With the arrival of Nvidia’s RTX video cards and support for real-time ray tracing in both the DirectX 12 and Vulkan graphics APIs, we’re looking at one possible future for graphics technology – but it is one mired in controversy because up until this week, only RTX GPUs could run DXR-enabled software, meaning only a very limited number of PC users could access ray tracing functionality. But now, the floodgates are open: Nvidia’s latest driver allows its 10 and 16-series GPUs to run DXR software too. But is any kind of playable experience possible on cards without RTX’s hardware accelerated support?
We tested DXR software across three cards that lack Nvidia’s RT hardware – specifically the GTX 1060 6GB (the least capable compatible card), the GTX 1660 Ti (Turing with RT cores) and the top-end gamer’s 10-series offering, the excellent GTX 1080 Ti. To get a sense of comparison against the RTX line, we opted to bench the RTX 2080 and the RTX 2060. It’s an interesting assortment of cards: the RTX 2080 typically runs non-RT workloads at a similar speed to GTX 1080 Ti, while the 2060 is the cheapest card in the RTX line.
But what we quickly discovered is that outside of specially prepared ray tracing workloads, gaming benchmarks flatter to deceive the older cards. What quickly became evident is that RT workloads introduce such an enormous variability into gameplay that canned benchmarks only have limited relevancy compared to an actual experience of playing the game, and sometimes there’s basically no correlation at all. Ray tracing can add a baseline cost to a game, but in scenes heavy in RT components, a 10-series card crumples while a 20-series GPU powers through. As we discovered, what you test in-game needs to be carefully selected.
First up though, it’s worth explaining what sets an RTX card apart from a GTX product and by extension, how ray tracing works. First of all, the GPU creates the structure that the rays will be shot at – the Bounding Volume Hierarchy, or BVH. Then the rays are shot at the BVH – and that’s what Nvidia’s RT core accelerates. Finally, there’s another expensive step: denoising. Because only a relatively tiny amount of rays can be shot, the results you get back are very patchy/noisy – this needs to be processed into something more visually palatable. The bottom line though? In three demanding processes, only one is accelerated by RTX – and the RT core can be emulated by compute shaders, which is how 10-series cards can be brought into the mix.
And to be clear here, the entire ray tracing procedure is carried out by Windows via the DXR API. DXR doesn’t hardware acceleration, your GPU just needs to have sufficient RAM and DX12 support. While 10/16-series ray tracing support is now enabled, there is nothing to stop AMD releasing their own DXR support for its existing line-up of graphics hardware – and owing to their heavy bias towards GPU compute, we could see some surprises there.