Rendering - Physically-Based Rendering

Global Illumination-Light Transport

The simulation of the physical propagation of light energy presents significant research challenges. The two most common physically-based rendering methods used today are stochastic ray-tracing [WHIT80] and radiosity [GORA84]. Although during the past fifteen years many improvements have been made, both techniques still neglect various significant mechanisms of light transport.

A general formalization of the rendering equation has been well known [KAJI86], but until recently neither the processing power nor a sufficiently accurate reflection model has been available to perform predictive simulations. We are now using a density estimation framework that splits light transport and lighting representation into separate computational stages [WALT97A].

This physically-based method supports characterization of error and can handle complex geometries and general reflectance functions.

In the transport stage, we first simulate the flow of light between surfaces using Monte Carlo particle tracing, without explicitly reconstructing the lighting on them. Since the intensity of the lighting on each surface is proportional to the ultimate density of light particles, local lighting reconstruction is a density estimation problem [SILV86]. Although computational requirements are enormous, both the particle-tracing and density-estimation phases can easily exploit coarse-grained parallelism, thus reducing computation time.

We use the light measurement laboratory again to compare the resulting simulated radiant energy on an image plane with measured values at full dynamic range and infinite resolution. The last stage of realistic image synthesis then incorporates human perceptual factors to generate a visual image.