ATI’s Radeon 9700 (R300) – Crowning the New King

SMOOTHVISION 2.0 – Multisampling, Finally

When the Radeon 8500 was first released we were very disappointed to discover that ATI’s SMOOTHVISION Anti-Aliasing was a supersampling algorithm and did not employ the more efficient multisampling technique that NVIDIA had already started to use.

We have explained this in the past but the way ATI’s supersampling (Rotated Grid Super-Sampling – RGSS) works is by rendering multiple copies of the scene and shifting each copy by a certain amount off the center. These copies were then blended together to produce the final image. This approach provides excellent texture resolution and gets rid of aliasing at the same time; the downside is that it does so at a huge fill rate penalty, since you’re rendering the scene multiple times.

Multisampling, on the other hand, works by looking at the z-value (depth) of each pixel. Depending on the amount of the pixel that is covered, a weighted average between the foreground and background colors is computed and that’s the final color of the pixel. The benefit of this approach is that you are not rendering multiple copies of a scene at the cost of losing the texture resolution benefits of supersampling and at the expense of even more z-buffer accesses.

The R300 supports both supersampling and multisampling (a change from the R200, which only supports supersampling), but at this point ATI is only going to expose multisampling in the drivers; this may change between now and the first R300 cards’ shipping date.

We mentioned that multisampling increases the number of z-buffer accesses, but because the R300 is equipped with Z Compression, these accesses do not eat into the GPU’s memory bandwidth very much. ATI quotes 50 – 75% z-sample compression using HyperZ III’s Z Compression in their tech-docs.

While we did not get much time to evaluate the image quality of ATI’s multisampling algorithms, ATI insists that it will be higher quality than NVIDIA’s implementation in situations where transparent textures are used. The best example of this is in the DM-Antalus level in UT2003; the gorgeous grass in the level is simply a high resolution texture that is alpha blended, so you can see what’s behind it. According to ATI, NVIDIA’s multisampling algorithm will merely ignore aliasing within these polygon edges due to their transparency whereas the R300 will not. We will be able to provide screenshots in a couple of weeks, once we get our hands on a board for a longer period of time.

SMOOTHVISION 2.0 supports 2X, 4X and 6X AA modes on the R300.