voxel model and its extensions (Requicha, 80; Foley and al, 90). In a CSG representation, the primitives are combined by union, intersection or difference operations to which it is necessary to add transformation operations: rotation, translation or scaling (figure 3). The problem is therefore to find a possible decomposition of the object into primitives.

U

Figure 3 : Example of an object represented with the CSG model The voxel model decomposes a solid into a set of cubical cells, called voxel, lying on a 3D grid. This model is not adequate to database techniques (Meier, 86). It is very often used in medicine (Ayache, 93) and in geology (Jones, 89). The Octree model (Hunter, 78) is a 3D generalization of the quadtree model (Samet, 84). This is a recursive decomposition of the space into eight cells of equal size (called octants) until the cells are entirely inside or outside the object. This decomposition process is represented by a tree of eight degree in which the root node corresponds to the whole object and non terminal nodes to the octants for which an additional subdivision is necessary (Samet, 84; Laurini and Milleret, 86) (figure 4).

Object

4 4 0

34 1

36

35 31

32 33

1

2

3

4

2 0

1

2

3 4

5 6 7

5 6 7

Figure 4 : Representation by the Octree model The problem with these decomposition models is to find a compromise between the accuracy and the data volume. Furthermore, this accuracy is inevitably restricted to some object shapes. This is why hybrid models are based on several

models in order to benefit from their advantages while limiting their disadvantages. This is the case of the extensions like "vector octrees", "extended octrees", or "polytrees" (Carlbom and al, 85; Navazo an al, 86; Jones, 89). For more information on 3D models, see (Cambray, 94).

3. The modelling of free spaces Architecture or town planning projects are not only concerned with the representation of solids, but they are also concerned with the representation of free spaces existing between the structures of civil engineering. This part deals with this representation for a closed and encumbered environment. This same subject has been dealt in the mobile robotics area for the problem of path planning. Indeed, an autonomous robot, to be controlled, needs a relatively accurate model of its environment. In this context, few works have studied this modelling in 3D except (Delpy, 95) that will be described in section 4.2. Most of free space models are based on a 2D description of the environment. The path planning is equivalent to searching a path between an initial position and a final position that avoids obstacles (Crowley, 86; Habib and Asama, 91; Badcock and al, 93). Generally, this search leads to a search in a graph (with usual algorithms as A* or Dijkstra) that describes free positions in the considered environment. Most of the methods lie on a decomposition of the 2D space into cells (convex, regular, irregular or Voronoï diagram cells) (example in figure 5a) (Laumond, 83; Iooss and al, 88; Jouvencel and Simphor, 91). In general, cells are linked together by a graph as a connectivity graph like the one represented in figure 5b. 1 s 1 2

2

4 3 10

18 17

24

20

5

9 11

13 12

19

3

15

15 16

4

g

14 23

25 26

6 21 22

16 17

18 13

10

9

12

11 14

19 8

7

Figure 5a : space decomposition into cells

24 25

5

20 23 21

8

6 22 7

26 Figure 5b : Its connectivity graph

These methods are sufficient for local retrievals in slightly encumbered spaces. On the contrary, they are ineffecient in more complex environments (like an industrial building). Semantic information concerning these spaces are not or slightly taken into account. The 3D topological information is non-existant. Complex environment examples are: a house with mezzanines and duplex, a nuclear power station, or a subway station with corridors, staircases, and different floors. It is important to know the height of doors to determine if they are obstacles for a mobile robot for example. Likewise, the knowledge of the ramp slope to a given room allows to know if a particular robot can take this slope. A model of buildings integrating the 3D representation of free spaces and their semantic description has been proposed in (Delpy and Zeitouni, 93). This model is not restricted to mobile robotics applications, but is general for other uses as we will underline it in the following sections. For a complete survey of free space modelling, we refer the reader to (Delpy, 95).

4. Models for the building We distinguish two categories of works. The purpose of the first one is an aid to architectural design, such as the automatic resolution of spatial allocation. Combining CAD with expert systems is obviously a very interesting solution. The second category comes from the need of data exchange in large architectural projects. It aims to define standard models for the building. 4.1 System Aided Architectural Design The modelling in system aided architectural design is often based on a declarative approach. It describes architectural objects and the design process (or design rules). These knowledges are used by the software, usually an expert system, to propose solutions for architectural design. The Archipel system (Maculet, 91) proposes a space subdivision (to be built) by automatically placing simple parallelepiped boxes and by respecting declared constraints of placing. The ROOS1 system (Flemming and al, 89) belongs to the same category. The Remus project aims to visualize or to simulate facades allowing therefore to represent the architectural and urban morphology (Mahbous and al, 93; Gamsau, 93). It uses an architectural knowledge base compound of typical architectural objects (bay, wall, windows, etc.) and of a set of production rules acting on these objects. These objects can be composed of and located in relation to other objects.

This system is specific because it does not represent the building interior but it represents its external geometrical shapes. 4.2 Conceptual description model of buildings The second group of work has been initiated in the framework of the program "Informatique et Productique Bâtiment " (IN.PRO.BAT) launched by the Architecture and Building Plan ("Plan Construction et Architecture") in 1985 and continued by the Data Structuring Group (GSD) (GSD91; LMB, 94). An expert committee has, since, been constituted within the AFNOR (French Organization for standards) (AFNOR) to propose, within the STEP project, a standard of technical data exchange in this area. The purpose is descriptive contrarily to the abovementioned models that helps to produce draughts. The free space notion is called "SPACE" by the GSD. It is defined as the interior of a hollow volume that is available for a given use. This can be especially a room. The notion of separator between two spaces allows to delimit them. A partition is a separator of rooms. These partitions can be opened by openings such as doors, windows, or air vent (figure 6).

Separator

Partition bounds Room

Opening

bored by +1

Equipment is located inside

is adjacent to

Furniture is located inside Figure 6 : Partial building model by GSD It is possible to classify rooms according to their use, for instance a room used for inhabiting; a technical room (e.g., boiler room, maintenance room), or a common room (e.g., entrance hall, staircase, landing). Gathering spaces into zones and zones into divisions have also been defined (Dubois, 90). They use gathering criteria linked to the semantics of these spaces (e.g.: a commercial zone).

The context of the Environmental DataBase (EDB) project of EDF (the French company of Electricity) is the navigation simulation in order to prepare maintenance operations in nuclear power stations. The EDB model is conceived around a topological graph (figure 7) called Volume Adjacency Graph (VAG). This model is based on the following entities: room, separator, passage point, and obstacle (figure 8). Contrarily to free space models described in section 3, the VAG allows for both the three dimensions and the semantical description of objects (Delpy and Zeitouni, 93). Functions of path search, according to user criteria, are solved using this structure (Delpy and al, 94). The result is a path represented by a list of rooms. For instance, in a Virtual Reality application, this allows to obtain the list of geometrical primitives that are visible from the user location.

Li

L3 d

L0

e c

L1 L2

b

Room (or local)

Si,p Label : Separation, passage point Obstacle L0 S1,a L1 S4,d

a Figure 7a : 2D map of a building

L3

S2,b

S5,e S3,c

L2

Figure 7b : the corresponding VAG

Local-1 S1-P1

Staircase-1

Local-2

Inclusion, S2,P2

Virtual separation Figure 7c : 3D environnment

Figure 7d : the corresponding VAG

Building composed of

+1

Room

+3

Obstacle

Separation

described by

described by Geometric described representation by set of +1

Vertical separation

bored by

Horizontal separation Opening

door

window virtual

Primitive

+1

3D location

+1

Virtual

Other separation

Wall

1-2

delimited by

Ceiling

contains

Floor

includes

+1

orientation parameters Figure 8 : The object model of EDB

Research on modelling are going on, as in the framework of the European project "COMBINE" (Dubois and Parand, 93) whose ambitious aim is to define the basis of a model of buildings that covers the whole aspects required by the design.

5. Application to Virtual Reality The Virtual Reality (VR) is a concept increasingly used in varied areas, that can either be architectural applications - especially design and simulation -, or other areas as surgery (Burdea and Coiffet, 93), art (Miller and al, 92) or flight simulators. Thus, in the interior design, the Virtual Reality is a new way for architects that allows the interactive exploration -by immersion- of buildings before their construction (Brooks and al, 92). Likewise, this virtual exploration is

particularly useful for inaccessible or hostile environments (as nuclear power stations). In robotics, it helps both in the design of robots and in the description of poorly known environments. Some say that virtual worlds are representation tools that enable a better communication, others say that is a new artistic mean. Finally, the Virtual Reality appears to be a new man-machine interface. In all cases, it is important to correctly manage data because the very large number of polygons to process leads to a slow visualization and is therefore incompatible with the desired interactivity. It is obvious that all the areas do not have the same needs in terms of accuracy and therefore data volumes. In either architecture, or robotics, or simulation systems in complex or poorly known environments, it is essential to have an accurate 3D modelling. 3D DataBases have to be managed with new techniques. The aim of such techniques is to take into account a minimum of geometry at a given moment according to the position of the observer. We call it segmentation of the model. Among the different approaches (Burdea and Coiffet, 93) that are possible, we find: 1 the anticipation of the observer displacement (memory allocation) 2 the incremental loading and division of the database (segmentation into cells) (Pimentel and Teixera, 93; Airey and al, 90). 3 the detail level of the modelling is adapted for objects that are assumed to be far from the observer (segmentation into details) (Latham, 93). 4 an hybrid solution that is based on memory allocation methods and on segmentation into cells: the step-by-step loading (Delpy, 95; Fertey and al, 95). In the solution proposed in section 4, it is necessary to astutely divide the data of the environment into different groups and to manage these data according to the user displacement. For that, we have adopted the EDB model described in the previous section. The topological graph of the EDB stores and manages the set of connection links between rooms. This allows, on the one hand, to search an optimum path that conducts the user and, on the other hand, to solve the segmentation by loading only the group of data corresponding to the rooms that are connected to the user position. The topological graph can thus be queried in real time by the Virtual Reality software.

6. Conclusion This paper allows us to make a survey on CAD models, on free space description methods, on building models, and on their application to virtual scenes. These methods mostly have two common points. The first one is the decomposition

into cells, that can either be 3D primitives in CAD, or 2D (free or occupied) cells in a navigation context, or rooms in a building, or, virtual subspaces resulting from the segmentation of too voluminous spaces in VR. The second common point is the management of structural or topological links between these cells. Furthermore, these areas are linked and complementary. Concerning architectural data modelling, there have been different evolution stages. At the beginning, it only represents 2D, afterwards 3D, draughts (using some CAD models of solids). Then, it was enhanced, either with knowledges enabling the automatic creation of 3D scale models, or with the semantics adapted to the civil engineering domain and the topological information, providing then a powerful tool of data auto-description and analysis. This auto-description is essential with view to technical data exchange in engineering projects. The present situation in this domain enables to perceive some orientations. The emergence of objects databases and the evolution towards concurrent engineering in Computer Integrated Manufacturing (CIM) will certainly encourage to use kernels of standard models in Technical Objects Management Systems. Design or simulation systems will be considered as applications of these systems and could extend the model to their own domain.

References AFNOR, Association Française de NOrmalisation, Building Construction Core Model, ISO/TC184/SC4/WG3/N342. Airey J., Rohlf J., and F. Brooks Jr., 1990, Towards Image Realism with Interactive Update Rates in Complex Virtual Building Environments, Computer Graphics, ACM, Vol.24, N°2, p 41-50. Ayache N., 1993, Volume Image Processing: Results and Research Challenges, INRIA Research Report, n° 2050, September 1993. Badcock J.M. and al, 1993, An autonomous robot navigation system - integrating environnemental mapping, path planning, localisation and motion control, Robotica, vol 11, p 97-103. Brooks F. and al, 1992, Walkthrough Project, Final report to the National Science Foundation, TR92-026, University of North Carolina, 24 p. Burdea G., Coiffet P., 1993, La Réalité Virtuelle, Edition Hermès, Paris. de Cambray B., 1994, Etude de la modélisation, de la représentation et de la manipulation de l'information spatiale 3D dans les bases de données géographiques (Modelling, Representing, and Manipulating 3D Spatial Information in Geographical Databases), PhD dissertation, University of Paris VI. Carlbom I., Chakravarty I., Vanderschel D., 1985, A Hierarchical Data Structure for Representing the Spatial Decomposition of 3D Objects, IEEE Computer Graphics and Applications, vol. 5, April 1985, p 24-31.

Crowley J.L, 1986, Navigation and world modelling for a mobile robot: a progress report, CAD & Robotics in architecture & construction. Delpy T., Zeitouni K., 1993, A Graph Model to Describe Topological Relationships in a 3D World, Third International Conference on Computers in Urban Planning and Urban Management, Atlanta, USA, Klosterman (Ed.), p 151-166. Delpy T., Brillault B., Zeitouni K., 1994, A Topological Database to Simulate 3D Robotics Navigation, Third International Conference on Automation, Robotics and Computer Vision (ICARV'94), Singapore. Delpy T., 1995, Modélisation et conception d'une Base de Donnée Environnementale pour de la Simulation de Navigation 3D en Environnement Complexe (Modelling and Design of an Environmental Database for the 3D Navigation Simulation in Complex Environment), PhD dissertation, University of Paris VI,. Dubois A.M., CSTB, 1990, Prototyping approach of multi-actors computer aided design for buildings in the frame of CIBAO project, Proceedings of MICAD 90, Paris. Dubois A.M., Parand F., 1993, "COMBINE Integrated Data Model", CIBSE National Conference. Fertey G., Delpy T., Lapierre M., Thibault G., 1995, An industrial application of virtual reality : An aid for designing maintenance operations in nuclear plants, 4th International Conference : Interface to real and virtual worlds, Montpellier. Flemming U., Coyne R.F., and al, 1989, A Generrative Expert System for the Design of Building Layouts (Final Report), Report N° EDRC 48-15-89, Carnegy Mellon University. Foley J. D., van Dam A., Feiner S., Hughes J., 1990, Computer Graphics, Principles and Practice, Addison-Wesley Systems Programming Series, Second edition. Gamsau, 1993, Groupe d'études pour l'Application des Méthodes Scientifiques à l'Architecture et l'Urbanisme, Rapport d'Activités 1990 - 1993, Activity Report. GSD, Groupe Structuration de Données, 1991, Synthèse des modèles conceptuels développés dans le cadre de la recherche bâtiment en France, N°10 de la Collection Recherches du PCA (Plan Construction et Architecture). Habib M.K., Asama H., 1991, Efficient method to generate collision free paths for autonomous mobile robot based on new free space structuring approach, IEEE/RSJ International Workshop on Intelligent Robots and Systems IROS'91, p 563-567, Nov 3-5 1991, Osaka, Japan. Hunter M., 1978, Efficient Computation and Data Structures for Graphics, Ph D dissertation, Departement of Electrical and Computer Science, Princeton Universuty, Princeton, NJ. Iooss E, Boulle M, Tournassoud P, 1988, Etude d'un sous système de navigation pour un robot mobile, INRIA Research Report , n°783, INRIA Sophia Antipolis. Jones, C. B., Data Structures for Three-Dimensional Spatial Information Systems in Geology, International Journal of Geographical Information Systems, vol. 3, n° 1, 1989, p. 15-31.

Jouvencel B, Simphor J.E, 1991, The variable modelling of mobile robot environement, IEEE/RSJ International Workshop on Intelligent Robots and Systems IROS'91, pp 1044-1051, Osaka, Japan. Latham R., 1993, What Isn't New in VR, Proceedings of VR System's 93 Conference, New York, p 255-258. Laumond J.P, 1983, Model structuring and concept recognition: two aspetcs of learning for a mobile robot, Proceedings of the eighth IJCAI 83, p 839-841, IJCAI, William Kaufman inc. Laurini R, Milleret F, 1986, Les clefs de Peano: un nouveau modèle pour les bases de données multidimensionnelles et les bases d'images, 2eme journées, conférence Bases de Données Avancées, Giens, France. Rocquencourt, France: INRIA, p 211-230. Lienhardt P, 1991, Topological models for boundary representation : a comparison with n-dimensional generalized maps, Computer-Aided Design, vol. 23, n° 1, January 1991, p 59-82. LMB, La Lettre des Modèles pour le Bâtiment N°3, 1994, publication of the GSD group, November 1994. Maculet R, 1991, Archipel : Intelligence Artificielle et Conception Assistée par Ordinateur en Architecture, Phd dissertation, University of Paris VI. Ben Mahbous M., Quintrand P., Zoller J., 1993, From cartography to 3D representation of urban morphology, AREC-DAO conference, Barcelone. Meier A., 1986, Applying Relational Database Techniques to Solid Modelling, Computer-Aided design, Butterworth & Co Editeurs, vol. 18, n° 6, Juillet-Août 1986, p. 319-326. Miller G and al, 1992, The Virtual Museum : Interactive 3D Navigation of a Multimedia Database, Journal of Visualization and Computer Animation , vol 3, pp 183-197, John Wiley and Sons. Navazo I., Ayala D., Brunet P., A Geometric Modeller Based on the Exact Octtree Representation of Polyhedra, Computer Graphics Forum, n° 5, p. 91-104. Pimentel K, Teixeira K, 1993 Virtual Reality : Through the New Looking Glass , Windcrest McGraw-Hill, New York. Requicha A.G., 1980, Representations for Rigid Solids: Theory, Methods, and Systems, Computing Surveys, vol. 12, n° 4, December 1980, p. 437-464. Samet H., 1984, The Quadtree and Related Hierarchical Data Structures, ACM Computing Surveys, vol. 16, n° 2, p. 187-260.