There is no sure fire way to detect exactly when VBlank will occur because Windows doesn't expose a VBlank interrupt, and displa […]
There is no sure fire way to detect exactly when VBlank will occur because Windows doesn't expose a VBlank interrupt, and displays/GPU aren't necessarily required to generate one anyway (in addition, 'current scanline' information as given by e.g. IDirect3DDevice9::GetRasterStatus may not be accurate). As a result, programs generally poll for VBlank or rely on Direct3D/OpenGL to do it for them.
Programs present video frames during VBlank to avoid tearing, since the monitor will happily switch to the new frame mid-draw. With the compositor in Windows Vista and later versions of Windows, these programs will still detect VBlank and only present frames during it, as they think they are presenting video frames directly, when in reality the video frames are feeding into the compositor first. Frames sent to the compositor (from any running programs on the PC) will be queued up by the compositor, and merged together to be swapped/copied into place during VBlank.
Problems that can occur with this system:
1) A program polling for VBlank may miss composition. This will cause the frame to be queued up for the next composition, meaning the previous frame will be shown twice as long.
2) Worse, the next frame may not miss composition, and end up overwriting the previously queued up frame - so you end up with a duplicate frame followed by a skipped frame.
3) The program's VSync implementation may naturally fail to detect VBlank (which has only a short duration), causing it to wait until the next VBlank and risk problems 1 and/or 2.
4) These problems may even combine to generate a 'perfect storm' of duplicate and/or missed frames.
As you can see, this polling setup is not ideal, and far worse when a compositor is present. There are multiple problems that can cause a new video frame to fail to be displayed, causing a previous frame to be displayed for longer than intended and potentially skipping the new frame altogether!
The solution is to work with the compositor instead of against it. Present a new video frame immediately and afterward, call a command that will block until the compositor has completed it's task (DwmFlush). This will ensure that at most 1 new video frame is presented to the compositor between each VBlank period. As long as the compositor is active, you also won't have to worry about polling for VBlank yourself anymore.
Of course, since Windows XP doesn't have a compositor and not all users on Vista and 7 run with the compositor enabled, you will need to retain the old method of VSync detection as a fallback. But the changes required will be minor additions to your existing code, so why not handle things this way under Vista and newer?