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NVIDIA’s GeForce 7800 GTX |
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Walk Softly… |
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by Josh Walrath |
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Improvements in Antialiasing Many users and tech writers have been bemoaning the fact that NVIDIA has not significantly changed its multisampling unit since the days of the GeForce 3. While improvements have been made over the years, the basic functionality is nearly the same. Last year we received a bit of an upgrade with a rotated grid multisampling algorithm at 4X, but the unit on the NV40 was essentially the same as that in the NV3x series. Now it seems that some further upgrades have occurred with NVIDIA’s multisample unit. While the unit has not received a major makeover in terms of overall design, it has been upgraded with some very interesting new algorithms that should make this old dog get up and show some impressive tricks. NVIDIA’s AA unit does not feature more than 4 subsamples per pixel, and the sampling pattern is not programmable (unlike ATI’s AA unit). The first major upgrade is that of implementing gamma-corrected AA to all of its modes. This is something that ATI implemented with the introduction of the R300/9700 Pro, and has been a long time in coming with NVIDIA. Now at 2X and 4X AA settings, the NVIDIA and ATI output should be nearly indistinguishable to each other. To get past the 4 subsample limitation, NVIDIA implemented mixed mode antialiasing some years back. This was not very popular as these mixed modes were performance hogs. Mixed modes use a combination of supersampling with multisampling applied. For example the 8XS mode featured a 2x supersampling with 4x multisampling. This gave excellent edge antialiasing as well as color texture filtering. Supersampling also has the advantage of antialiasing alpha textures. The 7800 GTX and SLI setups will also officially support the higher mixed mode settings in later driver releases. It seems that NVIDIA did not stop with the mixed mode idea, as they have implemented two very exciting features into the G70’s AA repertoire. Transparency Adaptive Supersampling and Transparency Adaptive Multisampling are designed to detect when alpha textures that need antialiasing are present in a scene. To give the average user an idea what an alpha texture really is, consider that most games will use textures instead of true models for fine and complex models. An example of this would be a chain link fence. It makes no sense for the developer to individually model and texture each strand of wire on the fence, so instead they utilize a flat texture that has the chain links “drawn” into it. Unfortunately the gaps in the chain links are actually alpha textures (transparent). Since multisampling only does edge antialiasing on a polygon, the interior details of that polygon’s texture are not antialiased. So jaggies are very apparent in these types of textures. NVIDIA’s Transparency Adaptive Supersampling actually supersamples only the alpha texture. It does not supersample the entire scene, so performance is not nearly as negatively affected as with full scene supersampling. The driver apparently identifies the texture that has the specific alpha component that causes aliasing. TAS performs a higher quality antialiasing of alpha textures as compared to its counterpart, Transparency Adaptive Multisampling. TAM does not provide as much quality as TAS, but it appears to be faster. NVIDIA has not gone into detail about how exactly it is able to use multisampling on alpha textures.
This is an excellent example of a alpha texture. Note that the chain links are jagged, but the billboard's edges behind it are smooth.
With Transparency Adaptive Supersampling enabled, we see the links in the alpha texture get smoothed out. This allows fo a much more visually stimulating scene, which in turn can help with a title's immersiveness. Another area that has is left unclear about NVIDIA’s antialiasing unit is that of HDR. With the NV4x generation of chips, when FP16 frame buffer effects are enabled, antialiasing is disabled. These effects include OpenEXR HDR effects, such as those done in Far Cry and Splinter Cell: Chaos Theory. NVIDIA has not stated (and I don’t physically have a 7800 GTX here to investigate) if antialiasing will be enabled with FP16 frame buffer effects. With the extra pixel units, as well as the redesigned texture units, AA should be available, but I am not certain at this time. So while NVIDIA’s antialiasing unit does not appear to have been radically redesigned, it has been given some very interesting and useful features that the end user can really enjoy. I myself cannot wait to try the TAS/TAM features out on many titles such as Far Cry, Thief: Deadly Shadows, Half Life 2, and other games which have typically had their overall visual appeal diluted by the use of poorly antialiased alpha textures.
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