As well as affording us an indepth look at Intel's forthcoming GPU, Larrabee, this year's Special Interest Group on GRAPHics (SIGGRAPH) conference has seen nVidia showing off a real-time ray tracing demo to attendees. The aim of the demo, though, seems not so much to have been to prove that ray-tracing is possible on the GPU, but rather that it is still extremely unviable.
The demo itself comprised a render of a green Bugatti Veyron at a resolution of 1,920 x 1,080 running at 30fps. In order to run in this way, though, nVidia needs an extremely powerful system: namely an nVidia Quadro Plex 2100 D4 Visual Computing System, which packs in a pair of dual-GPU Quadro FX 4700 X2s. In simple terms: a heck of a lot of GPU processing power.
Despite this huge amount of processing power available, nVidia still isn't able to produce a photorealistic image in real-time. In order to run at 30fps at 1080p, the demo uses three bounces for each ray (i.e. each beam of light reflects off three surfaces before 'hitting' the screen). To create a realistic image, more reflections need to be calculated - but doing so would turn the simulation into a slideshow.
nVidia does have the edge over Intel's CPU-based Quake ray-tracing demo, which only ran at 720p, between 14 and 30 FPS and needed a four-socket quad quad-core system to run. But, as I said, nVidia is pointing out that ray-traced games are still some way off, not that GPUs ray-trace better than CPUs. Despite the hype around Larrabee's potential use as a ray-tracing card.