SLI: Myth and
Reality - Part 2
Last week, we took a quick look at what NVIDIA's SLI
has to offer. In that article, we concluded that while SLI
does work, you should not expect tremendous gains in frame
rates. In average, SLI can only offer around 30 % increase,
occurring mostly when AA and AF are enabled. So, getting a
single faster card is always better than putting two slow
cards in an SLI setup. There are several reasons why is
this so. First, the increase in performance with a faster
single card is at least the same or even better and you'll
see that increase both with and without AA and AF. Second,
a single card will work with every game unlike SLI which
can only work its muscles in games that are pixel shader or
fillrate bound. Third, you'll save some space inside your
PC for additional expansions, not to mention some power
savings as well.However, performance is not the only feature of SLI. In addition to multi rendering, SLI can be used in SLI antialiasing mode. In this mode, the two graphics cards will perform antialiasing with slightly different sample patterns and then combine the results. Once you've enabled SLI antialiasing in the driver panel, you have a choice of either SLI 8x or 16x AA. Just like the name implies, 8x means 8 samples are used to compute an antialiased pixel - this works much like this: each graphics card will perform 4x AA, with the second card using a slightly different sample pattern and 'blends' them together. So, 8x SLI translates to 4x per card while 16x translate to 8xS. You might ask why is this a big deal. Well, since technically each card is only rendering 4x AA, with SLI 8x you can get 8x AA quality at close to 4x AA performance levels. Furthermore, since we're not computing two pixel samples (remember the 'S' in 8xS), SLI 8x will probably be faster than 8xS even in SLI multi rendering mode. We'll be looking at this in the later parts of the article.
We also chose a different (well, two actually) card for this article - the GeForce 7800GTX. With our setup, this card is mostly system limited when running without AA and AF. That's perfect for this article because we want to focus on AA and AF. Remember, from the first article we saw that SLI multi rendering mode works best with AA and AF. Most people who bought the 7800GTX expect to run it with AA and AF, however even a single 7800GTX can still be too slow in some games. So, it will be interesting to see whether or not a pair of 7800GTX will be able to offer higher frame rates in SLI multi rendering mode. This will be the focus of the first part our performance evaluation.
Image Quality Comparison
We're going to look at what SLI 8x has to offer, not just in performance but quality as well. So, we will be looking at several image taken from different AA modes, But before we do that, let's take a look at their sample patterns. Below, you can see the different sample patterns from each mode we will be testing in this article - left to right they are 4x, 8xS, SLI 8x and SLI 16x.
If you look at SLI 8x sample pattern, you'll see it doesn't look like 8xS at all. That's normal since we know it's based on 4x sample pattern. You can definitely see the resemblance. Overall, SLI 8x sample pattern has more 'slant' than 4x, so it should help more when antialiasing nearly horizontal lines. At least that's the theory. Let's see how it translates in real life. Below you'll see screenshots taken (in order) with 4x, 8xS and SLI 8x in Call of Duty.
Did you notice any difference? The most obvious is the fence on the left - 8xS by far has the best quality. Not surprising since this mode also perform supersampling which help antialiased transparent textures. That's about it actually, we can see no significant differences between 4x, 8xS and SLI 8x. There are some very subtle differences if you look very close, but they are not noticeable in gameplay. Let's see the next game.
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