nVidia GeForce GT200 Series Anti-Aliasing Investigation

Coverage Sampling Anti-Aliasing (CSAA)

The 8000 series also introduced a new type of anti-aliasing called CSAA, using a new sample type called coverage. Coverage sampling is based around multi-sampling but further decouples information by removing Z (depth), stencil and color information, leaving only a yes/no value as to whether the sample lies within a polygon or not. These values then affect the outcome of the final blended color. Generally this technique is very good on first-class polygon edges where the 16x setting can rival the quality of 16xMSAA, but at a fraction of the performance hit and storage requirements.

However since additional Z/stencil values aren’t actually taken or stored (only whether the sample falls inside a polygon), CSAA loses effectiveness on edges shared by multiple polygons, and on stencil shadows. Should coverage sampling fail for a particular pixel, the scheme falls back to its base multi-sampling level.

Here are the sample grids for the three CSAA modes (thanks to Beyond3D for the original images):

Again the blue dots are multi-samples, while the red dots are coverage samples. Based on this information we can see that:

8x = 4xMSAA + 4xCSAA.
16x = 4xMSAA + 12xCSAA.
16xQ = 8xMSAA + 8xCSAA.

Note that nVidia counts multi-samples twice in their papers, once as multi-samples and once as coverage samples, so they list 8x as having eight coverage samples and 16x/16xQ as having sixteen. While this is technically correct, it’s also ambiguous so it makes sense to count samples based on their highest common denominator.

Most games cannot activate these modes directly, but there are three ways you can use them. Firstly, you can force them directly from the control panel. This should work in most games where forced AA works.

Another way is to set the control panel to enhance and set one of the three CSAA modes. Then set the game to use any level of AA (it doesn’t matter what it is), and the driver will give the application your AA level but only on surfaces the game asks for. Thus the enhance setting allows the user to gain the benefit of AA modes games don’t know about, without overriding how games perform AA on their surfaces. Note that enhance only works if the game sets a genuine MSAA level through the Direct3D/OpenGL API, so it won’t work with games that use custom shader-based AA.

And finally, a handful of games like Crysis and Half-Life 2: Episode 2 can set CSAA modes directly from the game.