Custom Filter AA

One feature ATI originally introduced in R600 was custom filter AA. We didn’t look at the feature in our initial R600 launch story, as drivers with custom filter AA support weren’t available at the time, but we wanted to take a brief look at the technology as it is implemented on the Radeon HD 4800s.

Normally when AA is applied, a box filter is used. With newer ATI drivers however, end users can run custom ATI filters to deliver improved AA quality over the traditional box filter. In particular we’re using ATI’s edge detect filter, which harnesses the GPU’s shaders to analyze the image and apply additional filtering on all the edges. According to ATI, the edge detect filter is capable of delivering AA that is similar to the image quality achieved if three times the number of samples had been taken using a conventional box filter. In laymen’s terms, if 4xAA is enabled with the edge detect filter turned on, the AA quality would be the equivalent of 12xAA without the filter, and 8xAA with edge detect delivers visuals equivalent to 24xAA. We’ve provided screenshots that illustrate this:

4xAA	4xAA with edge detect

8xAA	8xAA with edge detect

4xAA



4xAA with edge detect (12xAA)



8xAA



8xAA with edge detect (24xAA)


As you can see in the 400% zoomed screenshots, the edges of the power cord have quite a few jaggies under conventional 4xMSAA, and to a lesser extent with 8xAA. The jaggies are considerably smoother once the edge detect filter is applied though, you can really see this in the final screenshot, where 8xAA with edge detect is used to produce “24x”AA.