That the framerate difference between ray tracing a scene at 1080p and 4K would be 25%, which is what you were suggesting wasn't it?
If we forget any potential optimisations, then it would not be unreasonable to expect that the raytracing portion of frame generation would indeed quarter versus 1080p at 4K, if only due to the increase in pixels. But that tells us nothing about the actual overall framerate difference, because raytracing takes up an unknown portion of the total time it takes to generate a frame.
Put it this way:
Using this link for 1080ti performance on Witcher 3 we can see that at 1080p, the card gets 140fps, whereas as 4k it gets around 66fps.
https://www.guru3d.com/articles-pages/geforce-gtx-1080-ti-review,26.html
Now, using those numbers, let's do some calculations, remembering that for 60fps, a single frame has to complete in 16.8ms.
- 140fps at 1080p means that each frame is taking 7.1ms to render.
- Lets add raytracing onto that, and say that it takes another 3ms to complete.
This works out to a combined total of 10.1ms to render a single raytraced frame at 1080p and would equal 99FPS.
- 66fps at 4K means that each frame is taking 15.2ms to render.
- And for the sake of argument, let's say at 4K is 4 times slower than it was at 1080p, making the raytracing take 12ms to complete.
That's a combined total of 27.2ms, which equals around 37-38fps for fully raytraced 4k Witcher 3 at 4K.
So that's 99FPS with RT on at 1080p, and 38FPS with RT on at 4K.
The point being that even if raytracying a scene is 4 times slower at 4k versus 1080p, that doesn't mean that the overall framerate would also be four times slower, since it would depend entirely upon the proportion of raytracing versus rasterisation, which right now, we have no idea on.
I'm really sorry if i'm articulating this poorly, hopefully that makes sense.
I'm by no means an expert on modern GPU architectures, but I'm a professional mobile/app developer who has worked on several quite large scale 3D projects, so working out rendering pipelines and calculating performance is something I do quite a bit.
An a loosely related note, I wouldn't be surprised if NVIDIA's raytracing implementation allows for a variable resolution which is completely decoupled from the main rendering/rasterisation resolution, meaning that raytracing can be completed in 1080p while the rasterisation is still 4k. Given that shadows and lighting don't necessarily need the same level of fidelity and detail as textures and geometry, this would seem like an obvious optimisation to me. If NVIDIA themselves aren't baking it in, then I suspect games developers and the big engines will certainly offer something similar.
But yer, I will be giving a vid or written review (probably the latter) with benchmarks and I bloody hope there is some serious gains over my current 1080Ti
.... Although im currently playing through AOE3 on mine.... I think its only using about 30% of the GPU at 4K!!
