Dissertation: Modular Radiance Transfer

Abstract:

Indirect illumination is a highly desirable feature of realism in computer generated imagery. However, its global nature makes it difficult to compute in real-time. Despite the difficulty, many games are currently attempting to produce global illumination solutions using varying techniques. Most are simple techniques, unlike the fully dynamic systems talked about in academia. Games generally use precomputed global illumination stored in light maps (otherwise known as static baked global lighting) or ambient occlusion as well as placing nonshadowing static lights to fill in areas of shadow.

In live-action films, reflectors and bounce cards are used to reflect additional light back into the scene. These planes and simple shapes only approximate indirect light from geometry in the real-world, but offer a high level of control that allow directors of photography to produce desired results. In digital film production, simple-shaped lights are commonly used to allow artists to quickly iterate and achieve a desired look. The ease-of-use and controllability of these approximations outweighs their physically incorrect nature.

We believe that by following this insight we have created a new, simple, global illumination method that could be applied to current games, where global illumination is to be decomposed and calculated on simple shapes and then reconstructed to generate pleasing visual results. We have developed a technique that creates a set of bases in which we calculate indirect lighting. Our approach is very efficient and uses very little data and a quick, one-time, scene-independent precomputation step. It also allows real-time computation of approximate indirect light and is designed with rapid iteration of light design in mind. We precompute light transport operators (LTOs) for a handful of simple canonical “shapes,” then interactively warp and combine these shapes, along with their LTOs, to more complex geometry. These shape proxies are used to model direct-to-indirect transport, which is then applied as a light map to the actual scene geometry. The flow of indirect light between shapes is modeled with lightfields, and all computations are performed in very low-dimensional subspaces (Chapter 3: Modular Radiance Transfer). We generate another set of LTOs to capture the finer details onto, off of, and blocked by objects within our dictionary (Chapter 5: Delta Radiance Transfer). To improve the visual results of these low-frequency calculations we also introduce a novel screen space ambient occlusion method called Volumetric Obscurance (Chapter 6) that, when paired with MRT and DRT, generates visually plausible, real-time (on the order of milliseconds) indirect illumination.

Modular Radiance Transfer (MRT), Delta Radiance Transfer (DRT), and Volumetric Obscurance (VO) result in plausible, dynamic global-illumination effects, rendering at high frame rates with low memory overhead. Our methods have been shown to scale from high-end to mobile platforms and, like precomputed radiance transfer (PRT), provide smooth results that respond to dynamic changes in lighting.