In the 3D accelerators of today there are a few different approaches to anti-aliasing and we're going to have a look at two of them. First out is polygon edge anti-aliasing:

Polygon edge anti-aliasing is what we commonly see when anti-aliasing is switched on in any given Direct3D title. Accelerators such as the RIVA TNT2 support this feature. When using PEAA the accelerator or the driver or the application determines which triangle edges to anti-alias and then renders these edges again with a slight offset and lower opacity to produce a smoothing effect. The end result is a smoother image and no ugly jagged polygon edges.

While this feature is widely supported today, performance when anti-aliasing is enabled is downright pathetic, and if not, the results are. For example I've tried Rollcage on my TNT with anti-aliasing and the performance drop was amazing. Running speed is about 60 fps at 800 x 600, which turns into about 10 fps, 15 tops, at the same resolution with anti-aliasing enabled. Needless to say the game was hardly playable, but it did look quite nice (2 x 2 AA was set in the drivers not the standard 2 x 1 which doesn't look too pleasing but offers better performance).