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Antialiasing image quality - Alpha transparency
Here's one final AA image quality example, focused on the methods that AMD and Nvidia have devised to handle the tough case of textures with alpha transparency cutouts in them. Nvidia calls its method transparency AA and AMD calls its adaptive AA, but they are fundamentally similar. The scene below, again from Half-Life 2, has two examples of alpha-transparent textures: the leaves on the tree and the letters in the sign. 4X multisampling is enabled in all cases. The top row shows images without transparency AA enabled. The second row shows the lower-quality variants of transparency/adaptive AA from Nvidia and AMD, and the bottom row shows the highest quality option from each.

For what it's worth, I took these screenshots on the Radeon HD 2900 XT with an updated driver that AMD says provides performance improvements in adaptive AA, so I believe this is a new algorithm.

Alpha transparency antialiasing quality w/4X AA
Radeon X1950 XTXGeForce 8800 GTXRadeon HD 2900 XT
Box filter
Radeon HD 2900 XT
Narrow tent filter
Radeon HD 2900 XT
Wide tent filter

Looks like you can get away with the lower quality adaptive AA method on the Radeon HD 2900 XT. If you combine it with a tent filter, the results are pretty good.