Scalable Content for Urban Applications

Juan Diego Toro*

Universidad de Los Andes

Pablo Figueroaφ

Universidad de los Andes

ABSTRACT We present an application that allows the acquisition of 3D models in an open format that adjust to the device’s requirements and specifications. By defining a format in X3D to describe each one of the kinds of objects we have considered as part of an urban model, such as blocks, buildings, cars, pedestrians, streetlights, and massive transportation, among others. This way, the application that requests the urban model can manipulate each of its components independently. The model stores not only graphic representation of objects, but also animation and setup of a certain scene. The application stores in the database scenes as independent objects that can be requested by applications. The application can deliver the same model in different LOD (level of detail) according to the device or application realizing the request. KEYWORDS: Scalable content, level of detail, urban, 3D model. INDEX TERMS: E.5 [Input/Output]: Data—Life Cycle; I.3.5 [Object hierarchies]: Computational Geometry and Object Modeling

1 INTRODUCTION Currently many applications require urban representation of real zones and even animation of many of its components. The main problem among applications is the processing requirements of the device that executes them. “Highpoly” models provide an attractive and precise appearance to applications, but low rendering power software or hardware might not be able to display fluidly these kinds of models. On the other hand, “lowpoly” models might be efficient for low power systems, but the quality is incredibly affected, and the capacity of high quality systems is misused. Therefore, models should be able to adjust to the system’s requirements and capability, but it also shouldn’t waste processing power in decimating highpoly models.

We propose a web service that allows other applications to request models, and according to their specifications, a 3D urban model is delivered in the LOD (Level of Detail) that fits most the system. The web service considers the client making the request and extracts the model information according to the LOD from a file that contains the model in its different LOD in X3D format.

2 RELATED WORK In this section we present various works that relate in the scalable content issues to our proposal.

Smith et al. [1] propose a data model called “infopyramid”, which contains multimedia scalable content such as audio, video, images, and text. This method allowed content to be available for pervasive devices like smart phones, PC, TV browsers, PDAs. The “infopyramid” is contains the four types of information as

horizontal content, and the height of the pyramid is given by the fidelity (i.e. quality) of the media object.

Phan et al. [2] present a middleware component called the Content Adaptation Pipeline that performs content adaptation on arbitrarily complex data. They propose an architecture that can handle a variety of data objects stored in a repository application server and requested by any client device.

Nebiker [4] introduces the DILAS project, which is focused on identifying the main requirements in 3D geoinformation services and allowing management and visualization of large landscape and city models. The main focuses of the project are a 3D object model with integrated support for LOD, the handling of multiple representations of 3D objects within the OGC SFS framework, and a quadtree-based multi-resolution access structure for 3D objects

Bradshaw [5] presents a method to recreate curved models of buildings and export them on a new VRML file. He applies VBA scripts on the ArcMap software to detect the curvature of the buildings from satellite images and apply them on the extruded polygon. However, these features require human interaction, as they are not yet available automatically.

3 FEATURES This section presents the different features the web service

provides for the other applications

3.1 Types of Models The application provides different types of objects, depending on the request the client realizes. The web service can provide 3 types of 3D models, according to the types of objects we have defined as part of our urban model.

The first type of object is the zone. Geodetic information describes a particular zone of the city, which is relatively small (10 to 15 blocks radius. Zones manage hierarchically blocks and buildings, allowing applications to manipulate the block as a whole or each building individually.

The second type of object is the model. This is a simple representation of any object (e.g. car, bus, pedestrian, etc.) in its various LOD in an X3D file.

Finally, we provide objects called scene. A scene contains all the elements to display an urban model. This includes zones, models and any additional animation. The scene provides identifiers for each one of the elements in the model.

3.2 Level of Detail The application considers for now 3 types of clients: Augmented reality, flash apps, and standalone. For each of these, a LOD is provided for each model, and each one has different characteristics. Figure 1 shows the difference between a city with high level of detail (left) and low level of detail (right) for the same structures.

* d-toro@uniandes.edu.co φ pfiguero@uniandes.edu.co

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IEEE Virtual Reality 201119 - 23 March, Singapore978-1-4577-0038-5/11/$26.00 ©2011 IEEE

Figure 1. Wireframe model of the same zone with high level of

detail (right) and low level of detail (left).

The Augmented reality LOD is simply a set of extruded polygons representing each one of the buildings. They don’t contain any textures, since it is intended to overlap the real view from the camera.

The flash app LOD consists of extruded polygons representing each one of the buildings. Additionally, the structures have textures mapped. Other objects such as cars, buses, streetlights, etc. are represented with lowpoly models.

Finally, the standalone LOD considers a computer with high (or average at least) rendering capabilities. Therefore, the buildings contain a larger number of vertices than previous LODs to assure that texture mapping is more accurate and less pixelated. The highest LOD models, along with their textures are delivered to this client, for high quality display.

3.3 Web Display Since the web application delivers files through the http protocol, it was necessary to provide a web interface to show the content of the server. Figure 2 presents the main page.

Figure 2. Application server's main page

The main page allows adding new objects to the server (top) as well as requesting content (bottom). The browser presents the models available for download. The user can select the textures attributes and press a submit button to receive the content.

4 APPLICATION STRUCTURE This section presents the internal concept of the web service application.

4.1 Application Concept The web service is in charge of storing 3D models in different LOD, making them available to different visual interfaces, such as AR, flash, or standalone applications. Figure 3 presents the basic structure of the model.

Figure 3. Web Application Concept

4.2 Architecture and Functionality The web service’s architecture is quite simple, as it contains very few components. The application is built using 5 main components: the web interface, the main application, the X3D extractor, the database, and the zip generator. These elements are the ones responsible to handle the request, the acquisition of the X3D file and its delivery along with its textures.

5 CONCLUSION We have presented a web application that allows the acquisition of 3D urban models in various LOD depending on the client’s hardware and software specifications. Using the X3D format, we can label these levels of detail, and recognize them to extract them form the file.

REFERENCES [1] Smith, J.R., Mohan R., Li, C. Scalable Multimedia for Pervasive

Computing. ACM Multimedia ’99. 1999. [2] Phan, T., Zorpas, G., Bagrodia, R. An Extensible and Scalable

Content Adaptation Pipeline Architecture to Support Heterogeneous Clients. Proceedings of the 22nd International Conference on Distributed Computing Systems (ICDCS'02) 2002.

[3] Oh, B.M., Cheng, M., Dorsey, J., Durand, F. Image-Based Modeling and Photo Editing. Proceedings of the 28th annual conference on Computer graphics and interactive techniques 2001.

[4] Nebiker, S. Multiscale representations for scalable and dynamic 3D geoinformation services. Workshop on Data Generalization and Data Mining, ISPRS Commission IV WG IV/3.Ottawa, Canada. 2002.

[5] Bradshaw, B. Using VBA and ArcMap to Create and Export 3D Buildings. Proceedings of ESRI User Conference 2003.

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