Elsevier

Computer-Aided Design

Volume 36, Issue 9, August 2004, Pages 821-833
Computer-Aided Design

The incremental editing of faceted models in an integrated design environment

https://doi.org/10.1016/j.cad.2003.09.009Get rights and content

Abstract

Faceted/meshed models play an important role in product development cycle. Due to the fact that such models typically involve huge amounts of data, the exchanging of them in network-based integrated design environments (IDEs) is a critical issue. In this paper, an approach known as incremental editing is presented to expedite such a data exchange process through tracking changed portions of a faceted model after it has been edited at one application, transmitting these portions, and finally embedding them into the associated faceted model at another application. This new approach is expected to bring two benefits to current IDEs: (a) the editing of a faceted model at an application can be done without completely reconstructing all its underlying facets, and (b) the consistency between faceted models in different applications can be maintained without repeatedly transferring large amounts of facet data over computer networks.

Introduction

Meshes have been used in a wide range of CAD/CAM/CAE (or CAX in short) applications, including graphics visualization, finite element analysis, computational fluid dynamics, rapid prototyping, etc. Meshes represent complex solids as collections of simple geometric elements, and may generally be classified as surface-based (consisting of triangular or quadrilateral facets filling the boundaries of solids) or volume-based (consisting of tetrahedral or hexahedral elements filling the volumes of solids). In this paper, a surface-based mesh is also referred to as a faceted (or meshed) model. Compared to volume-based meshes, faceted models appear to play a more important role in current industries, not only because they are relatively easier to compute and manage, but also they serve as the basis for volume-based meshes.

From the perspective of produce lifecycle management, product models always have to go through extensive modifications (in the context of this paper, only shape modifications are considered and hence the ‘product model’ is replaced by ‘solid model’) in various CAX applications before finally being manufactured. If a faceted model is involved in this cycle, it becomes unavoidable to repeatedly rebuild the facets to reflect the alterations in the solid model. Frequent rebuilding of facets is not an easy task especially in the presence of complex geometry. Assuming that a product is represented as a general solid model, one needs to first create the corresponding boundary representations (b-reps) and then generate facets by tessellating the faces (of the b-rep). This is typically a computationally intensive process, more so, in a collaborative or distributed integrated design environment (IDE). Such IDEs usually involve the simultaneous execution of a variety of applications that communicate with each other via computer networks. The changes made to solid or faceted models at one application result in modifying the solid and/or faceted models at other applications. This process of updating models across networks needs to occur quite frequently if consistency of shape information is to be maintained. Traditionally, the propagation of model changes between applications is achieved by repeatedly transmitting entire faceted models over the network and completely reconstructing the model data. This consumes large amounts of network bandwidth and introduces unacceptable latency. The contribution of this paper is to present an approach that (a) ameliorates the need for redundant rebuilding of facet models, and (b) reduces the amount of facet data being transferred over the networks.

The rest of the paper is structured as follows. In Section 2, relevant research is reviewed. Section 3 gives an overview of the proposed approach. The main procedures of presented approach are described in 4 Editing b-reps, 5 Generating faceted models, 6 Embedding, 7 Remote updating. Section 8 discusses some computational performance issues, followed by concluding remarks in Section 9.

Section snippets

Literature review

The research germane to the topic of this paper falls under three categories: (a) work describing the exchange of faceted models in IDEs, (b) the techniques of mesh generation, and (c) the techniques of faceted model transmission. In the following paragraphs, literature from the three categories will be reviewed and used as a basis to motivate the problem undertaken in this paper.

Overview of methodology

Without loss of generality, it is assumed that two disparate applications need to communicate changes in faceted models with each other (communication between more than two systems can be treated in a similar manner). The site at which the change is being made is referred to as the server and the site that receives the change is referred to as the client. Note that either site, at any instant, could be a server or client depending on whether it is editing and/or receiving the model. Thus, the

Editing b-reps

As a starting point, a b-rep model SSPrEBM at the server is supposed to be edited to make the SSPoEBM. This section will firstly explain some definitions as the mathematical basis of editing b-reps, then the processes of editing b-reps will be detailed.

Generating faceted models

Once the SSPrEBM has been edited, its corresponding faceted model SSPrEFM also needs to be modified. By comparing differences between the SSPrEBM and SSPoEBM, a new faceted model (named FCM) could be generated by remeshing only the changed faces of the SSPoEBM. Moreover, the FCM could be further used to update the SSPrEFM to yield a faceted model SSPoEFM, which maintains the shape conformity with the newly updated b-rep SSPoEBM. This section will firstly define some properties of faceted

Embedding

After generating the FCM, it is ready to make the SSPoEFM without remeshing the SSPoEBM. This could be done by embedding all the FM_virtual_faces of the FCM into the SSPrEFM to replace invalid facets. The procedure of embedding includes three steps (see Fig. 8 for an example): (a) removing changed FM_virtual_faces from the SSPrEFM, (b) combining FM_virtual_faces of the FCM with those of the SSPrEFM, and (c) merging FM_virtual_edges between the SSPrEFM and the FCM. These steps involve directly

Remote updating

Once the SSPoEFM is created at the server, the faceted model CSPrEFM (i.e. client-site pre-edit faceted model) also needs to be updated based on the information of the FCM. This is achieved by transmitting the FCM from the server to client and embedding it into the CSPrEFM (i.e. client-site pre-edit faceted model) to make the CSPoEFM (i.e. client-site post-edit faceted model).

The procedure of remote updating involves two processes: (a) pre-transmission process, which occurs before transmitting

Performance issues

The effectiveness of the proposed approach depends on the price one pays for additional computational complexity undertaken to reduce the amount of data being transmitted. The additional computational complexity occurs primarily in (a) extracting FM_virtual_edges of the SSPrEFM/CSPrEFM, and (b) embedding. This complexity depends to a large extent on the nature of the edit, the model being edited, and the way in which the model is being meshed. In this section, some performance issues are

Closing remarks

In this paper, the approach of incrementally editing faceted models was discussed, which enables faceted models be efficiently modified and updated in integrated design environments. The contribution of this approach is two-fold: (a) avoiding completely rebuilding entire facets after a faceted model was changed, and (b) avoiding repeatedly downloading entire facets to propagate changes. Furthermore, using the proposed approach, most of the heavy duty editing operations (e.g. editing of b-reps

Di Wu is a Software Engineer at SolidWorks Corporation, working on product design software. He received his MS degree in Mechanical Engineering from the University of Toledo in 2001. Prior to that, he also earned BE and MS degrees in Mechanical Engineering in 1994 and 1997, respectively. His research interests include: geometric and solid modeling, computer graphics, and scientific visualization.

References (16)

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Di Wu is a Software Engineer at SolidWorks Corporation, working on product design software. He received his MS degree in Mechanical Engineering from the University of Toledo in 2001. Prior to that, he also earned BE and MS degrees in Mechanical Engineering in 1994 and 1997, respectively. His research interests include: geometric and solid modeling, computer graphics, and scientific visualization.

Dr Radha Sarma is an Adjunct Associate Professor in the Mechanical Engineering Department at the University of Michigan at Ann Arbor. She obtained her Masters and PhD degrees from the Universities of Toledo and Michigan, respectively. Her research interests include solid modeling, surface modeling, CAD for MEMS, and CAM.

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