Rui Bastos, Wolfgang Stürzlinger

Department of Computer Science - University of North Carolina at Chapel Hill

June 1997

Rendering globally illuminated scenes with non-diffuse surfaces is computationally intensive. The resulting photo-realistic images are used by designers and advertisers. We present a technique to render globally illuminated glossy scenes at interactive frame rates.

The method proceeds in two phases: particle-tracing and rendering. The particle-tracing phase simulates and stores the distribution of light in the scene. The rendering phase uses this light distribution to dynamically reconstruct the light reaching the viewer as the viewpoint changes.

To simulate how light interacts with the scene, a number of photons is emitted from each light source, according to its power and directionality. Each photon is traced through the scene and reflected by surfaces until it is finally absorbed. Every time a photon hits a surface, absorption or reflection is decided randomly. If the photon is reflected, the material properties of the surface (BRDF) are used to determine its new direction and power. All photon hits are stored with corresponding incoming direction at the surface they hit.

The contribution of each photon hit to the illumination of a surface is approximated as a ‘splat’ (Gaussian kernel). The kernel width is determined by the density of photon hits for each surface. For non-diffuse surfaces, the viewing direction acts as a non-linear scale factor for the intensity of each splat. Accumulating all splats per surface gives a smooth reconstruction of the illumination.

Each splat is rendered as a textured triangle (shown on the right) lying on the surface hit by the photon and centered at the corresponding hit point. Splats are clipped to the visible portion of the surface and scaled according to the viewing direction. Accumulation of all splats overlapping at a given pixel gives the view-dependent radiance at that point. See figure below.

Rendering is optimized by splatting only the indirect illumination. The direct illumination component is approximated using hardware shadow-maps and simulated Phong-shading.

Image-sequence showing how the reflection in the larger glossy box changes with viewing angle. The red wall behind the viewer changes the color of the reflection on the glossy box in the background. These images have been rendered at 1 frame per second on a Silicon Graphics Onyx with Infinite Reality graphics. Performance is mainly determined by fill rate.