At the base level, DXR will have a full fallback layer for working on existing DirectX 12 hardware. As Microsoft’s announcement is aimed at software developers, they’re pitching the fallback layer as a way for developers to get started today on using DXR. It’s not the fastest option, but it lets developers immediately try out the API and begin writing software to take advantage of it while everyone waits for newer hardware to become more prevalent. However the fallback layer is not limited to just developers – it’s also a catch-all to ensure that all DirectX 12 hardware can support ray tracing – and talking with hardware developers it sounds like some game studios may try to include DXR-driven effects as soon as late this year, if only as an early technical showcase to demonstrate what DXR can do.
In the case of hitting the fallback layer, DXR will be executed via DirectCompute compute shaders, which are already supported on all DX12 GPUs. On the whole GPUs are not great at ray tracing, but they’re not half-bad either. As GPUs have become more flexible they’ve become easier to map to ray tracing, and there are already a number of professional solutions that can use GPU farms for ray tracing. Faster still, of course, is mixing that with optimized hardware paths, and this is where hardware acceleration comes in.