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Morphological AA - DirectCompute for Higher Graphics Quality

Another notable improvement in the new GPU is its support for a new type of full-screen antialiasing called Morphological Antialiasing (MAA or MLAA).

AMD’s official presentation doesn’t reveal the details of the new algorithm or its implementation in the new Radeon processor but we can find them in an Intel publication where this antialiasing method was employed for images rendered using ray tracing. We do not know how exactly this algorithm is implemented in the GPU but its basic principle should be the same for both CPUs and GPUs.

So, the MLAA algorithm finds certain structures in a rendered frame and blends colors at the edges of such structures according to a set of rules. The blending depends on the angle of view, color and other features of the structures. We assume that such rules can be set by the driver or even by the application and thus can be improved over time.

The MLAA algorithm resembles the edge-detect CFAA that was introduced as far back as the Radeon HD 2900 XT but there is one important difference. Instead of detecting edges which differ greatly in color and are positioned at certain angles, MLAA finds all adjacent differently-colored structures and identifies their special features. And the most important difference is that the edge-detect CFAA method uses pixel shaders and thus the entire rendering pipeline whereas MLAA relies on compute shaders which do not call for texture instructions and involve fewer data transactions.

UPDATE: 4 November, 2010
 

MSAA 8x

MLAA 8x

MLAA 8x + SSTAA

The good thing about MLAA 4x and MLAA 4x is that they, as expected, do not blur textures. The quality of MLAA 8x is akin to MSAA 8x on many surfaces and it comes with lower performance hit. Indeed, the morphological antialiasing works on all angles.

Unfortunately, there is a huge drawback of MLAA: it does not work on alpha-textures. For example, in case of Fallout: New Vegas we see that the fence and the trees are still aliased and some of the color information that could be seen in case of multisampling (twigs, wires, etc) is missing. This seems to be a problem of the algorithm or its particular implementation itself. In fact, even Intel's demos use traditional hardware antialiasing for transparent textures usually used for vegetations and small items. Therefore, in order to make MLAA work perfectly, one would have to enable hardware transparency antialiasing (TAA). In fact, as you see on one of the screenshots, the quality of morphological antialiasing with TAA is simply perfect, in fact, MLAA 8x + supersampling TAA provides almost better quality than MSAA 8x!

 

By the way, MLAA is not an exclusive feature of the Radeon HD 6800 series. Based on DirectCompute 11 and local data shares, this algorithm can run on any other AMD GPU that complies with the DirectX 11 specs. There is no theoretical obstacle to using it on the Fermi architecture, either.

 

New Anisotropic Filtering Algorithm

The improved anisotropic filtering algorithm must be noted, too.

Anisotropic filtering doesn’t have a serious effect on the performance of modern GPUs, so algorithms can be used whose filtering quality doesn't depend on the angle of inclination of the plane. AMD and Nvidia have both transitioned to high-quality anisotropic filtering already and the Radeon HD 6800 just improves the algorithm further to smooth out transitions between mipmaps so that they would be less conspicuous in textures with lots of small details.

 


Radeon HD 6800 series AF


Radeon HD 5800 series AF

As opposed to MLAA, the benefits of the new AF algorithm are easy to see. They won't be so clear in the process of actual gaming, yet a sharp-eyed gamer will surely spot the difference, especially as modern games offer a lot of scenes suitable for that.

Thus, the Radeon HD 6800 doesn’t look like a completely new product. It is rather a steady and evolutionary development of the successful Radeon HD 5800 architecture.

 
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