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Imagination Technologies

Geforce 6800 - the next generation

The NV40 line didn’t only introduce changes to the AA, but also to the AF algorithm. As time goes by AF algorithms have gotten increasingly adaptive. After the introduction of a basic hybrid bi-/trilinear texture filtering mode (often referred to as “brilinear”) in NV3x – which also carried over to NV4x – NV added also angle-dependancy to it’s algorithm.

In general I have nothing against clever performance improving ideas, as long as the image quality doesn’t get affected too much. That doesn’t mean that in the given case (same goes for the competition) angle-dependant AF is under all occasions ideal; in many cases it takes an experienced eye to notice differences. However there are some cases where the output is sub-optimal and the algorithm switching between 2x and 16x AF samples can get quite noticeable. It’s not that there wouldn’t had been alternative solutions to gain at least equivalent performance with higher quality AF algorithms, but that’s an entirely different chapter.

“Brilinear” on the other hand can be in my opinion much less annoying, and if anything else can be switched off in the driver control panel, would the user want to. Here’s to say that I have to applaud NVIDIA for implementing a high quality texture filtering setting in it’s control panel. More simply put: the more options the better and the user can freely decide what fits his needs most.

Let’s have a look on some AF modes with Demirug’s D3D AF Tester:

2x AF:

2x sample AF isn’t angle-dependant here. Sadly enough 2x samples are too few for today’s standards; even more with recent high-end class accelerators.

4x AF:

Here’s the spot where angle-dependancy actually begins, to end up like this with 16xAF.

16x AF:

That “flower” looks quite familiar to the competition’s AF output.

Anisotropic filtering is meant –as most if not all know – to be combined with Multisampling. Combining Supersampling with AF is of course delivering superior texture quality and allows more aggressive LOD values, without adding any texture aliasing to the scenery; sadly enough that combination can be rarely used, due to performance reasons. Besides there is a small portion of redundancy involved, since a fair amount of textures will end up being filtered twice.

Supersampling combined with AF:

1*2 SSAA or 4xS or 8x AA + 16x AF:

Output for the all three anti-aliasing modes is identical, since all the three use the same pattern and sample amount. If you compare it to the simple 16xAF output above, you’ll see the changes on the horizontal axis.

4x SSAA or 8xS + 16x AF:

Since plain 4x SSAA and 8xS use both the same portion of Supersampling, output is here identical too, between the two modes. If you compare it again to plain 16xAF, you’ll notice that the “flower” this time has shrunk towards the center of the tunnel. The shift in LOD equals –1.0, which is normal for 4x Supersampling.

The major problem with Supersampling is that it eats up a huge amount of fill-rate, doesn’t only affect performance, but also restricts to quite lower resolutions; restrictions that don’t apply to recent MSAA + AF implementations.

I’ll have a look at performance ratings and the usability of SSAA+AF combinations later on.

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 Geforce 6800 - the next genera...
 17 Seiten
 verfasst von Ailuros
 Donnerstag - 26.08.2004 - 18:20 Uhr
[1]  Introduction & Systemsetup
[2]  Antialiasing & Anisotropic Filt...
[3]  2. Anisotropic filtering
[4]  Synthetic Applications
[5]  F1 2002
[6]  Rallisport Challenge
[7]  Serious Sam: The Second Encount...
[8]  Need For Speed: Underground
[9]  Unreal Tournament 2004
[10]  Far Cry
[11]  Hybrid Super-/Multisampling Mod...
[12]  Hybrid Super-/Multisampling Mod...
[13]  2D, DVD / VIDEO Playback
[14]  Summary
[15]  Softmode
[16]  doom3
[17]  DVD