lower framerate input lag questions

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tygeezy
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lower framerate input lag questions

Post by tygeezy » 14 Oct 2019, 15:28

I'm curious about input lag with lower framerates. Most of the tests I see online are showing framerates with the lowest framerate tested being 60 fps. I've noticed that input lag caused from your display seems to be lower the higher the refresh rate is. This is going by rtings testing. If you are playing at a lower framerate and you have a variable refresh rate monitor since the refresh rate is matching the frame rate is your tv or monitor causing more input lag on that part of the chain? Would 35 fps vsync off on a 120 hz monitor have significantly lower input lag than 35 fps synced assuming we also aren't gpu bottlenecked (99 % gpu usage)?

I have a question about rtss as well. It's constantly mentioned that using rtss to cap your framerate over in game capper will induce a 1 frame input lag penalty. So at a lower framerate, say 35 fps, are we talking about a 29 ms input lag penalty for using rtss instead of in game cap?

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jorimt
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Re: lower framerate input lag questions

Post by jorimt » 14 Oct 2019, 17:09

Long story short, the lower the framerate, the higher the frametime, which means the longer each frame takes to render. Also, the lower the refresh rate, the slower the scanout speed, which means the slower each frame will be delivered (and the less frames will be delivered per second).

The monitor could have 0ms input lag, and that same monitor at a physical refresh rate of 60Hz is still going to have higher input lag than, say, if it were running at 144Hz, for the above aforementioned reasons.

As for RTSS, another long story short, it's basically input lag neutral. It doesn't "add" 1 frame over an in-game limiter, the in-game limiter can (at least a proper one) "reduce" input lag further than otherwise possible.
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tygeezy
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Re: lower framerate input lag questions

Post by tygeezy » 14 Oct 2019, 17:15

jorimt wrote:Long story short, the lower the framerate, the higher the frametime, which means the longer each frame takes to render. Also, the lower the refresh rate, the slower the scanout speed, which means the slower each frame will be delivered.

The monitor could have 0ms input lag, and that same monitor at a physical refresh rate of 60Hz is still going to have higher input lag than, say, 144Hz for the above aforementioned reasons.
Okay, so I have a scenario for you. at 1440 p 120 hz my tv VRR range is 48-120 hz. I've found out that I can really only run borderlands 3 at 52 fps if I want to stay at below 97 % usage 98 % of the time. Would I actually have lower input lag by capping my framerate at 48 fps to induce low frame rate compensation which will take my frame rate and double the refresh rate to 96 hz than keeping it at 52 fps/52 hz?

Edit:
Render time plus scan out would be 38.46 ms with 52 fps/52 hz while 48 fps/96 hz would be 31.23 ms.

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jorimt
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Re: lower framerate input lag questions

Post by jorimt » 14 Oct 2019, 17:23

tygeezy wrote:Would I actually have lower input lag by capping my framerate at 48 fps to induce low frame rate compensation which will take my frame rate and double the refresh rate to 96 hz than keeping it at 52 fps/52 hz?
No, because VRR doesn't (and can't) actually change the physical refresh rate of the panel (which is fixed to whatever maximum you currently have it set to), it, instead, actually "adjusts" the refresh rate by controlling how many times the scanout (the process that physically draws each frame, pixel by pixel, left to right, top to bottom on-screen) is repeated per second, not the delivery “speed” of each individual scanout cycle.

This “speed” is, instead, only dictated by the current physical refresh rate; at 120Hz VRR, the delivery speed of each frame is 8.3ms, regardless of framerate/render time.

So the only way input lag could be further reduced in this scenario, is if imposing a lower FPS limit would prevent your system from being GPU-bound (which you've already covered).
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tygeezy
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Re: lower framerate input lag questions

Post by tygeezy » 14 Oct 2019, 17:30

jorimt wrote:
tygeezy wrote:Would I actually have lower input lag by capping my framerate at 48 fps to induce low frame rate compensation which will take my frame rate and double the refresh rate to 96 hz than keeping it at 52 fps/52 hz?
No, because VRR doesn't (and can't) actually change the physical refresh rate of the panel (which is fixed to whatever maximum you currently have it set to), it, instead, actually "adjusts" the refresh rate by controlling how many times the scanout (the process that physically draws each frame, pixel by pixel, left to right, top to bottom on-screen) is repeated per second, not the delivery “speed” of each individual scanout cycle.

This “speed” is dictated by the current physical refresh rate; at 120Hz VRR, the delivery speed of each frame is 8.3ms, regardless of framerate/render time.

So the only way input lag could be further reduced in this scenario, is if imposing a lower FPS limit would prevent your system from being GPU-bound.
Did you see my edit with the frametime plus scan out? Wouldn't the physical refresh rate actually be 52 hz since it's matching the framerate at 52 and not the fixed refresh rate of 120? So that would mean 19.23 ms for scanout and not 8.3 ms.

Edit: I read it a couple more itmes and now it makes sense. Even at 52 fps where it displays 52 hz the delivery speed will be that of the maximum refresh rate of 120 hz which is 8.3 ms.

Thanks for clearing this up.

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Re: lower framerate input lag questions

Post by jorimt » 14 Oct 2019, 17:55

^ EDIT: I just saw your latest above edit (good to hear), but the below should clarify this even further.
tygeezy wrote:Did you see my edit with the frametime plus scan out? Wouldn't the physical refresh rate actually be 52 hz since it's matching the framerate at 52 and not the fixed refresh rate of 120? So that would mean 19.23 ms for scanout and not 8.3 ms.
I did see your edit, and you're complicating things bit.

With VRR, "Hz" is fixed to the currently set maximum physical refresh rate of your display. So if your physical refresh rate is 120Hz, regardless of framerate, regardless of VRR operation, your maximum refresh rate is still 120Hz, and frames are still scanning in at 8.3ms per. This is a fixed number that VRR (and the framerate) can't alter.

Again, what VRR can control is how many times the scanout cycle is repeated per second.

It uses this to (rapidly) time (or "pad") the span between individual scanout cycles; if the incoming frame has, say, a frametime of 20ms (50 FPS), at 120Hz, it will complete the frame scan (aka delivery) in 8.3ms (remember, at this point, the frame is already fully rendered and ready to be scanned in), and then prevent the next scanout cycle from starting until the next frame is fully rendered and ready to be scanned in (by whatever frametime that frame ends up being), repeat.

That is, really, quite literally it; With VRR, the display's delivery time is 1:1 matched to the frametime of each rendered frame in sequence, and in real-time. Hope that's a little clearer.
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tygeezy
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Re: lower framerate input lag questions

Post by tygeezy » 14 Oct 2019, 18:19

jorimt wrote:^ EDIT: I just saw your latest above edit (good to hear), but the below should clarify this even further.
tygeezy wrote:Did you see my edit with the frametime plus scan out? Wouldn't the physical refresh rate actually be 52 hz since it's matching the framerate at 52 and not the fixed refresh rate of 120? So that would mean 19.23 ms for scanout and not 8.3 ms.
I did see your edit, and you're complicating things bit.

With VRR, "Hz" is fixed to the currently set maximum physical refresh rate of your display. So if your physical refresh rate is 120Hz, regardless of framerate, regardless of VRR operation, your maximum refresh rate is still 120Hz, and frames are still scanning in at 8.3ms per. This is a fixed number that VRR (and the framerate) can't alter.

Again, what VRR can control is how many times the scanout cycle is repeated per second.

It uses this to (rapidly) time (or "pad") the span between individual scanout cycles; if the incoming frame has, say, a frametime of 50ms (50 FPS), at 120Hz, it will complete the frame scan (aka delivery) in 8.3ms (remember, at this point, the frame is already fully rendered and ready to be scanned in), and then prevent the next scanout cycle from starting until the next frame is fully rendered and ready to be scanned in (by whatever frametime that frame ends up being), repeat.

That is, really, quite literally it; With VRR, the display's delivery time is 1:1 matched to the frametime of each rendered frame in sequence, and in real-time. Hope that's a little clearer.
Thanks, I re-read it a few times and it made sense, but wouldn't a frametime of 50 ms actually be 20 ms? 50 fps should be 20 ms I thought.

Is there some sort of lag chain calculation to do a rough approximation of your input lag with your monitor or tv? The input lag of my tv is 6.6 MS at 120 hz using freesync (according to rtings) here's a link to these numbers. It's under the input section.

https://www.rtings.com/tv/reviews/samsu ... rison_2485

Would it be something like frametime + scanout + tv/monitors added latency + mouse/keyboard polling rate?

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jorimt
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Re: lower framerate input lag questions

Post by jorimt » 14 Oct 2019, 19:02

tygeezy wrote:wouldn't a frametime of 50 ms actually be 20 ms? 50 fps should be 20 ms I thought.
Thanks, yes, it was a typo. Correct, 20ms frametime = 50 FPS.
tygeezy wrote:Is there some sort of lag chain calculation to do a rough approximation of your input lag with your monitor or tv?
Without measurement tools? Not that I know of (maybe someone else here does).
Author: Blur Busters "G-SYNC 101" Series

Display: Acer Predator XB271HU OS: Windows 10 Pro MB: ASUS ROG Maximus X Hero CPU: i7-8700k GPU: EVGA GTX 1080 Ti FTW3 RAM: 32GB G.SKILL TridentZ @3200MHz

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