Multi-Resolution Modeling in Collaborative Design

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1 Multi-Resolution Modeling in Collaborative Design JungHyun Han 1, Taeseong Kim 1, Christopher D. Cera 2, William C. Regli 2 1 Computer Graphics Laboratory, School of Information and Communications Engineering, Sung Kyun Kwan University, Suwon, , Korea 2 Geometric and Intelligent Computing Laboratory, Department of Computer Science, Drexel University, Philadelphia, PA 19104, USA Abstract. This paper provides a framework for information assurance within collaborative design, based on a technique we call role-based viewing. Such rolebased viewing is achieved through integration of multi-resolution geometry and security models. 3D models are geometrically partitioned, and the partitioning is used to create multi-resolution mesh hierarchies. Extracting an appropriately simplified model suitable for access rights for individual designers within a collaborative design environment is driven by an elaborate access control mechanism. 1 Introduction In collaborative design, access control is mission-critical. Suppose a team of designers working collaboratively on a 3D assembly model. In collaboration, designers must interface with others components, but do so in a way that provides each designer with only the level of information he or she is permitted to have about each of the components. In the authors recent work [1], integration of multi-resolution geometry and access control models has been proposed. Its conceptual architecture can be stated as follows: An assembly model consists of a set of component parts, grouped into sub-assemblies. Each component part is represented as a NURBS-based boundary model. Design is performed collaboratively by a group of designers. A standard clientserver architecture is assumed, where the collaborative CAD server maintains and synchronizes the master design model. The collaborative CAD server tessellates the master model into polygon meshes. Multi-resolution mesh hierarchies are constructed. Depending on the accessibility privilege of a designer, an appropriately simplified model is extracted from the multi-resolution hierarchies, and provided to the designer at the collaborative CAD client. When a component part or sub-assembly gets modified, the server reconstructs only the corresponding (changed) portion of the hierarchy, and then passes these updates to the other clients according to their accessibility privileges. This paper proposes a new elaborate scheme for multi-resolution hierarchies and access control mechanism, which has demonstrated a better performance than the previous work of the authors [1].

2 2 JungHyun Han et al. 2 Related Work Out of the vast body of work on concurrent engineering and collaborative design, some of existing work most relevant to our efforts deals with real-time 3D collaboration and communication, including Distributed Virtual Environments (DVEs) [2, 4, 5, 9, 12], immersive environments such as CAVE [3], etc. These systems support real-time interaction, but do not necessarily support collaborative CAD. In CAD and collaborative design contexts, few research results on access control have been reported. A most relevant work in the domain of collaborative assembly design can be found in Shyamsundar and Gadh [15]. Their work could be taken as a simple implementation of information-hiding techniques, but lacks an elaborate access control mechanism. Further, they do not provide fine-grained levels of detail. Polygon mesh simplification has been adopted for generating multi-resolution models or various levels of detail (LOD). Rossignac and Borrel [8] proposed vertex clustering approach to mesh simplification, where the initial cluster is recursively divided into cells and vertices in a cell are replaced by a representative vertex. Luebke [11] extended the approach to hierarchical dynamic simplification(hds) with vertex tree. Runtime algorithm computes an active tree, which corresponds to an appropriately simplified mesh. Recently, vertex clustering has been adapted to out-of-core simplifications by Lindstrom [10] and Garland [7]. Their algorithms used quadric error metrics, which was originally proposed by Garland [6], for generating a representative vertex of each cell. 3 Role-Based Viewing 3.1 Role-Based View (a) (b) (c) Fig. 1. Role-based View Examples (from [1]) : (a) Original model, (b) Genus-reduced model, (c) Simplified model A role-based view is a tailored 3D model which is customized for a specific user based on the roles defining the user s access permissions on the model [1]. Consider the component shown in Figure 1-(a). Suppose that the designer of the part wants to hide the design details from other participating designers. Our solution to the problem

3 Multi-Resolution Modeling in Collaborative Design 3 is to present the component to others in some lower resolutions. Figure 1 shows three different resolutions or LODs. Figure 1-(a) is a full-resolution model, which the component s owner sees and may also be presented to, for example, project supervisors. The holes of the component might be critical features which should be hidden from other designers. Then, all holes are removed from the original model, and the model in Figure 1-(b) is presented. Suppose that the component is to be presented to a supplier from another organization. Then, the model in Figure 1-(b) can be again simplified to generate the crude model in Figure 1-(c), which just presents the outline of the component. Those are examples of role-based views. Our system provides an appropriate resolution to each designer according to the designer s roles. 3.2 Role-based Access Control Policies Among various access control policies commonly found in contemporary systems [14], we adopt a Role-based Access Control(RBAC) [13]. In RBAC, system administrators create roles according to the job functions in an organization, grant permissions (access authorizations) to the roles, and then assign users to the roles. The permissions associated with a role tend to change much less frequently than the users who fill the job function that role represents. Users can also be easily reassigned to different roles as needs change. These features have made RBAC attractive, and numerous software products such as Microsoft s Windows NT currently support it. Roles, R = {r 0,r 1,...,r m }, are abstract objects that define both the specific users allowed to access resources and the extent to which the resources are accessed. The engineers (designers, process engineers, project supervisors, etc.) correspond to a set of actors A = {a 0,a 1,...,a n }, each of which will be assigned to a set of roles. The entire assembly design is represented as a solid model M. Let b(m) represent the boundary of M. We define a set of security features, SF = { f 0, f 1,..., f k }, where each f i is a topologically connected point set on b(m) and SF = b(m). CAD models are often hierarchically represented, and Figure 2-(a) shows the group and security feature hierarchy of a certain assembly design example. g 0 g 1 g 2 f 0 f 1 f 2 f 3 f 4 f 5 r 0 w r(0.2) p - - r(0.5) - - r(1.0) r 1 r(0.2) w p - - r(1.0) - - r(0.5) r 2 p p w r(0.5) r(0.5) - r(1.0) r(1.0) - (a) Group Hierarchy (b) Access Matrix Fig. 2. Group Hierarchy and Access Matrix Example

4 4 JungHyun Han et al. 3.3 Access Matrix In our security framework, RBAC is implemented as an access matrix. Figure 2-(b) is the access matrix for the group and security feature hierarchy of Figure 2-(a). In this matrix, there is a row for each role, and a column for each group and security feature. For simplicity, only three roles, r 0, r 1 and r 2, are created. Suppose that actors designer 0, designer 1, and designer 2 are assigned to roles r 0, r 1, and r 2 respectively. Each cell of the access matrix is assigned read (with weight value in parenthesis), write, or pass (only for group) permissions. It is reasonable to assume that write permission of a feature is exclusively given to single role. In contrast, read permissions of a feature can be given to multiple roles. The pass permission of a group allows an actor to access its children, and the permissions associated with the children determine their readability/writability. Let us discuss the read/write permissions of r 0. It has write permission to g 0. Then, write permissions to g 0 s children, f 0 and f 3, are automatically set up. In contrast, the role r 0 has 20% Read permission to g 1. Then, r 0 cannot directly access g 1 s children, f 1 and f 4, and a low resolution model of g 1 will be presented to r 0. Finally, r 0 has pass permission to g 2, and the permissions associated with f 2 and f 5 should be checked: r 0 has 50% Read permission to f 2, and full read permission to f 5. Let us then discuss f 0. As explained above, write permission to f 0 is automatically given to r 0, and r 0 will edit NURBS model of f 0. (Of course, f 0 is 100% Readable to r 0.) The role r 1 does not have read permission to f 0, but has 20% Read permission to its parent g 0. Therefore, f 0 itself is not visible to r 1, but a low resolution version of combination of f 0 and f 3 will be presented to r 1. In contrast, r 2 has 50% Read permission to f 0, and will see an appropriately simplified version of f 0. 4 Role-Based View Generation 4.1 Multi-resolution Mesh Hierarchy It is more storage-efficient to have a single dynamic/continuous hierarchy rather than multiple discrete LODs. A continuous hierarchy guarantees extremely fine granularity in the sense that a distinct LOD can be presented to each actor. Vertex-clustering-based octree structure, vertex tree, is a good choice for the continuous hierarchy. In collaborative design, a security feature is assigned a design space, which limits the boundary the feature can reach. In 3D design, the design space is represented as an axis-aligned box. The box is used as the initial cluster of vertex-clustering. The cluster is recursively partitioned into 8 cells until each cell contains a single vertex. For each cell, a representative vertex is computed using fundamental quadric [7]. In this way, a vertex tree is constructed per a security feature. The leaf nodes provide the finest version of the security feature model, and internal nodes (with representative vertices) constitute simplified versions. The quality of the coarsest mesh (of a vertex tree) can be determined, for example, by specifying the number of vertices of the coarsest version. We also construct a vertex tree per a group. For each child of the group (whether a sub-group or a security feature), the vertices of the coarsest mesh is collected. A collection of such vertices (from all

5 Multi-Resolution Modeling in Collaborative Design 5 children) constitutes the finest mesh of the group. The initial cluster of the group is the bounding box of its children s design spaces, and the vertex tree of the group is constructed with the same strategy used for building security features vertex trees. 4.2 Role-based Viewing within RBAC The problem of how much of a security feature/group is made visible to a role is reduced to the task of what subtree of its vertex tree to select, or how to choose a set of boundary nodes [11] of the vertex tree. Note that the minimum quadric error of the vertex tree, e min, is found from the leaf nodes of the vertex tree. Similarly, the maximum error of the vertex tree, e max, is found from the coarsest mesh s vertices of the vertex tree. [e min, e max ] is then normalized into [0,1]. Such a normalized error is depicted for each node in Figure 3. (The vertex tree is an octree, but depicted here as a binary tree just for easy illustration. For implementation purpose, the error values of all root nodes are made 1.0.) (a) Vertex Tree (b) Boundary Nodes with α=0.36 (c) Boundary Nodes with α=0.46 Fig. 3. Vertex Tree and Boundary Nodes Examples As shown in Figure 2-(b), the read permission is associated with weight value. Let us call it α. We use α to extract an appropriately simplified mesh from the vertex tree. The vertex tree is traversed to select boundary nodes which bound a subset of nodes whose error values are greater than or equal to 1-α. The boundary nodes lead to a simplified mesh, and is transmitted to clients. Figure 3-(b) shows the boundary nodes determined by α=0.36(1-α=0.64), and Figure 3-(c) by α=0.46(1-α=0.54). As 0.46 is larger than 0.36, a higher resolution should be presented for the case of α=0.46. Therefore the boundary nodes of α=0.46 lies lower than that of α=0.36. If α=1.0, the leaf nodes are collected to provide the full resolution version. If α=0.0, the current implementation completely hides the feature, but we could present the coarsest mesh of the security feature or replace the feature with its convex hull or bounding box. It is implementation dependent.

6 6 JungHyun Han et al. 4.3 Design Change Support We support adding, deleting, and editing NURBS patches. In vertex tree, we preserve links between vertices and NURBS patches, i.e. which patch a vertex is from. Using these links, design change is propagated as follows. Step 1: For each NURBS patch being removed or edited, all vertices from the patch are removed from the vertex tree. When a vertex is removed, its parent is marked to be updated. Step 2: Each patch being added or edited is tessellated, and all new vertices from the patch are added to the vertex tree (always expanding the leaf nodes). When a new leaf node is added, its parent is marked to be updated. Step 3: Selecting the nodes marked to be updated, recursively update representative vertices in a bottom-up fashion. Suppose that, for a node, the distance between the old representative vertex and the updated one is smaller than a threshold value. Then, computing new representative vertices for the node s ancestors may be skipped. It saves computation in Step 3. Note that the design space is fixed for a design session, and so is the initial cluster of the vertex tree. Therefore, internal nodes of the vertex tree are never deleted or added. Only the leaf nodes come and go, and the above three-step algorithm works very efficiently. 5 Implementation and Results To test the approach proposed in this paper, a prototype system has been simultaneously developed using OpenGL on Solaris and Windows, and using both Mesa and nvidia s native OpenGL drivers on Linux operating systems. (a) Security Features (b) Group Hierarchy Fig. 4. Security Features and Group Hierarchy on Piston Assembly Figure 4-(a) shows a piston assembly model with 10 security features. (The model is illustrated from two viewpoints.) Figure 4-(b) shows its hierarchy. Figure 5 is the access matrix for the assembly.

7 Multi-Resolution Modeling in Collaborative Design 7 The role r 0 is in charge of designing piston pin f 6 and piston body f 7, which are grouped under g 1. Write permission to g 1 is assigned to r 0, and therefore r 0 can edit both of f 6 and f 7. (Of course, r 0 sees the full resolution models of f 6 and f 7.) Observe that r 0 has pass permission to g 2 and therefore the permissions associated with its children should be checked. 100% Read permission is associated with the connecting rod f 3, but just 50% Read permission is with f 8 and f 9. As a result, the full resolution model of f 3 and simplified models of f 8 and f 9 will be presented to r 0. In contrast, r 0 has 10% Read permission to g 0 and therefore crude models of g 0 will be presented to r 0. The rolebased view rendered for r 0 is illustrated in Figure 6-(a). The other role-based views for r 1 and r 2 are depicted in Figure 6-(b) and -(c) respectively. g 0 g 1 g 2 g 3 f 0 f 1 f 2 f 3 f 4 f 5 f 6 f 7 f 8 f 9 r 0 r(0.1) w p r(1.0) r(0.5) r(0.5) r 1 p r(0.0) r(0.5) p r(1.0) r(1.0) r(0.5) - w r(1.0) r 2 p p p r(0.5) - r(0.7) r(1.0) w - - r(1.0) r(0.7) r(1.0) r(1.0) Fig. 5. Access Matrix for the group hierarchy in Figure 4 : r 0 for Piston Designer, r 1 for Crank Shaft and r 2 for Connecting Rod (a) View Generation for r 0 (b) View Generation for r 1 (c) View Generation for r 2 Fig. 6. Role-Based Viewing for Piston Assembly 6 Conclusions This paper has presented a new technique, role-based viewing, for collaborative 3D assembly design. By incorporating security with collaborative design, the costs and risks incurred by multi-organizational collaboration can be reduced. Aside form digital 3D watermarking, research on how to provide security issues to distributed collaborative design is largely non-existent. The authors believe that this work is the first of its kind in the field of collaborative CAD and engineering.

8 8 JungHyun Han et al. Acknowledgements This work was supported in part by the grant No from the Basic Research Program of the Korea Science & Engineering Foundation. Additional support has been provided by US National Science Foundation (NSF) Knowledge and Distributed Intelligence in the Information Age (KDI) Initiative Grant CISE/IIS ; CAREER Award CISE/IIS and Office of Naval Research (ONR) Grant N References 1. C. Cera, T. Kim, J. Han and W. Regli.: Role-based Viewing Envelopes for Information Protection in Collaborative Modeling. In : Journal of Computer-Aided Design Special Issue on Distributed CAD and Collaborative Design (to appear) 2. J. W. Barrus and R. C. Waters.: Supporting Large Multiuser Virtual Environments. In : IEEE Computer Graphics and Applications Vol. 16(6) (1996) C. Cruz-Neira, D. J. Sandin, and T. A. DeFanti : Surround-screen projection-based virtual reality : the design and implementation of the cave. In : Proceedings of the 20th annual conference on Computer graphics and interactive techniques. ACM Press (1993) J. M. S. Dias, R. Galli, A. C. Almeida, C. A. C. Bello, and J. M. Rebordao. : mworld: A Multiuser 3D Virtual Environment. In : IEEE Computer Graphics and Applications, Vol 17(2). (1997) H. Eriksson. : MBONE: The Multicast Backbone. In : Communications of the ACM, Vol. 37(8) Augest (1994) M. Garland and P. S. Heckbert : Surface Simplification Using Quadric Error Metrics. In : Proceedings of the 24th annual conference on Computer graphics and interactive techniques. ACM Press. Addison-Wesley Publishing Co. (1997) M. Garland and E. Shaffer. : A multiphase approach to efficient surface simplification. In : IEEE Visualization. (2002) R. Jarek and P. Borrel. : Multi-Resolution 3D Approximations for Rendering Complex Scenes. In : In B.Falcidieno and T.L.Kunii, (eds). Geometric Modeling in Computer Graphics. Springer-Verlag. (1993) S. Jayaram, U. Jayaram, Y. Wang, H. Tirumali, K. Lyons, and P. Hart. : VADE: a Virtual Assembly Design Environment. In : IEEE Computer Graphics and Applications, Vol. 19(6) (1999) 10. P. Lindstrom. : Out-of-core simplification of large polygonal models. In K. Akeley (eds), Siggraph 2000, Computer Graphics Proceedings.ACM Press / ACM SIGGRAPH / Addison Wesley Longman. (2000) D. Luebke and C. Erikson. : newblock View-dependent simplification of arbitrary polygonal environments. Computer Graphics Vol. 31, Annual Conference Series (1997) M. Macedonia, M. Zyda, D. Pratt, P. Barham, and S. Zeswitz. : NPSNET: A Network Software Architecture for Large-Scale Virtual Environments. In : Presence, Vol. 3(4) (1994) R. S. Sandhu, E. J. Coyne, H. L. Feinstein, and C. E. Youman. : Role-Based Access Control Models. In : IEEE Computer, Vol 29(2) (1996) R. S. Sandhu and P. Samarati. : Access Control: Principles and Practice. In : IEEE Communications Vol. 32(9). (1994) N. Shyamsundar and R. Gadh. : Internet-based Collaborative Product Design with Assembly Features and Virtual Design Spaces. In : CAD, Vol. 33. (2001)