Section: Scientific Foundations
Keywords : lighting simulation, rendering, visibility, partitioning.
Lighting Simulation and Rendering
- Global illumination :
direct and indirect illumination computation.
computation of an image of a virtual world as seen from a camera.
- Partitioning :
subdivision of a 3D model into cells.
- Client-server :
a server contains complex 3D scenes, a client sends requests for objects to the server.
- Level of detail :
an object is represented with a mesh at different resolutions.
A global illumination model describes the light transport mechanism between surfaces, that is, the way each surface interacts with the others. Therefore, the global illumination model is a key problem when accuracy is needed in the rendering process (photorealism or photosimulation). As global illumination is a computation intensive process, our research consists in making it tractable even for large and complex environments.
Another objective is to propose a new navigation system built upon our client-server framework named Magellan . With this system one can navigate through 3D models or city models (represented with procedural models) transmitted to clients over a network. Regarding procedural models, their geometry is generated on the fly and in real time on the client side. These procedural models are described using an enhanced and open version of the L-system language we have developed. The navigation system relies on different kinds of preprocessing such as space subdivision, visibility computation as well as a method for computing some parameters used to efficiently select the appropriate level of detail of objects.
To attain realism in computer graphics, two main attempts have been adopted. The first one makes use of empirical and ad-hoc illumination models. The second one makes use of the fundamental physical laws governing the interaction of light with materials and participating media. It integrates characteristics of the human visual system, in order to produce images which are exact representations of the real world. Our work follows this second approach and relies on the real aspects of materials and on the real simulation of global lighting using physics-based reflection and transmission models as well as a spectral represention of the emitted, reflected and refracted light powers. Unfortunately, global illumination is still a demanding process in terms of memory storage and computation time. Our objective is to rely on the radiance caching mechanism and on the performance of the new graphics cards to make interactive global illumination possible even for complex scenes.
In case of real-time remote navigation, transmission and real-time visualization of massive 3D models are constrained by the networks bandwidth and the graphics hardware performances. These constraints have led to two research directions that are progressive 3D models transmission over Internet or local area network and real-time rendering of massive 3D models.
In regard to progressive 3D models transmission, one can suggest the use of geometric levels of detail (LODs). Indeed, as soon as one LOD is selected according to its distance from the viewpoint, the finer LOD is prefetched over the network. In the same spirit, one can select the LOD of 3D objects to be transmitted based on the available bandwidth, the client's computational power and its graphics capabilities. Our work makes use of these two approaches.
As for real time rendering of massive 3D models on a single computer, one can find many solutions in the literature. The most commonly used solution consists in subdividing the scene into cells and computing a potentially visible set (PVS) of objects for each view cell. During walkthrough, only the PVS of the cell containing the current viewpoint is used for rendering. Our system for interactive building walkthrough follows this approach.