It seems that Microsoft is less than enthused about the prospect of ray tracing in the Xbox Series X, despite it being deemed the 'ultimate in realism.' In a Hot Chips (opens in new tab) deep dive on the AMD-powered GPU at the heart of the next-gen console, Mark Grossman, principal architect at Microsoft, has detailed the graphics silicon inside the Series X and just what role ray tracing has in it.
The short answer seems to be: not much.
The dual compute units (DCU) of the Big Navi-like GPU inside the Xbox Series X do have specific hardware dedicated to accelerating the real-time ray tracing process. But that seems to be the only change to the RDNA 2.0 dual compute unit compared with the first-gen ones found in the AMD RX 5700-series cards.
There are 26 of them (so 52 actual individual compute units), but from high-level a look at the GPU structure it seems like a pair of DCUs have been disabled in each chip. As a whole then, the full AMD GPU would have 56 compute units inside it, but maybe with such a large slice of 7nm silicon it makes sense to build some redundancy in there.
Because the next-gen Xbox GPU is built for DirectX 12 Ultimate—and therefore uses the DirectX Raytracing API—that means the dedicated hardware blocks in the AMD GPU will be aimed at accelerating the same bounding volume hierarchy (BVH) algorithms Nvidia's RTX-based graphics cards are pointed at.
"We've added hardware embedded in the compute units," says Grossman, "to perform intersections of rays with acceleration structures that represent the scene geometry hierarchy. That's a sizeable fraction of the specialised ray tracing workload, the rest can be performed with good quality and good real-time performance with the baseline shader and memory design.
"The overall ray tracing speed up varies a lot, but for this task it can be up to 10x the performance of a pure shader-based implementation."
But while Microsoft has indeed worked with AMD to ensure there is some level of ray tracing support inside the Xbox Series X GPU, Grossman doesn't seem to be that enthused about how readily it will be utilised.
"We do support DirectX Raytracing acceleration, for the ultimate in realism™, but in this generation developers still want to use traditional rendering techniques, developed over decades, without a performance penalty," says Grossman sadly. "They can apply ray tracing selectively, where materials and environments demand, so we wanted a good balance of die resources dedicated to the two techniques."
Maybe it's the fact that he's presenting to a screen, with an unknown number of virtual attendees sitting on the other side, or maybe he's really not expecting real-time ray tracing to be of much interest to developers in this coming generation either. Either way, we just want to give him a hug.