This paper introduces a new technique, called peer-assisted rendering, that aims to enable interactive navigation in a 3D networked virtual environment using a resource-constrained device, by speeding up the rendering. A resource-constrained client requests part of the rendered scenes from other peers with similar viewpoints within the virtual environment, and merges the rendered parts into its own view. This approach is more scalable than previous solutions based on server-based pre-rendering. The goal of this paper is to make a strong case for the feasibility of peer-assisted rendering through the following two messages. First, by analyzing a large number of user traces from a popular virtual world called Second Life, we show that there are surprisingly many users with similar viewpoints and encompass large number of common objects in their viewing areas, indicating that a client can potentially find multiple other peers that can assist in rendering. Second, by combining three different rendering methods, each contributing to rendering of different classes of objects in the scene, we show that it is possible for a client to render the scene efficiently with little visual artifacts.

3D triangular meshes are becoming an increasingly prevalent data type in networked applications such as digital museums, online games, and virtual worlds. In these applications, a 3D mesh is typically coded progressively, yielding a multiresolution representation suitable for streaming. While such progressive coding allows incremental rendering for users while data is being transmitted, it introduces dependencies between data, causing delay in rendering when packets are lost. This article quantitatively analyzes the effects of such dependency by modeling the distribution of decoding time as a function of mesh properties and network parameters. We apply our model to study two extreme cases of dependency in progressive meshes and show that the effect of dependencies on decoded mesh quality diminishes with time. Our model provides the expected decoded mesh quality at the receiver at a given time. Based on this expected value, we propose a packetization strategy that improves the decoded mesh quality during the initial stage of streaming. We validate the accuracy of our model under a variety of network conditions, including bursty losses, fluctuating RTT, and varying sending rate. The values predicted from our model match the measured value reasonably well in all cases except when losses are too bursty.

We demonstrate MobiTree, a system we built that allows progressive streaming and rendering of 3D tree models on a mobile phone. MobiTree enables user to retrieve not only typical media describing a species (such as text and image), but also a 3D model that yields detail information about the structure of the branches and the foliage. MobiTree adopts our previous proposed progressive representation to speed up display of trees at the mobile client, trading off latency and quality. Progressivity also allows MobiTree to flexibly adopt the level of details to the capability of the mobile devices.

Just as in the real world, plants are important objects in virtual worlds for creating pleasant and realistic environments, especially those involving natural scenes. As such, much effort has been made in realistic modeling of plants. As the trend moves towards networked and distributed virtual environments, however, the current models are inadequate as they are not designed for progressive transmissions. In this article, we fill in this gap by proposing a progressive representation for plants based on generalized cylinders. We model the shape and thickness of branches in a plant as Bézier curves, group the curves according to the similarity, and differentially code the curves to represent the plant in a compact and progressive manner. To facilitate the transmission of the plants, we quantify the visual contribution of each branch and use this weight in packet scheduling. We show the efficiency of our representations and the effectiveness of our packet scheduler through experiments over a wide area network.

International audience

3D triangular mesh is becoming an increasingly important data type for networked applications such as digital museums, online games, and virtual worlds. In these applications, a multi-resolution representation is typically desired for streaming large 3D meshes, allowing for incremental rendering at the viewers while data is still being transmitted. Such progressive coding, however, introduces dependencies between data. This paper quantitatively analyzes the effects of such dependency on the intermediate decoded mesh quality when the progressive mesh is transmitted over a lossy network, by modeling the distribution of decoding time as a function of mesh properties and network parameters. To illustrate the usefulness of our analytical model, we describe three of its applications. First, we show how it can be used to analytically compute the expected decoded mesh quality. Second, we study two extreme cases of dependency in progressive mesh and show that the effect of dependencies on decoded mesh quality diminishes with time. Finally, based on the model, we propose a packetization strategy that improves the decoded mesh quality during the initial stage of streaming.