Single page Print

Morphological AA: sounds cool, right?
The other new image quality enhancement arriving with Barts is a software-based antialiasing filter that AMD has dubbed morphological AA. AMD has been playing around with various custom, post-process AA filters for some time now, and this new one is the next step. Unlike some of AMD's past custom filters, morphological AA is based on a compute shader and, as I understand it, simply looks at a finished image, detects edges with rough transitions, and attempts to smooth them.

The advantages of this approach are several. The morphological filter has relatively low performance overhead, and because it's simply looking at a finished scene and doing its work, it will smooth out rough transitions even if they don't occur along polygon boundaries. The most widely used AA method, multisampling, simply won't address jagged edges within textures or the like. Also, because it's a post-process effect, morphological AA should be compatible with a wide range of games based on DX9 and up. Since this feature is implemented in DirectCompute, it will be available via a driver update for owners of existing 5000-series Radeons, as well as the 6850 and 6870.

AMD only got us a driver with morphological AA enabled a couple of (very busy) days ago, so we haven't had much time to play with it and form many impressions. We have produced some sample images, shown below, from morphological AA versus multisampling. You should know that the morphological AA sample image was produced by a tool from AMD that applies the filter to a screenshot. We had to use this tool because the post-process filter's effects don't show up in screen captures.


No antialiasing


4X multisampling


Morphological AA (without MSAA)


8X multisampling

These images from Bad Company 2 are a pretty good example of the problems with multisampled AA. Quite a few of the object edges in this shot aren't touched by MSAA, regardless of the strength of it. That seems to be a quirk of a lot of modern game engines, including this one. The result is that the sight and the top of the player's gun are smoothed nicely by MSAA, but very little else in the image is—not the foliage, nor the edge of the cliff curving through the top right portion of the image. By contrast, with morphological AA, a great many of the object edges in the scene are softened.

One disadvantage of morphological AA is that, since the filter operates on a completed scene, it lacks any sort of sub-pixel precision. The edge-detection algorithm must simply guess about the angles of the slopes it's smoothing. Look, for instance, at the right side of the middle tine of the sight on the player's gun above. Without AA, that tine is three pixels wide most of the way down and then fans out into a fourth pixel on the right. The morphological filter turns that into a fairly pronounced angled edge, while multisampling (especially at 8X) reveals that line runs very nearly straight up and down.

Ok, if you don't see that, I don't blame you. It's a bit subtle, but the limitation is a real one, and it may mean that object edges tend to crawl or warp while in motion. We need to spend more time with this feature to get a fuller impression of its worth.

The Catalyst drivers for the new Radeons have a couple of other changes, as well. In addition to morphological AA, the older edge-detect CFAA filter remains an option, but the narrow and wide tent filters are not available. Nalasco tells us the decision to remove the tent filters was a consequence of some performance improvements AMD recently made to its edge-detect filter that rendered the tent filters superfluous.

AMD has added another checkbox in the Catalyst Control Center titled "Disable surface format optimization." This language refers to the fact that AMD's drivers have been, for certain games, converting HDR textures into lower-precision formats in order to raise performance. Nvidia has been banging the drum about this issue for a while now, saying that it would never do such a thing. Only a handful of games seem to be affected, none of which we've used recently for performance testing, so we haven't spent much time worrying about it. (The list includes Dawn of War II, Empire Total War, Need for Speed: Shift, Oblivion, Serious Sam II, and the original Far Cry.) AMD claims image quality is not visibly reduced by this change, but it has decided to make the concession of letting the user disable this optimization if he wishes. Given the choice, we would prefer to let the game developers choose the appropriate texture formats, so we conducted all of our testing with this checkbox ticked.