Nvidia integrated FXAA on a driver based lvl in their recent driver versions, before you would have to use an inject/ hook dll which worked well (I posted a topic on here some time ago) but also had potential to cause issues in some games, aside from visual issues depending on the game as well in regard to cheat detection/ prevention. It also was not maintained beyond a certain point of basic function, so there were no further fixes or improvements made.
Driver based FXAA is the official, easy and save way to achieve FXAA and will be kept up to date and is very useful for games that don't implement any Antialiasing or have performance and/ or compatibility issues with more traditional ways of AA and/ or enforcing them.
FXAA itself was worked on by one of Nvidia's guys (Timothy Lottes) and made available for every dev or everyone really to use for free, no matter if ATI or whatever hardware was used. Although driver based FXAA is awesome, ingame implementations are preferable if done right (see skyrim for a negative example).
A lot of stuff was going to improve even further in FXAA Version 4 but at that point Nvidia apparently stepped on the breaks, realizing that kind of stuff had monetary value and also was becoming dangerously good in comparison to AA methods they wanted to work on with Timothy Lottes exclusively for their new hardware (which later became TXAA and was apparently partly based on results of work on FXAA4). Ultimately Version 4 was delayed and it's future is still uncertain today.
Another interesting point is now that with the new Warband updates shader files have been made customizable it might be possible to implement FXAA there.
Timothy Lottes said...
"might be possible to quickly prototype FXAA in a GL or DX11 game which provides editable shaders. You would need to find a late-in the-frame post processing shader which samples each pixel once, then run FXAA to "sample the pixel" with FXAA_GREEN_AS_LUMA and use in-pixel-shader texture size queries to produce the required inputs to FXAA (so DX9 is likely out). This would NOT be the correct way to integrate FXAA, but might work."
I don't know if anybody is working on that tho. That way it could be better adapted to the game and even higher quality could be achieved, although on the other hand, since the last code of FXAA openly available is FXAA 3.11 the now available Nvidia driver based FXAA which is closed source most likely has some further developement and advantages over FXAA 3.11 and unless Version 4 sees the light of day one would not have access to those improvements of course.
http://www.geforce.com/whats-new/articles/nvidia-geforce-301.24-beta-drivers-released/http://timothylottes.blogspot.de/2012/03/unofficial-txaa-info.html
