V-Sync - Problematic in Practice

To those who have sent performance info: thank you, but you probably won't hear for me for a week. I'm up to my eyeballs in reports and it's going to take a while to get through them.

I finally found the code that allows X-Plane to turn off V-Sync. This should help nVidia users who are having framerate problems.

The basic idea is this:

• X-Plane tells the graphics card to draw a lot of stuff.
• The video card accumulates this "todo" list and works on it while X-Plane runs.
• X-Plane indicates that the entire frame to be seen is done and tells the card to show the results.
• Eventually some time later the card finishes the todo list and then shows it to the user.

V-Sync relates to the question of when this last step happens. When V-Sync (vertical sync) is off, the card shows the results as soon as it is done drawing.

But when V-Sync is on, when the card finishes drawing the world, it then waits until the monitor is done drawing its frame, and then shows the results. Without V-Sync we can have a situation where the top half of the monitor is showing a new frame and the bottom half is showing an old frame. (This is called "tearing".)

So normally V-Sync is good because it prevents tearing. But the problem with V-Sync is that a frame can only be shown when the monitor refreshes. The video card has to wait until this happens, and this slows our framerate down.

In particular, most users have their monitors set to 60 hz. If X-Plane can only produce frames at 50 hz, the video card will have to further slow the framerate down to30 hz (one x-plane frame for every two monitor refreshes). If X-Plane falls below 30 hz, we end up with 20 hz (one X-Plane frame for three monitor refreshes), and if X-Plane goes below 20 hz, we would clamp at 15.

So when monitor refresh is on, there can be large framerate hits for small losses of performance in the sim, and a real risk of getting locked around 20 fps.

(The minimum framerate in X-Plane is intentionally set to 19 so that we won't fog up if the video card clamps us at 20 fps.)

So when beta 11 comes out, you may get some framerate back if you haven't already hacked your graphics card's control panel settings. If you still want v-sync, you can always set it this way in the control panel. But most users I've talked to are happy to have it off.

In an only vaguely related note, one of the reasons to have high frame rate is to have a smooth flight model. But Austin has now put a new setting in the operations-and-warnings dialog box: you can pick how many times per graphics frame the physics run. The normal ratio is 1:1, but for fighter and acrobatic pilots, you might find that you can get a nice feel at lower fps (20-30) by setting a higher ratio.