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Question about low persistence / G-Sync

Level 2
I was wondering a bit about the G-Sync / Free-sync monitors that are coming out. The ones that sync the display refresh rate with the games framerate output (is that even right? I'm not sure that's the exact phrasing).

Anyway, is Oculus compatible with that technology? Does low-persistency already cover the same issues, from a different perspective? Are they mutually exclusive?

I ask because, it seems to me like they suit different approaches. Oculus is all about pushing for the highest possible framerates, while G-sync apparently fits the opposite, allowing for smoother images even when framerates are lower. Which would be a godsend for performance savings when you render games at stereoscopic 3D.

So, any info on that? Thanks.

Level 15
No, they are not the same thing and are currently not compatible.

Low persistence pertains to how long a frame is displayed on the screen. It's basically just showing the image for a fraction of a second, and then showing black for the remainder of the refresh cycle. However, it is still updating at a constant rate (on DK2 it's 75Hz). This has the result of making the image appear more clear, as it reduces perceived motion blur.

With G-Sync/Free-Sync, the image is still updated as "sample and hold" like a normal display. Meaning the image is grabbed from the video card and shown on the screen until the next frame is ready. The difference is that the next frame can be shown at any point (up until the refresh max) and not just during the regular V-Blank point. Normally if a frame took longer than the refresh (for example 17ms on a 60Hz display) it would have to wait until the next refresh point, effectively dropping the refresh to 30Hz. With G-Sync/FreeSync, this is no longer an issue, as the display can update whenever the GPU is ready. This results in a smoother experience, especially at lower framerates (like, say, 45fps, which would normally be choppy).

I guess there may be clever ways to combine the two, but you would have to deal with variations in brightness. There are likely other technical hurdles but it would be interesting to think about.
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Level 2
Thanks cybereality. 🙂

From your answer, I assume at the moment there is no active research going into incorporating g / free-sync into Oculus then? 😕

I hope you guys consider this in the future. When you think about it, the entry barrier is very high right now. Guess it is not a problem for the enthusiasts who will buy the OR at launch and are willing to spend lots on highest-end rigs to power their VR experience. However it will be necessary to lower that barrier for massive adoption, and this could be one of the solutions to improve efficiency and provide good performance on less expensive systems (mobile, perhaps?).


Level 7

Oculus CTO, John Carmack was discussing this more than a year ago. He also hoped it would be somehow possible, but if we see nothing after a year, and this would give huge benefits in VR, then I'm afraid we should forget about it. Maybe when we'll get to the point where we have 180Hz HMDs, but if 1,5 year has passed and no 144Hz monitor that combines the two - then I think it's not going to happen in this decade. If ever.
Not an Oculus hater, but not a fan anymore. Still lots of respect for the team-Carmack, Abrash. Oculus is driven by big corporation principles now. That brings painful effects already, more to come in the future. This is not the Oculus I once cheered for.