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How does limiting 2 fps below refresh rate help mouse lag?

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How does limiting 2 fps below refresh rate help mouse lag?

Postby Dustmuffins » 03 Jan 2014, 10:47

This is something I've noticed a lot of twitch shooter gamers do. They will use Vsync with triple buffering and limit their FPS at 2 below refresh rate. They'll limit their fps @ 58 for a 60 hz monitor, or 118 for a 120hz monitor.

I can notice the difference when I limit my fps at 142fps on my 144hz monitor. It will actually feel like I'm running double buffering @ 144 fps without the nasty side effect of having FPS cut in half if it drops below 144.

How does this work? I assume it does something to keep one less frame in the frame buffer?
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Re: How does limiting 2 fps below refresh rate help mouse la

Postby Black Octagon » 03 Jan 2014, 12:18

They're preventing frames from being rendered too quickly for the next screen refresh and this having to wait for the next one. It's indeed to avoid a source of extra latency, albeit not 'mouse lag' as such

Sent from dumbphone (pls excuse typos and dumbness)
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Re: How does limiting 2 fps below refresh rate help mouse la

Postby Chief Blur Buster » 03 Jan 2014, 20:58

Dustmuffins wrote:This is something I've noticed a lot of twitch shooter gamers do. They will use Vsync with triple buffering and limit their FPS at 2 below refresh rate. They'll limit their fps @ 58 for a 60 hz monitor, or 118 for a 120hz monitor.

I can notice the difference when I limit my fps at 142fps on my 144hz monitor. It will actually feel like I'm running double buffering @ 144 fps without the nasty side effect of having FPS cut in half if it drops below 144.
Setting a frame cap slightly below the monitor refresh rate, have several beneficial reasons. However, sometimes it's an advice so often repeated, with the wrong explanations.

There are several (sometimes unrelated) reasons:

(1) Prevent a situation where the game is waiting for the monitor. Instead, the monitor waits for the game. Game-based framerate capping is more effective in some modes, such as GSYNC, and some games' triple buffering implementations. However, VSYNC OFF won't create this issue (however, there's other reasons there are still benefits; read blow).

(2) Sometimes you want to be about 2fps or 3fps away, rather than a close match or 1fps away.
(VSYNC OFF is never perfectly matched, otherwise it's called VSYNC ON -- synchronizsed to vertical blanking interval.)
During VSYNC OFF, if your refreshrate is too close to the framerate, you can have more noticeable tearing (rolling tearing effect) and amplified stutters from the harmonic between framerate and refreshrate.

(3) Some game engines bog down if you let them render uncapped. Input lag increases in this situation. (Example: The rendering thread starves the input-read thread). A frame cap fixes this.

(4) Motion clarity looks best at framerates matching refresh rate. But sometimes you don't want a near-exact match during VSYNC OFF. So a compromise is a reasonable close match. Even 118fps@120Hz (smoother motion) can look better than 77fps@120Hz (coarser motion)
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Re: How does limiting 2 fps below refresh rate help mouse la

Postby nimbulan » 04 Jan 2014, 04:41

My understanding of this is that it is related to pre-rendered frames. Basically with certain game/hardware/setting combinations, when the game is framerate capped by vsync, it will continue rendering more frames and queue them up rather than waiting for the display like it's supposed to. This obviously causes input lag. When the framerate is manually capped lower, it forces the game to obey that framerate rather than rendering ahead. The size of the frame queue is configurable in nVidia's drivers as well and it shouldn't be an issue when set to 1.

Again, this is just my understanding of the issue though I have never encountered it myself.
Chief Blur Buster wrote:(2) Sometimes you want to be about 2fps or 3fps away, rather than a close match or 1fps away.
(VSYNC OFF is never perfectly matched, otherwise it's called VSYNC ON -- synchronizsed to vertical blanking interval.)
During VSYNC OFF, if your refreshrate is too close to the framerate, you can have more noticeable tearing (rolling tearing effect) and amplified stutters from the harmonic between framerate and refreshrate.

Having a framerate slightly different from the refresh rate sounds like it would make rolling tearing worse, not better. Does this actually work?
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Re: How does limiting 2 fps below refresh rate help mouse la

Postby masterotaku » 04 Jan 2014, 07:18

nimbulan wrote:Having a framerate slightly different from the refresh rate sounds like it would make rolling tearing worse, not better. Does this actually work?

For fps capping, I'm using MSI Aterburner + RTSS.

With vsync off:
Playing at 75Hz, if I cap the fps to 74, the image tears once per second, from top to bottom. It's easily noticeable.
If I set cap to 75, there's usually a horizontal tearline appearing randomly. It sometimes synchronizes perfectly for a few seconds, and then the line appears for a while.

With vsync on:
I haven't tried it for a lot of time, but I think that capping the fps below the Hz of the monitor, caused a stutter per second, instead of tearing.

Personally, I prefer to set the fps=Hz, be it with vsync or not. Setting the prerendered frames to 1 in the nvidia settings helps A LOT when using vsync. The mouse feels much more responsive, being less frames behind.
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Re: How does limiting 2 fps below refresh rate help mouse la

Postby Vega » 04 Jan 2014, 13:40

Yes, setting pre-rendered frames to 1, games properly using triple buffering with V Sync = ON, and using a single GPU vs 2 or more all reduce input lag.

If properly setup and with a real fast machine, V Sync on isn't that bad.
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Re: How does limiting 2 fps below refresh rate help mouse la

Postby nimbulan » 04 Jan 2014, 15:20

Vega wrote:Yes, setting pre-rendered frames to 1, games properly using triple buffering with V Sync = ON, and using a single GPU vs 2 or more all reduce input lag.

If properly setup and with a real fast machine, V Sync on isn't that bad.

Yeah I've been using triple buffer vsync in all my games for many years and I rarely encounter significant input lag since I use pre-rendered frames = 1. Far Cry 2 was terrible about that and Skyrim definitely has more than most games, but I can't think of any other notable games at the moment. I noticed a MUCH larger decrease in input lag from upgrading my monitor to a VG248QE from an older 4 ms display than I ever did from turning vsync off.
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Re: How does limiting 2 fps below refresh rate help mouse la

Postby Chief Blur Buster » 04 Jan 2014, 17:34

nimbulan wrote:Yeah I've been using triple buffer vsync in all my games for many years and I rarely encounter significant input lag since I use pre-rendered frames = 1. Far Cry 2 was terrible about that and Skyrim definitely has more than most games, but I can't think of any other notable games at the moment. I noticed a MUCH larger decrease in input lag from upgrading my monitor to a VG248QE from an older 4 ms display than I ever did from turning vsync off.
This is quite a very noteworthy consideration. VSYNC ON 120fps@120Hz (consistent) can actually have less average total-chain input lag than VSYNC OFF ~60fps@60Hz.

Also, strobing and LightBoost looks much better with VSYNC ON if there's never any framerate slowdowns (perfect framerate=stroberate) than with VSYNC OFF. It's a compromise between latency and motion fluidity, as VSYNC OFF often has microstutters that interfere with motion blur reduction.

Interesting Video Game History Note: Although VSYNC ON has a bad reputation for input lag, it is noteworthy that VSYNC ON is what what used in many old pre-3D games. They synchronized motion to the refresh. For example, Super Mario Brothers on Nintendo was an always VSYNC ON game, and we never complained about input lag with Super Mario Brothers! Buttery smooth scrolling effect. Legacy games like those are mostly unbuffered (pre-drawn sprites and tiles). When we began to render 3D graphics, we needed buffers out of necessity (which added lag).
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Re: How does limiting 2 fps below refresh rate help mouse la

Postby RealNC » 04 Jan 2014, 17:54

nimbulan wrote:Yeah I've been using triple buffer vsync in all my games for many years and I rarely encounter significant input lag since I use pre-rendered frames = 1. Far Cry 2 was terrible about that and Skyrim definitely has more than most games

Skyrim is extremely weird in that regard. In 3rd person view, there's almost zero input lag. Mouse feels very snappy. Switch to 1st person, the mouse feels very "floaty". Try to explain *that* one.
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Re: How does limiting 2 fps below refresh rate help mouse la

Postby Chief Blur Buster » 04 Jan 2014, 17:55

RealNC wrote:Skyrim is extremely weird in that regard. In 3rd person view, there's almost zero input lag. Mouse feels very snappy. Switch to 1st person, the mouse feels very "floaty". Try to explain *that* one.

It's very possible it is game engine coding; the different view modes may have different timings of rendering relative to input reads (keyboard/mouse).
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