Why is 4xSS plus 4xMS equal to 16xAA? Because each supersample point contains 4 multisample points giving us 4 times the multisample points. The other advantage is that SSAA applies to the entire scene, so we get 4xSSAA applied to parts of the scene that would see no benefit from multisampling (the interior of polygons and textures).
This mode will not be a simple combination of two scenes rendered with the current 8xAA, but rather each card will render 4xSS + 4xMS. For alternate and split frame rendering, each card will be doing full 16xAA. This may also give us a glimpse into the future as each generation graphics cards continue to increase in power. Doing full 16xAA on each card means we could see this order of AA running on a single card in a generation or so.
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