Efficient ray traced soft shadows using multi-frusta tracing

Ray tracing has long been considered to be superior to rasterization because its ability to trace arbitrary rays, allowing it to simulate virtually any physical light transport effect by just tracing rays. Yet, to look plausible, extraordinary amounts of rays for effects such as soft shadows are typically required. This makes the prospects of real-time performance rather remote. Rasterization, in contrast, has a record of producing such effects in real-time through employing specialized and approximate solutions for individual effects. Though ray tracing may still be the right choice for effects like reflections and refractions, using specialized solutions for certain important effects also makes sense for a ray tracer. In this paper, we propose a special solution to ray trace soft shadows that is particularly targeted for Intel's Larrabee architecture. We use a specialized frustum tracing that traces multiple frusta of specialized "light-weight" shadow packets in parallel, while generating rays within each frustum on demand. The technique can easily be integrated into any packet ray tracer, and fits well into the wide SIMD and cache-size constraints of the Larrabee architecture. Our technique allows to reach rates of up to several dozen million rays per second per Larrabee core, outperforming traditional packet techniques by up to 6x. This high performance combined with a simple light-weight illumination filtering step allows to achieve real-time soft shadows for game-like scenes.