*Please note that if your mouse is lagging even with VSync turned off, then there is another issue going on and this doesn't really apply to you.*
Does tearing (http://en.wikipedia.org/wiki/Tearing) infuriate you? Want to use VSync to stop it, but don't want to suffer the mouse lag it causes? I might have figured out the cure for this dilemma.
I was in this boat for about a week after the demo came out. I was extremely unhappy to have a $1000+ PC specifically to play UT3 only to be forced to deal with horrendous tearing that made the game very unpleasant to watch. I was in a bind: play with tearing with no mouse lag or play without tearing and not be able to hit sh*t. Obviously I had to pick the latter if I wanted to be competitive, but I was determined to find a way to have my cake and eat it, too.
First off, the full version of UT3 may have a "ReduceMouseLag" option which could make the solution that follows irrelevent. I could play with VSync ON in UT2004 with ReduceMouseLag=True, and I didn't notice any mouse lag. But, if not.....
I spent probably a dozen hours dedicated to researching this problem and coming up with the solution. Nothing in my research fixed the problem by itself, but it does factor in the final solution.
I have an nVidia GeForce 8800 GTS 640MB. I don't know if this will work for ATi folks or not.
So now I will give you the final results of my hours of research/testing:
1) Turn VSync ON.
2) In UTEngine.ini, set "bMaxSmoothFramerate=True"
3) Set the "MaxSmoothFramerate" to a value that is very close to, but not above, your monitor's refresh rate. My monitor is a 20" widescreen with a refresh of 60.1 Hz. So I set "MaxSmoothFramerate=59.90".
4) Download Rivatuner and install it. http://www.guru3d.com/index.php?page=rivatuner
5) Find the "Pre-Render frame limit" option in Rivatuner and set it to something lower than 3. Don't use 0 though.....doing that caused UT3 to crash my PC immediately. (I'm actually not sure this is completely necessary, but I had already done it before finding the complete solution, so I just left it as it was)
6) Go to the "Tools" folder wherever you installed Rivatuner and open D3DOverriider.exe. Add UT3Demo.exe to the list and make sure all the checkboxes to force Triple Buffering are checked.
7) Now load up the UT3 demo and see how it goes.
For me, it eliminated the tearing and diminished mouse lag to the point that I can't notice it. I can't say I'm 100% sure that there is absolutely zero mouse lag, but it certainly is not noticable by me.
There were suggestions in my research about turning off Mouse Smoothing, but this didn't work for me. If VSync was enabled, the mouse lagged whether I had mouse smoothing on or not. You can try turning off mouse smoothing in addition to these other measures if you still notice lag, and see if it helps.
The triple buffering helps the situations in which your fps drops below the refresh rate. Normally, when VSync is enabled, your framerate will try to drop more dramatically because of the "double buffering" phenomenon. Assuming a refresh rate of 60Hz, triple buffering would make it so that your framerate doesn't drop to 30 fps just because your system can't quite render 60 fps. Without triple buffering, it wouldn't be possible to get a framerate of 50 fps for example. More info here: http://www.gamecritics.com/triple-bu...mance-for-free
You could leave VSync off and just use the MaxSmoothFramerate option to cap the framerate to your refresh, but this will not eliminate tearing. Tearing will be reduced (it won't be as bad as if you're getting 100 fps with a refresh of 60 fps), but not eliminated.
I can't explain why the mouse lags when you just enable VSync without capping off the framerate in the .ini file. I guess it is something to do with the combination of VSync converting the framerate down to the refresh rate. When the system isn't even trying to render more than the refresh rate, it doesn't have to do this. Maybe that's the reason.....I just know it worked.
Anyway, I hope this helps you like it helped me. It makes for a much more fluid gaming experience. If you try it, then make sure to post here and let us know if it worked for you.