So 220x4 ~ 900 million, or some 10%
Ah, so you ask 'Why only 10% of the advertised' 10 giga rays per second'?
Well, as i've learned here, this number comes from only primary rays on a single but detailed model with a simple shader that only displays surface normal of the hit point. And half of screen is empty. Something like that.
But those guys do not really want to use more rays. They want to show how far they can get in a worst case scenario of using only one path per pixel. This is their work, and they likely just keep this philosophy when releasing this as a playable demo. It is still mainly research.
On the other hand: As mentioned above, water already hurts performance, so one more path (or just reflection ray?) per pixel is already a performance problem it seems. Not sure though, and as said they could just rasterize primary visibility for a win.
you can rasterise textures and have them absolute quality instead of having to reconstruct them from noisy data
No, they certainly never need to reconstruct textures, and simple Quake textures have no impact on their algorithm anyways. They store and denoise incoming light, which is totally independent from the material. So the end result of denoising can be assumed to be a smooth signal, which is the primary idea here.
This is common for most GI tech, and i remember i've read this in their paper as well.
After this process is done, they just shade the textured buffer with the now known incoming light per pixel. So aldebo has no impact on denoising, but roughness has. High frequency variance in roughness makes denoising harder. (They have a newer paper about specular reflections, but roughness is still constant over large areas of surface.)
There are frequently missing shadows which is the one area raytracing shouldn't fail
If you go to their page and use the slider to alternate noisy input and denoised output, you see that denoising removes some things like contact shadows under a wall or AO kind of darkening corners. That's quite a loss, but for one path per pixel results are excellent. (Gamedevs would add ugly AO most likely
)