New SLI Anti-Aliasing Modes
To unlock the new SLI anti-aliasing modes available within the new Forceware v77.74 drivers we used for our testing (a newer version will likely be available on nZone.com), we had to enable the "Coolbits" tweak with a dword value of "8".
With the Coolbits registry value set accordingly, the new SLI anti-aliasing rendering modes are available from within the advanced global driver options. After selecting "SLI antialiasing" from within the "SLI Rendering Mode" drop-down menu, two new AA modes are made available, SLI 8X and SLI 16X. We should note that these two modes replace the traditional AA options available within NVIDIA's Forceware drivers when enabled, but they can be used in conjunction with transparency anti-aliasing should you own a pair of GeForce 7800 GTX cards. Owners of GeForce 6 cards will get SLI8X and SLI16X AA alone.
The two new SLI anti-aliasing modes work in a similar fashion. The slide above (provided by NVIDIA) somewhat explains how the process works. Essentially, each GPU renders a frame and applies the appropriate filtering, then the two frames are blended, and finally the blended frame is sent out to the display. In the SLI8X AA mode, NVIDIA's 4X multi-sample AA is applied; in SLI16X AA mode, 8xS (4X multi-sample / 2X super-sample) AA is applied. But there is a little more to the story.
The four slides above explain how NVIDIA's anti-aliasing samples are applied when in standard AA and SLI AA modes. Basically, the same sample patterns used in NVIDIA's traditional AA modes are employed in SLI AA mode, but they are offset before being blended, so they effectively compliment each other. The aggregate effect of applying 4X multi-sample anti-aliasing (4 AA samples / 1 texture sample) to the same frame twice, and then offsetting the pattern and blending the image is 8X AA (8 AA samples / 2 texture samples). Using the same logic, but applying it to SLI 16X AA, shows that 8xS AA (2X super-sample / 8 AA samples / 2 texture samples) results in an aggregate 16X AA (2x 8xS AA / 16 AA samples / 4 texture samples).
There are two side effects to these new SLI anti-aliasing modes. First, because AA is being applied to the frame being rendered on each GPU, and the frame needs to be offset and blended, additional memory is used as a buffer. So in some cases, the GPU may run out of frame buffer memory and have to swap into-and-out-of system memory, which will adversly affect performance. Secondly, due to the fact that each GPU is working on the same frame, there is no performance increase associated to running a pair of GeForce cards in a single system. SLI 16X AA and SLI 8X AA should be slower than a single card running in 4X mode because of this.
But SLI AA isn't about high-framerates in every game. The idea behind SLI AA is to improve image quality in games that will still run comfortably with the increased filtering being applied. Older games, or CPU bound games are the proposed candidates. For GPU bound games, it's better to just crank up the resolution and apply standard anti-aliasing. But with a CPU bound game, you can now crank the quality up even higher, and still maintain playable framerates in many cases.