AA Screenshots

First things first, while we’re using screenshots today to help illustrate ATI and NVIDIA’s dual-GPU AA rendering modes, we should reiterate that when you’re dealing with AA levels as high as we are in this article the differences between the AA levels are easiest to see when the game is running in motion rather than in a static screenshot. Screenshots just don’t do the AA modes justice, particularly at the higher AA levels.

We should also note that ATI and NVIDIA’s newer AA modes, while similar, don’t symmetrically correspond to each other. On NVIDIA setups for instance, two SLI antialiasing modes are available, an 8x setting and a 16x setting.

With the 8x setting, both GeForce cards in the SLI system utilize 4x multisampling, with each card using different rotated-grid sampling patterns which are blended together before being output to your screen. Under the new 16x setting, each card runs NVIDIA's 8xS mode with different sampling patterns. If you recall, 8xS combines NVIDIA's traditional 4xMSAA with 2x supersampling for sharper visuals.

ATI’s CrossFire implementation is somewhat similar. When running in CrossFire mode, both Radeon graphics cards use slightly different sampling patterns which are then blended together by the compositing engine found on the CrossFire master card to produce the final image, ultimately providing sharper visuals. CrossFire also supports 8x, 10, 12x, and 14x AA modes. For CrossFire’s 8xAA mode, both cards are running in 4xAA mode, for an effective equivalent of 8xAA likewise at 12xAA, both cards are running in 6x mode. The 10x and 14x modes mix multisampling with supersampling for even better image quality. At the 10x setting for example, the cards mix 8x multisampling with 2x supersampling, while 14x combines 12x multisampling with 2x supersampling.

For X1800 XT CrossFire, the compositing engine has a more powerful FPGA chip that more powerful than its predecessor. The new FPGA has been designed to handle the blending used for CrossFire’s Super AA mode, resulting in little or no performance hit for SuperAA in some cases. Let’s take a look at those screenshots though, first with 4xAA:

Radeon X1800 XT 4xAA/16xAF

GeForce 7800 GTX 512MB 4xAA/16xAF

Nothing much to say here, we’re starting with Lock On: Modern Air Combat first as it has proven to be a title that’s pretty good at illustrating the subtle variations in AA quality between ATI and NVIDIA hardware. We should also note that graphics quality settings were the same for both cards, with the screenshots taken using the game’s highest quality graphics settings. The heat haze from the engine exhaust didn’t show up properly in some of our ATI screenshots regardless of the method used to record screenshots, but the haze effect was definitely present on screen when the screenshots were taken. You'll also notice the banding on the tail of the F-15C (the swirly lines) this is another image capture bug that wasn't seen during actual gameplay. Under 4xAA, image quality from both cards is pretty close, each card has their areas that they render better than the other. Take a look at this example in favor of the Radeon X1800 XT:



Radeon X1800 XT 4xAA




GeForce 7800 GTX 512MB 4xAA


Look just behind the air intake, right where the intake meets the F-15C’s fuselage. This area of the screenshot clearly favors ATI. But what happens when you focus on a different portion of the same screenshot? Now let’s look at this screenshot taken from a different portion of the F-15C in favor of the GeForce 7800 GTX 512MB:



Radeon X1800 XT 4xAA




GeForce 7800 GTX 512MB 4xAA


In this screenshot, check out the trailing (back) edge of the F-15C’s wing, as well as the shadow underneath. This is a case that favors GeForce 7800 GTX 512MB (although the shadow underneath the F-15C’s wingtip looks better on X1800).