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NVIDIA GeForce 8800 GTX & 8800 GTS
by Damien Triolet
Published on November 13, 2006

If the Radeon started to support 6x multisample antialiasing a long time ago, the GeForce were limited to 4x. This limitation ends with the GeForce 8, which supports MSAA 8x and can also couple MSAA and HDR FP16 like the Radeon X1x00 (unlike the GeForce 7).

NVIDIA also implemented a new antialiasing mode called coverage sample (CSAA). This mode consists in improving the precision with which the various color samples are mixed to form the final image. For example, if 2 triangles cross the same pixel in MSAA 4x, they can be seen like if they cover 25 and 75 % of the pixel, 50 % each or 75 and 25 %. Colors are mixed in these proportions. The coverage sample is based on a standard MSAA buffer and on a second buffer with a higher resolution (8x or 16x), which do not focus on the color. There is only a boolean value that indicates whether the triangle covers this area or not.

This technique has some limitations, because with the coverage sample buffer, it isn't possible to precisely know the interactions between numerous triangles or triangles that are contiguous or cut into each other. As the Z-Buffer stays in the standard resolution, it isn't possible to know which triangle is on top. NVIDIA had to restrict this coverage sample buffer utilization to pixels that represents 2 spaced triangles. If this isn't the case, the CS buffer data is ignored. When it is taken into consideration, in theory, it provides a result similar to its resolution since the mixing of samples is made in that resolution. If we take the above example, the triangle that covers 30% of the pixel will only have 25% of the weight during MSAA 4x mixing since it will be the closest possible approximation in this mode. In MSAA 4x + CSAA 16x, it weighs 31%, which is a better approximation.

CSAA will only be of use in specific cases, but the counterpart is that it requires much fewer resources than a higher MSAA mode considering that it consumes much less bandwidth and never requires additional calculations. In terms of memory, MSAA4x occupies 256 bits per pixel. With CSAA 16x on top (16 boolean values), it "only" increases to 272 bits (actually probably a bit more than that but it stays cheap in term of memory used).

In practice, we measured the impact of these new options:

The impact on performances is from 15 to 20%, which is reasonable. However, the difference between CSAA 8x and 16x is small, and this limits the interest of the 8x mode.

This second test is identical, except we activated Transparency AntiAliasing (Adaptive AntiAliasing for ATI). Performances drop especially in MSAA 6x and 8x, which consequently have a calculation, respectively, of 6 and 8 samples of colors for the grass and other grids.
Antialiasing quality (without TAA / AAA)

GeForce 7 : without AA, 4x AA, 8xS AA


Radeon X1000 : without AA, 4x AA, 6x AA


GeForce 8 : without AA, 4x AA, 4x AA + 8x CSAA, 4x AA + 16x CSAA, 8x AA (8xQ), 8x AA + 16x CSAA
Antialiasing quality (with TAA / AAA)

GeForce 7 : without AA, 4x AA, 8xS AA


Radeon X1000 : without AA, 4x AA, 6x AA


GeForce 8 : without AA, 4x AA, 4x AA + 8x CSAA, 4x AA + 16x CSAA, 8x AA (8xQ), 8x AA + 16x CSAA

In our opinion, 4x MSAA with TAA or AAA should be used for a very good quality without an extreme performance cost.

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