NVIDIA GeForce GTX 480 Review

Image Quality Improvements

Even though additional geometry could end up adding to the overall look and “feel” of a given scene, methods like tessellation and HDR lighting still require accurate filtering and sampling to achieve high rendering fidelity. For that you need custom anti-aliasing (AA) modes as well as vendor-specific anisotropic filtering (AF) sampling and everything in between. As the power of GPUs rapidly outpaces the ability of DX9 and even DX10 games to feed them with information, a new focus has been turned to image quality adjustments. These adjustments do tend to impact upon framerates but with GPUs like the GF100 there is much less of a chance that increasing IQ will result in the game becoming unplayable.


Quicker Jittered Sampling Techniques

Many of you are probably scratching your head and wondering what in the world jittered sampling is. Basically, it is a shadow sampling method that has been around since the DX9 days (maybe even prior to that) which allows for realistic, soft shadows to be mapped by the graphics hardware. Unfortunately, this method is extremely resource hungry so it hasn’t been used very often regardless of how good the shadows it produces may look.


In the picture above you can see what happens with shadows which don’t use this method of mapping. Basically, for a shadow to look good it shouldn’t have a hard, serrated edge.


Soft shadows are the way to go and while past generations of hardware were able to do jittered sampling, they just didn’t have the resources to do it efficiently. Their performance was adequate with one light source in a scene but when asked to produce soft shadows from multiple light sources (in a night scene for example), the framerate would take an unacceptably large hit. With the GF100, NVIDIA had the opportunity to vastly improve shadow rendering and they did just that.


To do quicker, more efficient jittered sampling, NVIDIA worked with Microsoft to implement hardware support for Gather4 in DX11. Instead of doing four texture fetches per cycle, the hardware is now able to specify one coordinate with an offset and fetch four textures instead of having to fetch all four separately. This will significantly improve the shadow rendering efficiency of the hardware and is still able to work as a standard Gather4 instruction set if need be.

With this feature turned on, NVIDIA expects a 200% improvement in shadow rendering performance when compared to the same scene being rendered with their hardware Gather4 turned off.


32x CSAA Mode for Improved AA

In our opinion, the differences between the AA modes above 8x are minimal at best unless you are rendering thin items such as grass, a chain-link fence or a distant railing. With the efficiency of the DX11 API in addition to increased horsepower from cards like the GF100, it is now possible to use geometry to model vegetation and the like. However, developers will continue using the billboarding and alpha texturing methods from DX9 which allow for dynamic vegetation, but it will continue to look jagged and under-rendered. In such cases, anti-aliasing can be applied but high levels of AA are needed in order to properly render these items. This is why NVIDIA has implemented their new 32x Coverage Sample AA.


In order to accurately apply AA, three things are needed: coverage samples, color samples and levels of transparency. To put this into context, GT 200 had 8 color samples and 8 coverage samples which means a total rate of 16 samples on edges. However, this only allowed for only 9 levels of transparency. This lead to edges which still looked jagged and without proper blending so dithering was implemented to mask the banding.

The GF100 on the other hand features 24 coverage samples and 8 color samples for a total of 32 samples (hence the 32x CSAA moniker). This layout also offers 33 levels of transparency for much smoother blending of the anti-aliased edges into the background and increased performance as well.


With increased efficiency comes decreased overhead when running complex AA routines and NVIDIA specifically designed the GF100 to cope with high IQ settings. Indeed, on average this new architecture only loses about 7% of its performance when going from 8x AA to 32x CSAA.


TMAA and CSAA: Hand in Hand

No matter how much AA you apply in DX9, there will still invariably be some issues with distant, thin objects that are less than a pixel wide due to the method older APIs use to render these. Transparency Multisample AA (TMAA) allows the DX9 API to convert shader code to effectively use alpha to coverage routines when rendering a scene. This, combined with CSAA, can greatly increase the overall image quality.


It may be hard to see in the image above but without TMAA, the railing in the distance would have its lines shimmer in and out of existence due to the fact that the DX9 API doesn’t have the tools necessary to properly process sub-single pixel items. It may not impact upon gaming but it is noticeable when moving through a level.


Since coverage samples are used as part of GF100’s TMAA evaluation, much smoother gradients are produced. TMAA will help in instances such as this railing and even with the vegetation examples we used in the last section.