is "uncapped" VR a thing

[twistking]

Hello,
i often see people stating that they run VR at "60 - 80 fps" or similar. So they either give a range of fps, or they state fixed fps like 60, which does not relate to the native frefresh rate of any headset i know of.
What's going on here?
Is it technically possible to run VR with a capped or uncapped "arbitrary" framerate (without motion reprojection)? Would this not give a horrible experience with juttering due to frametimes "not matching" the HMD?
Would spatial reprojection even work correctly with unstable framerates?
Thanks.

[some1]

IIRC you can use OpenXR toolkit to limit fps to any value you want. But regardless of that setting, what the game shows is not what the headset displays. The headset works at fixed refresh rate no matter what, it will simply skip or prolong display of each frame to match the refresh cycle.

[j9murphy]

This is correct, "smoothness" is not only based on fps, but on whether or not those frames arrive at the same interval or not. (fps is to some extent  an average over some time interval)  anything over 30fps can be perceived as smooth if the frames arrive at precise intervals (until you add motion - panning your vision introduces motion blur at 30fps).  Essentially what we are trying to do is get frames at regular intervals at a high enough rate not to introduce motion blur. 

[twistking]

7 minutes ago, some1 said:

IIRC you can use OpenXR toolkit to limit fps to any value you want. But regardless of that setting, what the game shows is not what the headset displays. The headset works at fixed refresh rate no matter what, it will simply skip or prolong display of each frame to match the refresh cycle.

thanks for the reply.
ok. i can see that maybe someone would want to cap fps to half of HMD's native Hz on a 120Hz capable HMD, if the person does not want to use motion reprojection, but everything else seems to be a terrible idea: Lower than 60fps must be nauseating and every capped FPS that is not half/quarter of antive HZ must add stutter/judder.
Don't want to shame people that enjoy it that way, i'm just interested.

[twistking]

3 minutes ago, j9murphy said:

This is correct, "smoothness" is not only based on fps, but on whether or not those frames arrive at the same interval or not. [...]

I wonder if a VRR (variable refresh rate) would be a good idea. Sure, frames would be pumped out at irregular intervals, but at least there would be no additional stutter from temporal missmatch. Maybe spatial reprojection could not work correctly with VRR? I could imagine that it needs predictive frametiming...

[some1]

VRR is not easy. To my knowledge, all headsets use backlight strobing to reduce persistence and improve the perception of fluid motion. And it's hard to get the VRR working together with strobing. Few monitors on the market offer such option, and they're not perfect implementations either.

[BIGNEWY]

I am running my G2 at 90hz I dont cap my fps in DCS or NVIDIA control panel and I dont use any motion smoothness. 

Maybe I am just lucky but I dont have any issues, or my tolerance is much higher than others. 

[twistking]

3 minutes ago, BIGNEWY said:

I am running my G2 at 90hz I dont cap my fps in DCS or NVIDIA control panel and I dont use any motion smoothness. 

Maybe I am just lucky but I dont have any issues, or my tolerance is much higher than others. 

That's not what i mean. The G2 runs natively at 90Hz. So you're capped at 90fps no matter what. But that's very good of course, as long as your system can keep up. If your system could not keep up, it should automatically fall back to 45+45(motionRepro): Not ideal but probably still good if you can live with the artifacts from motion reprojection.
I was wondering how "bad" it would be to run with framerates below native HMD refresh without motion reprojection. Either by capping the ingame fps to some arbitary value below HMD refresh or by having fluctuating FPS below HMD refresh due to performance limits and disabled motion reprojection.

25 minutes ago, some1 said:

VRR is not easy. To my knowledge, all headsets use backlight strobing to reduce persistence and improve the perception of fluid motion. And it's hard to get the VRR working together with strobing. Few monitors on the market offer such option, and they're not perfect implementations either.

OK. Thanks. Did not think of that.

[CptBligh]

2 hours ago, twistking said:

 or by having fluctuating FPS below HMD refresh due to performance limits and disabled motion reprojection.

 

I assumed that's what Bignewy meant.  I run my G2 this way and it has never bothered me and I prefer it instead of 45FPS motion repro

[twistking]

Just now, CptBligh said:

I assumed that's what Bignewy meant.  I run my G2 this way and it has never bothered me and I prefer it instead of 45FPS motion repro

i see. so you have forced motionrepro off and just live with the slight jutter if you dip? in this case, spatial reprojection is still active though, correct?

[Phantom711]

Am 27.10.2023 um 15:15 schrieb twistking:

or they state fixed fps like 60, which does not relate to the native frefresh rate of any headset i know of.

Just for your info: HP G2 can run natively at 60Hz. The downside is, that there is some noticable flickering at the edges especially in very bright areas. Nevertheless I recently started to enjoy this mode, because just as you stated, as long as my system can provide enough FPS to stay above that (but would not be capable of staying at/above 90), the experience is nice and smooth.

For the cases where it still drops below, I compensate with MR.

Edited October 28, 2023 by Phantom711

[twistking]

1 hour ago, Phantom711 said:

For the cases where it still drops below, I compensate with MR.

Is the MR switching itself notable? I mean, do you notice everytime it occurs, or is ist that you eventually see those artifacts and therefore realize it must be on?

[Phantom711]

Hard to tell, because there are too many variables, because I just recently upgraded to a 4090 (from 3080ti) and I also just started to try out the 60Hz.

But since MR starts already when approaching the native resolution from above, it should actually not be noticable other than the artifacts, wobble etc…

Edited October 28, 2023 by Phantom711

[Phantom711]

And as for your initial question:

Yes, with MR off you will see stutters as soon as FPS drops below the native refresh rate. But this is not so much noticable as long as you just look mainly forward. As soon as you look at 3 or 9 o‘clock or there is fast lateral movement of another aircraft, it is very noticable. (My point of view)

[Rainmaker]

I run mine at 60 also with a 4090.  I end up around only 60% utilization and about 225W of power draw.  That's running down the middle of Las Vegas, etc.  Love having the overhead to work with.  Never see that side view distortion any longer which is awesome.