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Ray Tracing for Gaming Explored
Vigile brings us a follow-up to a discussion we had recently about efforts to make ray tracing a reality for video games. Daniel Pohl, a research scientist at Intel, takes us through the nuts and bolts of how ray tracing works, and he talks about how games such as Portal can benefit from this technology. Pohl also touches on the difficulty in mixing ray tracing with current methods of rendering. Quoting:
"How will ray tracing for games hit the market? Many people expect it to be a smooth transition - raster only to raster plus ray tracing combined, transitioning to completely ray traced eventually. They think that in the early stages, most of the image would be still rasterized and ray tracing would be used sparingly, only in some small areas such as on a reflecting sphere. It is a nice thought and reflects what has happened so far in the development of graphics cards. The only problem is: Technically it makes no sense."
Although I have a hard time arguing in the realm of 3D lighting, I will direct attention to the Beyond3D article, Real-Time Ray Tracing: Holy Grail or Fool's Errand?. Far be it of me to claim that this article applies to all situations of 3D lighting, it may be that Ray Tracing is the best choice for games, but I for one am glad to see an article that atleast looks into the possibility that Ray Tracing is not the best solution; I hate to just assume such things. Indeed, the article concludes that Ray Tracing has its own limitations and that a hybrid with rasterisation techniques would be superior to one or the other.
Demented But Determined.
well, if you look more closely, you would notice that the articles have the same author!
I think you're missing the point. The reason Quake 4 looks crap raytraced was because it wasn't written to be raytraced, no shaders are being applied (because they were all written for a raster engine) so of course it looks bad. This stuff is just research.
One of the biggest hurdle in game graphics is geometry detail. Normal mapping is just a hack to make things appear more detailed, it breaks down in some situations. Raytracing will allow *much* higher geometry detail than rasterisation. Better reflection, refraction, caustics and so on are just gravy.
they have a graph that very clearly shows raytracing at a performance advantage as complexity increases.
No, they have a graph that very clearly shows that raytracing while using a binary tree to cull non-visible surfaces has a performance advantage over rasterizing while using nothing to cull non-visible surfaces. Perhaps someday a raster engine will regain that advantage by using these "BSP Trees" as well.
Hybrids do make a lot of sense. The author's argument is the need for a spatial partitioning structure if one mixes ray tracing with rasterization. This is a no-brainer; you'd have such a structure anyway.
In fact, his points actually show why a hybrid is perfect: most surfaces are not shiny, refractive, a portal, etc. Most are opaque - and a rasterizer is much better for this (since no ray intersection tests are necessary). He shows pathological scenes where most surfaces are reflective; however, most shots do show a lot of opaque surfaces (since Quake 4 does not feature levels where one explores a glass labyrinth or something).
Yes, if a reflective surface fills the entire screen, its all pure ray tracing - and guess what, that is exactly what happens in a hybrid. Hybrid does not exclude pure ray tracing for special cases.
Ideally, we'd have a rasterizer with a cast_ray() function in the shaders. The spatial partitioning structure could well reside within the graphics hardware's memory (as an added bonus, it could be used for predicate rendering). This way haze, fog, translucency, refractions, reflections, shadows could be done via ray tracing, and the basic opaque surface + its lighting via rasterization.
Now, I keep hearing the argument that ray tracing is better because it scales better with geometric complexity. This is true, but largely irrelevant for games. Games do NOT feature 350 million triangles per frame - it just isn't necessary. Unless its a huge scene, most of these triangles would be used for fine details, and we already have normal/parallax mapping for these. (Note though that relief mapping usually doesn't pay off; either the details are too tiny for relief mapping to make a difference, or they are large, and in this case, traditional geometry displacement mapping is usually better.) So, coarse features are preserved in the geometry, and fine ridges and bumps reside in the normal map. This way, triangle count rarely exceeds 2 million triangles per frame (special cases where this does not apply include complex grass rendering and very wide and fine terrains). The difference is not visible, and in addition the mipmap chain takes care of any flickering, which would appear if all these details were geometry (and AA is more expensive than mipmaps, especially with ray tracing).
This leaves us with no pros for pure raytracing. Take the best of both worlds, and go hybrid, just like the major CGI studios did.
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