FSAA Sample Patterns

GeForce 6800 Ultra currently has 4 modes of FSAA enabled in the 60.72 drivers: 2X, 2X Quincunx, 4X and 8X. Let's take a look at the sampling positions of the the AA modes available using Colourless' DirectX FSAA Test Viewer. Each image shows three squares, the left one shows the geometry sample points (red squares), the right shows the texture sample points (green squares) and the middle square shows all the sampling points (geometry and texture) that occur for each pixel under that FSAA mode.
No AA
2X
2X Quincunx
4X
8X

The first image is just a reference, showing that both the pixel sampling positions and texture sampling position are in the centre of the sub-pixel without any FSAA. The 2X FSAA image shows that the texture sampling position is still one point in the centre of the sample, marking it as Multi-Sampling FSAA, but with two sub-pixel sample points at a 45° points, which is fairly common for this mode of AA. 2X Quincunx shows the same as 2X from these images, as this mode features a filter in the RAMDAC that combines samples from neighbouring pixels, which this application cannot pick up - the base sample points are shown to be the same as 2X FSAA though.

4X FSAA is where we see the difference from previous NVIDIA architectures. The AA application does show that GeForce 6800 has a "rotated" sampling position as the earlier documentation describes - matching the pattern that ATI and 3dfx have used before.

The 8X mode is somewhat curious. The application shows that this is a combination of 4X Super-Sampling and 2X Multi-Sampling. It's curious that NVIDIA would pick this mode as it is likely to have a higher performance penalty than 2X Super-Sampling and 4X Multi-Sampling. This may also be an indication that the sample positions for MSAA are not programmable but fixed, for if they were programmable in a sparse sampled grid a non regular pattern could be achieved by having one sample position on one pixel pipeline and another pattern on the other, so when they are combined they would form a larger, sparse sampled pattern.

Seeing as 4X FSAA is the newest modes available to NVIDIA hardware, here is a couple of shots with it enabled (also with 16X control panel Anisotropic Filtering):