ATi Radeon 4000 Series Anti-Aliasing Investigation

Custom Filter Anti-Aliasing (CFAA)

The 2000 series also gained CFAA, an extension of MSAA, and it’s also supported on the 3000 and 4000 series. CFAA uses MSAA as a base pattern, but also takes additional samples from adjacent pixels.

The advantage of CFAA is that it gains additional quality over regular MSAA without rendering the scene higher, thus saving memory storage and bandwidth. Unfortunately, narrow and wide tent take one and two samples from each adjacent pixel respectively, and this can cause blurring because they’re blending the internals of polygons, not just their edges.

Edge detect corrects this problem by using the shader to detect polygon edges and only sampling adjacent pixels in those situations. This increases the quality of polygon edges without any scene blurring. Additionally, there are more samples sourced from outside the pixel boundary than either narrow or wide tent use.

There are two edge detect modes: 12xAA, which is 4xMSAA + edge detect, and 24xAA, which is 8xMSAA + edge detect.

Unfortunately I’m unable to show you the sample patterns for CFAA; however this technical slide from ATi should give you an idea as to what is happening:

Most games cannot activate these modes directly, but there are two ways you can use them. Firstly, you can force them directly from the control panel by simply using 4xAA or 8xAA along with the edge detect setting. This should work in most games where forced AA works. The other way is to set the control panel’s AA level to application preference, but set the filter to edge detect. Then by setting the game to use 4x or 8x, you should get 12x or 24x respectively in most cases.

Note that it’s important to be using edge detect in the control panel in order to use these modes. If you use either of the tent modes, the scene will experience blurring because the blending won’t be restricted to polygon edges.