Mouse lag?

[r00k]

I've added a workaround for mouse lag with vsync on, but wondered what's the true problem with vsync and mouse input lag.

qboolean Host_FilterTime (double time)
{
double fps;
extern cvar_t vid_refreshrate;

realtime += time;

if (sv.active)
fps = max(10, host_maxfps.value);//QuakeC requires at least 10fps and no more than 72fps(originally) for physics.
else
fps = max(10, cl_maxfps.value);

if (vid_vsync.value)//Workaround for mouse lag issues...
fps = min((fps - 0.1), (vid_refreshrate.value - 0.1));

if (!cls.capturedemo && !cls.timedemo && ((realtime - oldrealtime) < (1 / fps)))
{
return false;
}
...

Is it because vsync ON sustains all input (on the hardware level), and the condition above passes true, thus drawing the frame++, reading/clearing the mouse buffer?

[mh]

So far as I understand it, by default Windows samples the mouse 40 times per second, which means that it will go out of sync with each pass through the main loop (unless refresh rate is a multiple of 40, of course).

I believe there is an API call to change the sampling rate, but I've never really gone looking for it.