Comment by brundolf | Hacker News Reader

Comment by brundolf | Hacker News Reader

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Very cool, but what I'd like to get a straight answer on is whether or not all lights/materials in this scene are ray-traced. Some things like the marbles' reflections obviously are, but there are lots of tricks that could be played to minimize the amount of other raytracing in the scene. On the other hand, if a scene with this high of a poly-count is 100% raytraced, that sets a dramatically higher bar than what's come before (the best that I've seen previously was the Quake 2 demo).

Edit: To be clear on what I mean, for those who haven't been following this stuff, most "real-time raytracing" that's started to crop up in real games over the past couple years has been limited to only a subset of lights/materials. A particularly shiny shield, a campfire, etc. And then the rest of the scene is rendered the traditional way. Even with Nvidia's dedicated hardware we can't (yet) just slap raytracing on an entire game and call it a day. But this demo makes it look like we may be much closer to that goal than I'd thought.

It’s all path traced (with denoising and DLSS upsampling I think).

The poly count is much less impressive than the number of different materials and number of light sources.

> The poly count is much less impressive than the number of different materials and number of light sources.

Is it? The basic tracing algorithms I'm familiar with scale linearly with the number of polygons (minus whatever can be culled out), and don't really care about the number of materials (other than for memory usage I guess) or light sources (because the rays come from the camera). I do know that for edge cases like refraction some more advanced algorithms will do partial forward-tracing from the light sources, so maybe that's what you're referring to. Though I didn't notice any refraction in this demo (it wasn't a big focus, at least).

All raytracing algorithms are based on some spatial subdivision structure (usually BVH based) which gets you O(logN) instead of O(N). Increasing poly count only gets interesting when you figure out how to compress truly huge scenes into a limited memory footprint.

Lots of different materials are difficult for GPUs because of divergence - if every ray spawned from a surface scattering event hits a different material you’ve lost all parallelism.

Lots of lights: Evaluating them all for a given surface scattering event is obviously an O(N) problem (for 10 lights no big deal, but what about thousands?), so you want to choose some small subset of “important” lights to consider. Making that choice is a hard problem because the correct choice depends on the product of the incident radiance from the light, the BSDF at the surface as well as the visibility function. There’s a lot of interesting research being done on this (Path guiding by learning the incident light field or its product with the bsdf, new sampling techniques, spatio-directional acceleration structures) at the moment.

Got it, that makes sense

Funny enough, the quake demo seems to have gotten better recently.

I couldn't use ray tracing on my 4k screen early this year with my 2070 RTX super. But now i can, they seem to have optimized it so the resolution can be different in different parts of the screen.

<30 fps suggests it might be all raytraced. Raytracing = unplayable in previous, this and probably two more generations.

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