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FSAA Quality and Performance

As we have already mentioned in our article called Highly Defined: ATI Radeon HD 2000 Architecture Review, although new anti-aliasing techniques using narrow tent and wide tend post-filters improve anti-aliasing of small details, they create a washed-out effect for the entire picture. In some cases it may result in unsatisfactory image quality. According to ATI, the new edge detect CFAA algorithm allows to avoid washed-out effect and retain excellent anti-aliasing quality of small objects.

To find out how true this statement is, we checked the anti-aliasing quality of contemporary ATI and Nvidia solutions in a blitz test session. For ATI we used traditional MSAA modes as well as multi-sampling mode with edge-detect filter. For Nvidia GeForce 9800 GTX we took screenshots for all MSAA and CSAA modes including 16xQ.

ATI Radeon HD 4850 
MSAA 4x

Nvidia GeForce 9800 
MSAA 4x

Nvidia GeForce 9800 
MSAA 4x + CSAA (CSAA 8x)

Both cards provide sufficient 4x multisampling quality to satisfy most gamers out there, although anti-aliasing of small objects and details could have been a little better.

We should note that CSAA 8x, which is supported by Nvidia GeForce 8/9 hardware, has nothing to do with true multi-sample AA 8x. In fact, this is one of the coverage-sampled antialiasing modes that stores only 4 color/Z samples, but uses 8 color lookups within its coverage sample. As you can see from the screenshot, this mode is practically the same as the regular MSAA 4x in quality. At least, it is extremely difficult to notice any differences in ideal conditions, not to mention a real game.

ATI Radeon HD 4850 
MSAA 4x + edge detect filter 
(FSAA 12x)

Nvidia GeForce 9800 
MSAA 4x + CSAA 
(CSAA 16x)

ATI’s edge detect CFAA 12x mode shows excellent results, outperforming MSAA 4x in anti-aliasing quality. You can clearly see it on the columns in the screenshot from TES IV: Oblivion. Adaptive filter does its job and the edges do indeed look much neater, without any washed-out effect. Now we have to find out how resource-hungry this mode is and if it can be used in games without any performance losses that could threaten comfortable gaming.

In the meanwhile, CSAA 16x mode turns out closer to the classical MSAA 4x than to MSAA 8x, because “16” in its name refers only to the coverage resolution, just like the “8” in CSAA 8x. Polygon edges are detected better, however the final pixel color can be calculated more precisely only with fully-fledged MSAA 8x.So, the anti-aliasing quality is much higher in the latter case.

ATI Radeon HD 4850 
MSAA 8x

Nvidia GeForce 9800 
MSAA 8x (CSAA 8xQ)

In MSAA 8x mode anti-aliasing is way better although it is hard to notice at first, especially in real games rather than on static screenshots.

Nvidia’s mode called “8xQ”, which is indeed a classical MSAA 8x, provides the same great anti-aliasing quality as the similar mode supported by ATI Radeon HD.

ATI Radeon HD 4850 
MSAA 8x + edge detect filter 
(FSAA 24x)

Nvidia GeForce 9800 
MSAA 8x + CSAA 
(CSAA 16xQ)

Csaa_16xQ/cfaa_24x

 

Although ATI’s edge detect 24x mode uses the same number of color and Z samples, it demonstrates much better anti-aliasing quality than Nvidia GeForce’s CSAA 16xQ. The “smart” post-filter allows it to ensure practically ideal anti-aliasing quality, but at what cost?

Unfortunately, the price is way too high: enabling edge detect FSAA 12x cuts the performance of ATI Radeon HD 4850 in half, and enabling 24x mode makes the performance 4 times slower. At the same time, the owners of multi-GPU systems with ATI Radeon HD 4850/4870 will certainly appreciate super-quality FSAA mode. With three or four GPUs in the system the performance will rise to acceptable level and the image quality will be truly unmatched.

Nevertheless, it makes perfect sense to enable MSAA 8x on ATI Radeon HD 4850 even in 1920x1200 (at least it is true for Half-Life 2 Episode 2). However, the same mode on Nvidia GeForce 9800 GTX pushes the average performance level close to the minimum.

 
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