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Section: New Results

Keywords : Virtual urban environments modeling, L-systems, Rewriting Systems, real-time visualization.

Modeling and display of virtual urban environments

Participants : Kadi Bouatouch, Julien Perret, Stéphane Donikian.

We have designed and implemented a software for creating cities using procedural models based on our new scripting language.

Modeling large virtual environments addresses several issues related to the complexity of the model and the data volume. Indeed, such models are as delicate to acquire or design than to use. In terms of modeling, these models require heterogenous data such as GIS information, terrain elevation, traffic network, buildings geometry, etc. Furthermore, considering the multiple scales in play and the diversity of the objects involved, few modeling techniques are suitable for such a task as designing a virtual city. Moreover, large models suggest large amounts of data. Thus, these models need large storage capacities, are hard to maintain (and/or update), and can be difficult to render in real-time; let alone through a network.

We proposed a functional extention to L-systems, namely FL-systems. L-systems offer a powerful mechanism for biologically-motivated modeling. It has been proved useful for plants, trees and street networks description. Despite numerous extensions, L-systems remain essentially used for plant-modeling purpose. FL-systems have been proved suitable to model urban features such as buildings, street networks, street lamps, etc. This representation is interesting for several reasons. First, as a grammar-based mechanism, it acts as a data compression scheme. It is therefore easier to transfer through a network as well as simple to evaluate in a lazy fashion. Second, it operates as a data amplifier: a single FL-system can generate a large diversity of models if provided different sets of parameters or using probabilistic rules. Finally, it offers a new modeling technique.

We proposed a new caching mechanism for FL-systems: the FL-systems Intelligent Cache. This new cache operates during the rewriting process of an FL-system. Based on dynamic dependancy calculus, this cache takes in account the formal properties of the FL-system using it. This cache is therefore able to compute which rules will generate similar results upon rewriting. While rewriting, it checks in the previously rewritten terms if there is one similar and if so, uses it. This cache has two main goals: it fastens the rewriting process of FL-systems and allows an implicit procedural instancing of objects. The instancing of geometric objects makes the rendering of the scenes faster whereas its implicit nature takes its responsability off the modeler's shoulders.

Figure 10. A city model
Image/city

Such methods are currently being integrated inside a modeling software dedicated to urban environments such as shown in figure 10 . (F)L-systems, as well as other rewriting methods allow a novel modeling process. Furthermore, this multi-scale design process is enhanced by automatic generation capabilities and real-time visualization techniques.


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