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

Björn Blissing bjorn.blissing at vti.se
Sat Apr 18 07:30:14 PDT 2015

Jan Ciger wrote:
> I find these techniques to be really fragile kludges and not really
> worth the effort most of the time - the saved 16ms of latency is
> imperceptible for the most people and most LCDs have response times
> (the time it physically takes to change the colour of a pixel) around
> 8ms+ anyway, so pushing down to ridiculously low limits is pointless -
> you won't be able to see it.
> If you really want lower latency, render at a higher frame rate which
> will benefit the user more, giving them much smoother experience.
> However, even there it is a battle with diminishing returns - it is
> rarely useful to go over 60fps and it will cost you tremendously if
> you do - both in the code and in the hw, because you will need a new,
> high speed, monitor.
> Hopefully the GPU vendors will provide us with some more reasonable
> APIs to manage latency in the near future, as the need is now clearly
> visible with the push for low latency VR.

Although very interesting, this discussion regarding minimum latency is a bit of a tangent from my initial questions. Rendering with VSync Off is not normally what you do in a production environment. The interesting case is when you render with VSync enabled. Then I can live with 1-2 frames of (consistent) latency, given that the scene complexity is within what is possible to render within one frame.  But with my starting point (OSG 3.2.1 running with DrawThreadPerContext and NVIDIA default settings) was latencies of 5-6 frames, which is way to much. Especially when you consider that all I rendered was ONE quad. I have a hard time to accept that driver issues causes latencies of these magnitudes. 

Regarding rendering at higher frame rates. I will try to repeat my measurements on a couple of 120 Hz monitors next week. These monitors also support Nvidia G-sync, so I will try that as well. And of course I will measure both the Oculus Rift DK1 and DK2 (the last one both in extended and direct mode).


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