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AMD DX11/tessellation

post #1 of 4
Thread Starter 
What's the verdict? I've seen some confusion on these forums about how AMD cards handle DX11 (Crysis 2) and tessellation. I've got a 570 but I'm contemplating swapping for a 6950 2GB since I'm seeing a couple games get dangerously close to my 1280MB limit. But I don't want to be screwed over when DX11/tessellation-filled Battlefield 3 comes out in October.

If there ARE legit issues - is it solvable via drivers, or is it a hardware limitation?
post #2 of 4
Quote:
Originally Posted by TMallory View Post
What's the verdict? I've seen some confusion on these forums about how AMD cards handle DX11 (Crysis 2) and tessellation. I've got a 570 but I'm contemplating swapping for a 6950 2GB since I'm seeing a couple games get dangerously close to my 1280MB limit. But I don't want to be screwed over when DX11/tessellation-filled Battlefield 3 comes out in October.

If there ARE legit issues - is it solvable via drivers, or is it a hardware limitation?
Its the way Nvidia and ATi handle tessellation. BF3 should be no problem with ATI cards.
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Ishimura
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post #3 of 4
Cayman GPUs have a series of tessellation (and generally speaking, geometry) upgrades in contrast of it's notoriously imbalanced predecessor, Cypress. The initial change in tessellation efficiency & throughput came with the 6800 release. AMD stood back from the Cypress architecture & were able to tweak the fundamental system without implementing a full-scale overhaul. Fundamentally similar or not, though, the upgraded unit was not identical.


Without increasing the triangle per clock geometric capability of the chip, which would fundamentally provide a top-to-bottom (irregardless of factor) boost in tessellation benefit, there were two ways Barts could substantively improve performance against Cayman. The first & simplest was implementing higher default clocks, such that a theoretical maximum of 700 million triangles per clock becomes 900 million without a solitary modification of the tessellator itself. The second change came by means of increasing the efficiency of the tessellator at factors common in contemporary titles (factors 1-16). As you can see from the following graphic, the most impressive optimizations directly influence this range specifically,


Beyond this factor, though, the tessellator & single primitive capability of the architecture (and not the hull/domain shaders) cause the bottleneck, & the slight graphed advantage becomes exclusive of the increase in clock speed. For reference, at 1920x1200 & Normal tessellation levels in Unigine 2.1, the 6870 ousts it's direct predecessor 5850 by 35%. Nothing to sneeze at.

Further to that point, optimizing the 7th generation,tessellator required addressing the primary bottleneck @ low to moderate factors which, as aforementioned, were the hull & domain shaders. This is relevant for the Cayman architecture because, it's important to know, Cayman doesn't just double the capacity of Cypress. It doubles the much improved Barts system, which equates to a 3-4x theoretical advantage relative the 5800 series.


The tessellation capability of these cards is really good. Even at high tessellation factors, the additional units hold their own and absolutely cream preceding AMD hardware. The Crysis 2 debacle isn't indicative of weak tessellation performance at all; but of a bizarre design choice in tessellating the ocean when you're nowhere near it, & pushing extremely high factors on objects that don't result a distinguishable IQ improvement. The 11.8 Catalyst driver, also, has bumped up AMD performance by 10-15%, allowing for very playable experience @ ultra settings (in spite of the Crytek de-optimization patch). AMD hardware obviously hasn't and isn't ascending to the theoretical maximum of Nvidia cards, because it's totally superfluous as a gaming benefit at this point. Taken in context of high-end DX 11 titles on the market, the AMD cards are performing very well & improving as VLIW is further optimized. Don't believe the hooey about AMD having poor tessellation performance.
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post #4 of 4
Thread Starter 
I'm guessing the 69xx series have the same improvements that were incorporated into the 68xx series?

I went with a 570 because I got a little paranoid (illogically so). Now I'm seeing a couple of my games use close to, or all 1280MB of my 570. I was going to add a second one, but if I'm already maxing out my VRAM I'd heavily consider the 6950 2GB. Would save me money, but I wanted to make sure I wouldn't have any Crysis 2 DX11-like debacles with BF3. Seems like I'd be fine though.
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