Hmm, when i am using FXAA i get a black screen except for the rear-view mirror? This happens on a GTX 570, 2560x1440 res.
I've always got the same sort of thing when I set it in-game. This build, I set it in the launcher and the whole screen was black when the game launched and it wasn't responding according to Task Manager. Tried several times.
Unless I am mistaken (and I might be), I think HDR has to be enabled for FXAA to work without black screens. At least that is the only way I can get it to work on my system.
