[osg-users] Enabling Vsync gives dramatic increase in latency

[Jan Ciger]

On Fri, Apr 17, 2015 at 6:10 PM, Björn Blissing <bjorn.blissing at vti.se>
wrote:

>
> But my main point still stands; it is possible to record latencies that
> are close to the scanout time of the screen with VSync Off (albeit for very
> simple rendering).
>

Yes, but not consistently for every frame. Assuming that each frame you are
drawing takes the same time, you need "to get lucky" for that to happen and
then the next frame is going to have a much higher latency. When you do get
lucky, the next frame buffer swap will happen slightly earlier now relative
to the start of the scanout, because you didn't start drawing after the
vsync event but earlier. This will keep going on for several frames,
shifting the buffer swap point earlier and earlier relative to the scanout
cycle until you, again, hit that sweet spot where you measure the low
latency. Your measurement shows this "beating" between the scanout/display
update frequency and your buffer swaps (when you are starting the timer, I
presume) quite nicely.

The best you can do from the latency point of view is to synchronize to the
scanout using the fences. Then you will have always at most one frame
latency (the GPU is scanning out the previous frame from the front buffer
while you are drawing the current one in the back buffer) if you can fit
your drawing calls within the duration of a single frame.

Don't get confused by the "late latching" tricks that Oculus is promoting.
That has nothing to do with display latency but user input/tracking one.
There they incorporate the tracking data at the very end of the rendering
process just before the buffer swap, achieving very low apparent input
latency relative to when the frame is going to be scanned out. But the
display latency (the time from buffer swap to pixels changing on the
screen) is unaffected by that.
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