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Inside the SLI antialiasing modes
In this first driver to expose the feature, SLI AA is presented among the list of SLI rendering modes because that's literally what it is: a new SLI rendering mode, distinct from the other options, including split-frame rendering (where the screen is subdivided horizontally between the two GPUs) and alternate-frame rendering (where the GPUs trade off rendering full frames in an interleaved fashion). NVIDIA says SLI AA works in all 3D games, whether they use the Direct3D or OpenGL API. SLI AA distributes the load between two graphics cards by having each card render half of the samples necessary for the final, antialiased image and then combining them together.

Take, for instance, the case of the 8X SLI antialiasing mode. This mode is actually a doubled-up version of NVIDIA's 4X multisampled antialiasing method. The diagram below helps illustrate how it works.


Inside 8X SLI AA. Source: NVIDIA.

Each card renders the scene with 4X multisampled antialiasing, but their sample positions are slightly offset, or jittered, to make them distinct from one another. The two resulting sets of samples are then combined, yielding eight coverage samples and two texture samples, which are blended to achieve the final pixel color. The result should be the equivalent of NVIDIA's familiar "8xS" antialiasing mode, except that the sampling patterns for SLI AA are a little bit different.

The SLI 16X AA mode is essentially the same thing, except it uses a jittered and doubled version of the 8xS mode to achieve four texture samples and sixteen coverage samples. We can take a look at the resulting sample patterns by using a simple AA pattern testing application. The red dots below represent coverage samples, and the green dots represent texture samples.

 4X 6X/8X 16X
GeForce 6800/7800

 
SLI Antialiasing 

Radeon X850 XT PE

 

To clarify, the row labeled "GeForce 6800/7800" shows sample patterns for those cards' regular, non-SLI AA modes. The 6X/8X column shows patterns for NVIDIA's 8xS mode for the GeForce 6800/7800 cards, for the SLI 8X mode, and for ATI's 6X multisampled mode on the Radeon X800 series.

The sample pattern of the SLI 8X mode is pretty much exactly what NVIDIA's diagram told us to expect. NVIDIA says this pattern is superior to the 8xS mode, shown directly above it in the table, because the samples in SLI 8X mode are not vertically aligned.

SLI 16X's pattern is consistent with a doubled and offset version of 8xS, as expected.

These are real, robust antialiasing modes. The SLI AA modes involve multiple texture samples, so there is an element of supersampling to them. That means the antialiasing effect will not be confined entirely to object edges, as pure multisampled AA modes typically are (more or less). SLI AA is true full-scene antialiasing, and object interiors are modified, as well.

Also, the jittering of NVIDIA's base patterns serves to create non-grid aligned sample patterns for SLI AA. That's good, because the human eye is very adept at pattern detection, and defeating that ability is one of the keys to good antialiasing. However, NVIDIA's GPUs appear to have limited programmability when it comes to AA sample patterns, and that weakness creates some potential problems here. The pairs of samples in 16X SLI AA, for instance, are spaced very closely together, probably because the 8xS sample pattern leaves very little room for an offset within the boundaries of a pixel. This tight proximity of sample points will probably limit the effectiveness of 16X SLI AA somewhat, despite its very high sample size.

ATI's antialiasing hardware is more flexible, as the funky sparse sample pattern of the Radeon X850 XT's 6X mode hints. The ability to vary sample patterns on the fly has allowed ATI to create its temporal AA feature, and it will likely allow CrossFire's Super AA mode the luxury of superior sample patterns.

Setting some performance expectations
Because the SLI AA modes are, in one sense, simply another means of splitting up the rendering workload between two cards, SLI AA won't necessarily always provide a magical performance boost over existing SLI rendering modes. The SLI AA modes may or may not be more efficient than other SLI modes at divvying the work between two GPUs, depending on the nature of the application. I believe that alternate-frame rendering (AFR) will still likely be the best overall SLI mode for raw performance, because with AFR, the GPU can split up both the vertex and pixel processing work in a way that benefits overall performance. Like split-frame rendering (SFR), the SLI AA modes should scale only in terms of pixel throughput.

The primary benefit of the SLI AA modes, in fact, isn't raw performance, but the ability to use the additional fill rate of an SLI system to improve image quality. The 16X SLI AA mode, in particular, is more intensive than any single-card AA mode, and it should be more of a performance drain, as well. In games where SLI's extra pixel-pushing power would otherwise be essentially wasted, 16X SLI AA will be a nice bonus.

With that said, there are a number of comparisons we'll want to make when looking at the benchmark numbers in order to get a handle on SLI AA's performance picture. Among them:

  • SLI AA modes versus single-card equivalents — Yes, there should be some overhead to SLI operation, but it will still be interesting to see how the SLI AA modes compare to their single-card equivalents. For example, how does 4X AA on a single GeForce 7800 GTX match up to SLI 8X AA on a pair of 7800 GTX cards in SLI? Or single-card 8xS mode versus SLI 16X mode? With a second card taking on the additional sampling workload, one would expect them to perform fairly similarly.

  • SLI with antialiasing versus equivalent SLI AA — SLI 8X mode, I'm looking at you. How does your performance compare with the regular 8xS AA mode running on a pair of cards in SLI? Is it more efficient to use the default SLI rendering mode (whether it be SFR or AFR, depending on the profile in the NVIDIA drivers) with 8xS AA, or is SLI 8X mode faster? NVIDIA says SLI 8X mode has the advantage of smaller memory footprint, because each card is only doing 4X AA. This 8X mode could be the sweet spot for SLI AA.

  • SLI 16X AA versus utterly, incorrigibly CPU-bound games — Some games, including notables like Half-Life 2, we've found are almost completely performance limited by CPU or system throughput, not by the graphics card. This is particularly the case, quite often, with a couple of GeForce 7800 GTX cards in SLI. Very few games need all of that power, so their performance often doesn't change as we crank up the resolution and visual quality. We've even resorted to testing at ultra-high resolutions like 2048x1536, just to show performance differences between high-end graphics solutions, but those comparisons aren't always entirely fair due to GPU architecture limitations.

    So, can 16X SLI mode tease out some contrasts between, say, a pair of 6800 Ultras and a pair of 7800 GTX cards? More importantly, can either of those configs run today's games at playable frame rates with SLI 16X AA?

Those are some of the things we'll want to consider. Now, let's see how SLI AA actually looks onscreen.