Based on a Half-Life 2 forum thread called For anyone with questions as to why AA wasn't turned on in the videos...
there's a story up about anti-aliasing support on (thanks Tiscali Games
and X-bit labs
quoting Valve's Gary McTaggart (second-hand) as saying: "At the present,
you wouldn't want to run the game with AA enabled. . you'd see some pretty bad
artifacts. We may come up with some way to work around the hardware problems in
the future though so that you can use AA." A couple of further quotes offer
outlook on to what degree this can be addressed on current hardware:
aren't likely to fix the problem, with the exception of the ATI 9500-9800.
There's hope there for being able to use FSAA properly. You are out of luck on
NVidia unless either NVidia or us come up with some clever way of solving this
The problem is specifically with multisample antialiasing on any card that uses
multisample antialiasing. If you use supersampling you should be fine. Actually,
this problem has existed for any game that uses light maps that are packed into
a subrect (like Quake 3, etc). You will be able to turn FSAA if you like (in the
control panel for your video card), but it's likely to have artifacts on
triangle boundaries. We don't recommend turning multisample antialiasing on for
cards that don't have centroid sampling. . .your mileage may vary.