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Anisotropic Filtering: New Method, Old Optimizations

We were surprised to find a new method of anisotropic filtering (AF) implemented into the NV40. Why change or invent something new when their existing AF method, realized in GeForce FX series chips, gives good quality and high performance?

The developers of the NV40 have another opinion. Without long descriptions, we just want to show you screenshots of a scene from 3DMark03, which is specifically designed for estimation of the texture-filtering quality. The screenshots were taken at the highest anisotropy level; the scene used a combination of tri-linear and anisotropic filtering and MIP levels were highlighted. The screenshots for the GeForce 6800 Ultra are to the left; for the GeForce 5950 Ultra – in the middle; for the RADEON 9800 XT – to the right:

GeForce 6800 Ultra GeForce FX 5950 UltraRADEON 9800 XT
2x
4x
8x
16x

The pictures produced by the GeForce 6800 Ultra and RADEON 9800 XT are surprisingly similar-looking! The NV40 seems to be using the same or similar AF method that graphics processors from ATI employ, with all its highs and lows. This method is fast and good on “convenient” angles, but there are also “inconvenient” angles of textures when the level of anisotropy is reduced and the quality of textures degenerates.

Implementing the new AF method, the developers didn’t forget about the optimizations that came along with the older method like simplification of tri-linear filtering, texture sharpness analysis, reduction of the anisotropy level and forced texture compression. The screenshots show that the RADEON 9800 XT uses true tri-linear filtering along with anisotropic one in the highest-quality mode, while the GeForce 6800 Ultra uses a simplified form of tri-linear filtering.

To see the influence of those optimizations on the quality of textures, we took the same scene and varied quality settings in the graphics card drivers.

GeForce 6800 Ultra, tri-linear filtering, modes from right to left: “High Quality”, “Quality”, “Performance”, and “High Performance”:

High QualityQualityPerformanceHigh Performance

It seems like we’ll never again see true tri-linear filtering performed by NVIDIA’s graphics processors: the GeForce 6800 Ultra uses a combination or bi- and tri-linear filtering even in the “High Quality” mode.

The same modes (“High Quality”, “Quality”, “Performance”, “High Performance”) with 16x anisotropic filtering:

High QualityQualityPerformanceHigh Performance

The positions of the borders of the MIP levels didn’t change – that is, the level of anisotropy remained the same. Well, there should have been no reduction of AF level since the texture in the scene is sharp – the algorithm that thinks over the possibility of reducing the AF level doesn’t allow the graphics processor to be lazy in this case.

The borders of the MIP levels become sharper as we go from “High Quality” to “High Performance” until tri-linear filtering disappears completely.

The last scene is from the OpenGL game “Serious Sam: The Second Encounter”. When taking the screenshots, we selected the maximum graphics quality settings in the game, controlling anisotropic filtering through the driver’s control panel.

So, the conditions are the same: the NVIDIA GeForce 6800 Ultra GPU performs 16x anisotropic filtering, with tri-linear filtering and MIP-levels highlighting enabled. The four screenshots refer to the four driver settings: “High Quality”, “Quality”, “Performance”, and “High Performance”.

High QualityQualityPerformanceHigh Performance

There’s little difference between the images of “High Quality” and “Quality” modes. Only the remotest MIP-levels bear some traces of divergence.

When we switch to the “Performance” mode, the border lines between MIP levels become sharper. The non-colored stripes that correspond to the second texture layer become closer to the viewer. This means that the anisotropy level is reduced for the second texture layer.

The anisotropy level is reduced further for the second texture layer in the “High Performance” mode and the scene at large loses its original appearance due to forced texture compression.

Thus, the NV40 core has inherited all the set of optimizations we know from GeForce FX series chips. Is it good or bad? Well, there’s nothing wrong about it as you can always have the highest-quality picture by choosing “High Quality” in the driver settings. But if the performance of the NV40 is not enough for a given game (this scenario seems fantastic today), you can sacrifice the visual quality for more speed.

The new AF method realized in the NV40 is practically identical to the one ATI Technologies uses in its chips, so all known pros and contras of ATI’s anisotropic filtering can now be applied to the GeForce 6800 Ultra. By the way, that’s why we don’t compare images without highlighted MIP levels as the GeForce 6800 Ultra and the RADEON 9800 XT output textures of the same quality, while the characteristic artifacts of the simplified tri-linear filtering of the NV40 are rarely perceptible and you can only see them in dynamic scenes.

 
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