RDNA 4's Unreal Engine 4 Ray Tracing Stutters Traced To NVIDIA Engine Branch
The initial story prompted much finger-pointing in various directions, namely but not only AMD's driver team, but the facts are reasonably simple. This all started with a video from DigitalFoundry, where the site's team took a look at a handful of games that exhibited the issue on an RX 9070 XT card. At the time, the community's running theory was that AMD had some nasty driver bug unfixed, that could hypothetically reveal itself in games not tested for the video.
The Youtube channel Tech Yes City (TYC) then did some additional digging by testing Hellblade: Senhua's Sacrifice and The Ascent in depth. Both games exhibited serious stuttering on the RX 9070, up to several seconds long in Hellblade. As the investigation went on, TYC found out that these two games, although built on Unreal Engine 4, used a NVIDIA-optimized fork of the engine for their RT effects, called NvRTX. This is in contrast to the standard implementation that used the vendor-agnostic DirectX Raytracing (DXR).
"Zero" is one heck of a 0.1% FPS low.
If we had to guess, this is likely because when those games were developed, the NvRTX branch offered a handful of improvements over UE4's stock RT implementation, like real-time global and direct illumination, DLSS, and improved denoising. The video goes on to point out that while it's true that AMD's drivers have a bug handling this case that begs for a fix, it's also a fact that the number of affected games fairly small, and enabling RT effects in these games comes at a steep performance penalty of 2x to 3x the frame rate regardless of GPU vendor.
That ultimately means that relatively few gamers with RX 9070 or cards would even enable the feature to begin with, and the combination of all these factors results in a very small subset of end-users affected. Interestingly enough, commenters on DigitalFoundry's video stated that they didn't encounter this issue when playing some of tested games under Linux.
TYC goes on to compare Epic's Lumen software-approximation lighting as a much faster alternative to full ray-tracing, pointing out that in some games the image quality difference is fairly small, though in our personal experience that's definitely not a universal statement. It's likely that AMD will fix this bug in relatively short order, and this story is an interesting dig into the ins and outs of contemporary game engines.
