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A dip in Ray tracing PC: Radeon RX 6800 XT vs GeForce RTX 3080



A dip in Ray tracing PC: Radeon RX 6800 XT vs GeForce RTX 3080


AMD’s brand new RDNA 2 architecture has arrived for desktop PCs through the RX 6000 graphics card line – and is an important launch, bringing full hardware support to the DirectX 12 Ultimate feature set, including the DXR API. Yes, ray tracing is now an integral part of AMD’s high-end parts, and we wanted to know how effective the rendering of Radeon technology is on the RX 6800 XT – and how it compares to its closest rival, the RTX 3080. The benchmarks showed that Nvidia’s second generation technology is faster than AMD’s debut offering, but what’s the full story here?

To help put this together, XFX sent me an overclocking unit of the RX 6800 XT from its factory, specifically the Speedster Merc 319 edition of the board. Using its own tri-fan cooler design, this monstrous card provides boost clocks at 2340MHz, about five percent higher than the reference model, but I’ve seen it routinely reach clocks in the 2400MHz range and beyond. Although the XFX board has a new brand, the design philosophy has a lot in common with its previous ‘THICC’ range. The refrigerator is certainly similar, and does a good job of keeping temperatures in the low 70 Celsius range under load. However, as I mentioned, it is a little beastly in terms of form. Certainly in terms of its granulometry, it is necessary to ensure that it has adequate clearance in your case – it is about 34cm or 13.5 inches long!

In terms of the actual ray tracing metrics, it’s best to see the video here to see how I tested individual RT effects and how they’re handled in each of our competing GPU architectures, but the key objective of this test was to isolate individual RT pipeline phases to see how Nvidia and AMD behave, and to do so in the context of three key RT effects: shadows, reflections and global lighting.

Typically, in any RT scenario, there are four steps. To begin with, the scene is prepared on the GPU, filled with all objects that can potentially affect ray tracing. In the second stage, the rays are fired into the scene, passing through it and tested to see if they reach the objects. Then there is the next step, in which the results of the second step are shaded – like the color of a reflection or whether a pixel is in or out of the shadow. The final step is denoising. As you can see, the GPU cannot send unlimited amounts of rays to be tracked – only a finite amount can be tracked, so the end result looks quite noisy. Denoising softens the image, and produces the final effect.

Thus, there are numerous factors at play in dealing with RT performance. Of the four steps, only the second is hardware accelerated – and the actual implementation between AMD and Nvidia is different, with GeForce cards having additional hardware “grunts”. RDNA 2 calculates the passage of lightning in computing units, introducing competition for resources, while Nvidia does this in a specialized processor within the RT core. The first configuration phase may have significant CPU requirements, while the shading and denoising phases may have specific preferences for certain GPU architectures. For example, Quake 2 RTX and Watch Dogs Legion use a denoiser built by Nvidia and although it was not designed to malfunction on AMD hardware (which Nvidia would not have had access to when coding it), it was certainly designed to work as well as possible on RTX cards.

Regardless, in the video, I intend to be comprehensive in addressing the entire ray tracing on both architectures, covering a number of effects. The lightning-streaked shadows are tested in Call of Duty: Black Ops Cold War (a title sponsored by Nvidia) as well as Dirt 5 (supported by AMD). I take a look at the ray tracing reflexes in Ghostrunner in Unreal Engine 4, where I can examine the effect with some degree of tuning, and of course, the Watch Dogs Legion reflexes are also placed under the microscope. I chose this because AMD RT hardware is used in the consoles to produce the effect, more through modding, I can access both the console and the Nvidia denoisers. With global lighting tracked by rays, 4A Games’ incredible Metro Exodus is tested in depth, while I look at a more extreme example through Quake 2 RTX – which now works on both AMD hardware and Nvidia RT, thanks to the integration of completed Vulkan RT extensions.

So, what is the takeaway? I think there are some intriguing results here. Lightning shadows are generally undemanding in features on both the RX 6800 XT and RTX 3080 – with the RTX 3080 seeing minimal gains in lower scenarios, which then increase as the quality of ray tracing is increased in more challenging scenarios high, in a game like Call of Duty Black Ops. For ray tracing reflections, the effect is much more demanding on GPU hardware, but the visual victory is more pronounced in many scenarios. The greater the randomness of the reflected rays and the greater the number of rays fired, the greater the RTX 3080 compared to the RX 6800 XT, reproducing almost half the time in certain configurations. The RTX 3080’s efficiency advantage diminished after a certain turning point, and I saw the same thing with global lighting: the RTX 3080 could render the effect almost half the time on Metro Exodus, or even a third time on Quake 2 RTX, but increasing the amount of rays after he saw the RTX 3080 having less advantage.

In general, from these tests it appears that the simpler ray tracing is, the more similar the rendering times for the effect are between competing architectures. The Nvidia card is arguably more capable across the RT landscape, and the RTX 3080 appears to experience less dramatic performance losses as the complexity of ray tracing increases, but at the less complex end of the scale, AMD is competitive. However, Spider-Man: Miles Morales on PlayStation 5 demonstrates that Radeon ray tracing can produce some impressive results in more challenging effects – and that is using a GPU that is significantly less powerful than the 6800 XT. And with that in mind, we need to accept that ray tracing on the PC side is still in its infancy, especially when running on AMD hardware. At this point, I can only present general conclusions from a representative sample, but still small. So far, we’ve only seen shadows of RT in AMD-sponsored titles, and I’m looking forward to seeing how future titles have developed in collaboration with Team Red in demanding RT effects. Although ray tracing has been with us in the PC space for over two years, the story is just beginning – and I can’t wait to see what comes next.