While announcing the CrossFire, ATI revealed the SuperAA 10x and 14x, a high quality anti--aliasing that consists in calculating an image on each card with a slight offset and then fusing them together. NVIDIA was noticeably annoyed to see their rival offer more options with its own multi-GPU technology. At the 7800 GTX press conference they wagered that they could develop a better anti-aliasing by updating the drivers and this even before the first CrossFire card was available. They won their wager!
SLI antialiasing is available in 8x and 16x mode. Each consists in mixing 2 images, one in AA 4x the other in AA 8xS, and each being calculated by a GPU and with a slight offset. The main card is responsible for combining the two images. In ATI notation, the SLI 8x mode corresponds to a "SuperAA 10x", the SLI 16x to a "SuperAA 20x".
Positioning of samples of the different FSAA modes :
From top to bottom: 4x, 8xS, SLI 8x, SLI 16xBetween the 8xS and SLI 8x modes, the position of samples varies and it’s difficult to say which is better. Overall, the 8xS has a better repartition of "geometric" samples while the SLI 8x mode has a better disposition of color samples. Of course, this all depends on the angle and it’s difficult to judge based on the position of samples. These two modes have two color samples, which mean that in addition to reducing aliasing on the edges of polygons, they also reduce it on the entire image (textures, shaders etc.). In the case of SLI 16x, 4 color samples are calculated, which increased quality even more.
Quality of the different FSAA modes :
From left to right : 4x, 8xS, SLI 8x, SLI 16xHere, it seems the 8xS mode offers better quality than SLI 8x. The 4x is, of course, below and the SLI 16x above these two.
One of the disadvantages of the 8xS mode, which this test does not show very well, is that certain parts of the image are blurry, most notably text :
The SLI 8x doesn’t have this problem.
Performances
These new antialiasing modes come with a price! Compared to 8xS in SLI, SLI 8x reduces performances by 40-50 %. This is quite significant and rendering it useless in Doom 3, and actually makes it unusable. It’s pointless to speak of SLI 16x performances, only useable in very low resolution, and which wouldn’t interest users who have 2 GeForce 7800 GTX in SLI.
In going from AA 4x on a single card to 8x SLI, there is a difference in performance of more than 50%. We could have thought this difference would be smaller. The mixing of the two images is very resource hungry. It’s the same between AA 8xS with a single card and 16x SLI 16x with a 30% decline in performance.
With Far Cry, the cost of SLI anti-aliasing is felt less, but the conclusions are the same as with Doom 3. The exception comes when we compare the 8xS in SLI to 8x in SLI. Contrary to Doom 3, which uses AFR, Far Cry functions in SFR in SLI and we can see that antialiasing 8xS doesn’t gain very much from SLI. The SLI 8x mode takes an advantage here with gains of 15 to 35 %. Unfortunately, the 8xS, SLI 8x and SLI 16x modes don’t offer great performances, and so they are of little interest.
If NVIDIA did win its bet, there is something else to be said for the poor performances of the new anti-aliasing modes in the latest games that use advanced graphic engines. On the other hand, they do allow a slight quality gain in older or less advanced games.
How do the SLI AA modes compare to the ATI’s SuperAAs? In terms of quality they should be close with a slight advantage to NVIDIA in SLI 16x. On the other hand, ATI uses an external component to mix the 2 images, which would, in theory, mean that no resources would be consumed, unlike what happens in SLI AA. So although NVIDIA can probably improve performances via future drivers, improving memory management, synchronization of the GPUs etc, ATI could hold a clear advantage in performance. Of course this is all theory. For what actually happens, we’ll have to wait until ATI finally decides to commercialize the CrossFire or at least let the press test this product, which was announced almost two months ago…
Activation of SLI AANVIDIA has just made available the non-WHQL drivers via its site, nZone, which handles SLI AA, corrects bugs and gives SLI support for the GeForce 6600 LE. This SLI support is done without the little connector and therefore via PCI Express, like for GeForce 6600.
Download it here
ForceWare 77.76.
Support of SLI AA isn’t accessible in default. They have to be activated
Coolbits. Then in the driver’s advanced settings, go into
SLI Rendering Mode and select
SLI Antialiasing instead of
Autoselection., which will allow the SLI AA modes in antialiasing parameters to come up. All of this is not practical, but NVIDIA has promised to have it better integrated in future drivers.
