[Info] Explanation of FPS vs. Refresh Rate - Overclock.net

Semper Fidelis · 08-12-08

OK, well... first off I have to say that I did not write this. It was written by Arkalius over at HardForum. I am just showing you his article and adding a little of my own insight on the subject.

OK, here is the article.

Quote:

I recently learned that how I thought vsync worked was wrong, and now knowing the way it really does work, I think it would be worthwhile to make sure everyone here understands it.

What is VSync? VSync stands for Vertical Synchronization. The basic idea is that synchronizes your FPS with your monitor's refresh rate. The purpose is to eliminate something called "tearing". I will describe all these things here.

Every CRT monitor has a refresh rate. It's specified in Hz (Hertz, cycles per second). It is the number of times the monitor updates the display per second. Different monitors support different refresh rates at different resolutions. They range from 60Hz at the low end up to 100Hz and higher. Note that this isn't your FPS as your games report it. If your monitor is set at a specific refresh rate, it always updates the screen at that rate, even if nothing on it is changing. On an LCD, things work differently. Pixels on an LCD stay lit until they are told to change; they don't have to be refreshed. However, because of how VGA (and DVI) works, the LCD must still poll the video card at a certain rate for new frames. This is why LCD's still have a "refresh rate" even though they don't actually have to refresh.

I think everyone here understands FPS. It's how many frames the video card can draw per second. Higher is obviously better. However, during a fast paced game, your FPS rarely stays the same all the time. It moves around as the complexity of the image the video card has to draw changes based on what you are seeing. This is where tearing comes in.

Tearing is a phenomenon that gives a disjointed image. The idea is as if you took a photograph of something, then rotated your vew maybe just 1 degree to the left and took a photograph of that, then cut the two pictures in half and taped the top half of one to the bottom half of the other. The images would be similar but there would be a notable difference in the top half from the bottom half. This is what is called tearing on a visual display. It doesn't always have to be cut right in the middle. It can be near the top or the bottom and the separation point can actually move up or down the screen, or seem to jump back and forth between two points.

Why does this happen? Lets take a specific example. Let's say your monitor is set to a refresh rate of 75Hz. You're playing your favorite game and you're getting 100FPS right now. That means that the mointor is updating itself 75 times per second, but the video card is updating the display 100 times per second, that's 33% faster than the mointor. So that means in the time between screen updates, the video card has drawn one frame and a third of another one. That third of the next frame will overwrite the top third of the previous frame and then get drawn on the screen. The video card then finishes the last 2 thirds of that frame, and renders the next 2 thirds of the next frame and then the screen updates again. As you can see this would cause this tearing effect as 2 out of every 3 times the screen updates, either the top third or bottom third is disjointed from the rest of the display. This won't really be noticeable if what is on the screen isn't changing much, but if you're looking around quickly or what not this effect will be very apparant.

Now this is where the common misconception comes in. Some people think that the solution to this problem is to simply create an FPS cap equal to the refresh rate. So long as the video card doesn't go faster than 75 FPS, everything is fine, right? Wrong.

Before I explain why, let me talk about double-buffering. Double-buffering is a technique that mitigates the tearing problem somewhat, but not entirely. Basically you have a frame buffer and a back buffer. Whenever the monitor grabs a frame to refresh with, it pulls it from the frame buffer. The video card draws new frames in the back buffer, then copies it to the frame buffer when it's done. However the copy operation still takes time, so if the monitor refreshes in the middle of the copy operation, it will still have a torn image.

VSync solves this problem by creating a rule that says the back buffer can't copy to the frame buffer until right after the monitor refreshes. With a framerate higher than the refresh rate, this is fine. The back buffer is filled with a frame, the system waits, and after the refresh, the back buffer is copied to the frame buffer and a new frame is drawn in the back buffer, effectively capping your framerate at the refresh rate.

That's all well and good, but now let's look at a different example. Let's say you're playing the sequel to your favorite game, which has better graphics. You're at 75Hz refresh rate still, but now you're only getting 50FPS, 33% slower than the refresh rate. That means every time the monitor updates the screen, the video card draws 2/3 of the next frame. So lets track how this works. The monitor just refreshed, and frame 1 is copied into the frame buffer. 2/3 of frame 2 gets drawn in the back buffer, and the monitor refreshes again. It grabs frame 1 from the frame buffer for the first time. Now the video card finishes the last third of frame 2, but it has to wait, because it can't update until right after a refresh. The monitor refreshes, grabbing frame 1 the second time, and frame 2 is put in the frame buffer. The video card draws 2/3 of frame 3 in the back buffer, and a refresh happens, grabbing frame 2 for the first time. The last third of frame 3 is draw, and again we must wait for the refresh, and when it happens, frame 2 is grabbed for the second time, and frame 3 is copied in. We went through 4 refresh cycles but only 2 frames were drawn. At a refresh rate of 75Hz, that means we'll see 37.5FPS. That's noticeably less than 50FPS which the video card is capable of. This happens because the video card is forced to waste time after finishing a frame in the back buffer as it can't copy it out and it has nowhere else to draw frames.

Essentially this means that with double-buffered VSync, the framerate can only be equal to a discrete set of values equal to Refresh / N where N is some positive integer. That means if you're talking about 60Hz refresh rate, the only framerates you can get are 60, 30, 20, 15, 12, 10, etc etc. You can see the big gap between 60 and 30 there. Any framerate between 60 and 30 your video card would normally put out would get dropped to 30.

Now maybe you can see why people loathe it. Let's go back to the original example. You're playing your favorite game at 75Hz refresh and 100FPS. You turn VSync on, and the game limits you to 75FPS. No problem, right? Fixed the tearing issue, it looks better. You get to an area that's particularly graphically intensive, an area that would drop your FPS down to about 60 without VSync. Now your card cannot do the 75FPS it was doing before, and since VSync is on, it has to do the next highest one on the list, which is 37.5FPS. So now your game which was running at 75FPS just halved it's framerate to 37.5 instantly. Whether or not you find 37.5FPS smooth doesn't change the fact that the framerate just cut in half suddenly, which you would notice. This is what people hate about it.

If you're playing a game that has a framerate that routinely stays above your refresh rate, then VSync will generally be a good thing. However if it's a game that moves above and below it, then VSync can become annoying. Even worse, if the game plays at an FPS that is just below the refresh rate (say you get 65FPS most of the time on a refresh rate of 75Hz), the video card will have to settle for putting out much less FPS than it could (37.5FPS in that instance). This second example is where the percieved drop in performance comes in. It looks like VSync just killed your framerate. It did, technically, but it isn't because it's a graphically intensive operation. It's simply the way it works.

All hope is not lost however. There is a technique called triple-buffering that solves this VSync problem. Lets go back to our 50FPS, 75Hz example. Frame 1 is in the frame buffer, and 2/3 of frame 2 are drawn in the back buffer. The refresh happens and frame 1 is grabbed for the first time. The last third of frame 2 are drawn in the back buffer, and the first third of frame 3 is drawn in the second back buffer (hence the term triple-buffering). The refresh happens, frame 1 is grabbed for the second time, and frame 2 is copied into the frame buffer and the first part of frame 3 into the back buffer. The last 2/3 of frame 3 are drawn in the back buffer, the refresh happens, frame 2 is grabbed for the first time, and frame 3 is copied to the frame buffer. The process starts over. This time we still got 2 frames, but in only 3 refresh cycles. That's 2/3 of the refresh rate, which is 50FPS, exactly what we would have gotten without it. Triple-buffering essentially gives the video card someplace to keep doing work while it waits to transfer the back buffer to the frame buffer, so it doesn't have to waste time. Unfortunately, triple-buffering isn't available in every game, and in fact it isn't too common. It also can cost a little performance to utilize, as it requires extra VRAM for the buffers, and time spent copying all of them around. However, triple-buffered VSync really is the key to the best experience as you eliminate tearing without the downsides of normal VSync (unless you consider the fact that your FPS is capped a downside... which is silly because you can't see an FPS higher than your refresh anyway).

I hope this was informative, and will help people understand the intracacies of VSync (and hopefully curb the "VSync, yes or no?" debates!). Generally, if triple buffering isn't available, you have to decide whether the discrete framerate limitations of VSync and the issues that can cause are worth the visual improvement of the elimination of tearing. It's a personal preference, and it's entirely up to you.

Now, here are a few common questions and misconceptions you guys may have about this that I believe I can answer.

It is impossible to get more FPS than your refresh rate on your monitor.
This is not true. If you use VSync, you will lock your FPS to your refresh rate. The thing is if you have it off, your GPU can render as many frames as it can before your monitor refreshes. Now, here is the thing. Let's say the refresh rate on your monitor is 60Hz, and you are getting 120FPS in your game. So, with VSync off, your GPU will render 2 frames before your monitor updates, so your monitor will update every other frame basically. (Frame 1, 3, 5, 7, etc.) So in reality it is actually skipping frames. Now, tearing will only occur if your GPU is rendering frames proportionally to your monitor's refreshes. Lets say your refresh rate is 60Hz and your FPS is 100. In this case, your GPU will render 1 frame and 2/3 of another frame, and that will cause a tear. I am not 100% sure on this one, but it will make sense that depending on the percentage of the next frame that is rendered, will tell where the tear will actually be on your screen. For example if you do 1 and 2/3 frames, the tear would be roughly 2/3 down your screen.

Should I use VSync or not?
Well, I would say if you have the hardware to handle it, you may as well. What I mean is if you have a high end CRT with 100Hz+ refresh, then it would probably be better to use VSync. But even if you have 100Hz refresh rate I would not use VSync if your actual FPS is over 100, so if you have VSync off and your FPS is 200 or more, I would just leave it off. Also, if you have a refresh rate of lets say 100Hz, but your video card can only do 90 FPS, your GPU can only do 9/10 of every frame, and you would only render 1 frame every 2 refreshes, and that would cut your frame rate down to 50. You can improve that by enabling Triple Buffering however, but it will eat more of your resources.

More will be added as it comes up.

mnishimura00 · 08-12-08

cool, i always wondered what exactly it did. i never use it anyways, everyone always said to disable it, which i did.

Crazy9000 · 08-12-08

You actually seem to have some of your explanation completely backwards

. The article makes it very clear that if you are getting over the refresh rate you will want to have vsync on, as it's 100% better unless the framerate drops below the refresh rate.

If you are getting 200fps with a 60hz refresh, you will want to have vsync on, as your FPS shouldn't ever drop below 60. However, if you are getting 65 fps with a 60hz refresh rate, you will probably want vsync off, as the FPS will drop below 60 at times.

Semper Fidelis · 08-12-08

Quote:

Originally Posted by Crazy9000

You actually seem to have some of your explanation completely backwards

. The article makes it very clear that if you are getting over the refresh rate you will want to have vsync on, as it's 100% better unless the framerate drops below the refresh rate.

If you are getting 200fps with a 60hz refresh, you will want to have vsync on, as your FPS shouldn't ever drop below 60. However, if you are getting 65 fps with a 60hz refresh rate, you will probably want vsync off, as the FPS will drop below 60 at times.

That wasn't my point of this thread. My point was saying that with vsync off you can get any amount of FPS no matter what your monitor refresh rate is.

The only way vsync is better is if you absolutely can't stand tearing, personally I don't notice the tearing, so it doesn't bother me.

Crazy9000 · 08-12-08

Quote:

Originally Posted by Semper Fidelis

That wasn't my point of this thread. My point was saying that with vsync off you can get any amount of FPS no matter what your monitor refresh rate is.

The only way vsync is better is if you absolutely can't stand tearing, personally I don't notice the tearing, so it doesn't bother me.

You can "get" the FPS, but they are not being displayed. You monitor is only looking for data at the refresh rate, and is only going to be grabbing 60 frames per second at 60hz.

When you have 120fps at 60hz and vsync off, your monitor will be displaying 60 frames per second on itself. If you are getting tearing, you're getting part of one frame and part of another, but the chopped frames are still only taken at 60 per second. If you are not getting tearing, the monitor is still only displaying 60 frames per second, as that's all it's going to the video card to ask for. The extra 60 frames are simply discarded.

Semper Fidelis · 08-12-08

Quote:

Originally Posted by Crazy9000

When you have 120fps at 60hz and vsync off, your monitor will be displaying 60 frames per second on itself. If you are getting tearing, you're getting part of one frame and part of another

But 120FPS is 100% faster than the 60Hz, so the GPU is getting 2 frames done between monitor updates, so it is showing 60 frames, but it is showing frame 1, 3, 5, 7, etc.

So even though it is only displaying 60 frames, there are a full 120 frames processed.

Kip69 · 08-12-08

And 60 frames are wasted. Thats the point we are trying to make. The screen is the bottleneck. The extra power in the GPU (and to a lesser extent the CPU) are only good if it keeps the minimum FPS at about the max screen refresh and you don't get tearing if the FPS goes higher.

Semper Fidelis · 08-12-08

Quote:

Originally Posted by Kip69

And 60 frames are wasted. Thats the point we are trying to make. The screen is the bottleneck. The extra power in the GPU (and to a lesser extent the CPU) are only good if it keeps the minimum FPS at about the max screen refresh and you don't get tearing if the FPS goes higher.

The frames are not wasted. With vsync on you only process 60 frames, with it off you process 120, but you only see 60 of them. But the last frame processed is 119 with it off versus 60 with it on.

Crazy9000 · 08-12-08

Quote:

Originally Posted by Semper Fidelis

But 120FPS is 100% faster than the 60Hz, so the GPU is getting 2 frames done between monitor updates, so it is showing 60 frames, but it is showing frame 1, 3, 5, 7, etc.

So even though it is only displaying 60 frames, there are a full 120 frames processed.

Over time, the same number of frames are being shown, so it really doesn't make a difference; if you don't get tearing, there's really no disadvantage to leaving vsync off, so might as well I guess.

Semper Fidelis · 08-13-08

Quote:

Originally Posted by Crazy9000

Over time, the same number of frames are being shown, so it really doesn't make a difference; if you don't get tearing, there's really no disadvantage to leaving vsync off, so might as well I guess.

I do get tearing, but I am not distracted by it while playing my games.

And also, I don't know why you guys keep saying the same number of frames are processed. Here is what I am trying to say.

With VSync

Quote:

1
monitor refresh
2
monitor refresh
3
monitor refresh
4
monitor refresh
5
monitor refresh
6
monitor refresh
7
monitor refresh
8
monitor refresh
9
monitor refresh
10
monitor refresh
11
monitor refresh
12
monitor refresh
13
monitor refresh
...
56
monitor refresh
57
monitor refresh
58
monitor refresh
59
monitor refresh
60

Total frames: 60 (frames displayed: 60)

No VSync

Quote:

1
2
monitor refresh (displays frame 2)
3
4
monitor refresh (displays frame 4)
5
6
monitor refresh (displays frame 6)
7
8
monitor refresh (displays frame 8)
9
10
monitor refresh (displays frame 10)
11
12
monitor refresh (displays frame 12)
13
14
monitor refresh (displays frame 14)
15
16
monitor refresh (displays frame 16)
17
18
monitor refresh (displays frame 18)
19
20
monitor refresh (displays frame 20)
21
22
monitor refresh (displays frame 22)
23
24
monitor refresh (displays frame 24)
25
26
monitor refresh (displays frame 26)
...
111
112
monitor refresh (displays frame 112)
113
114
monitor refresh (displays frame 114)
115
116
monitor refresh (displays frame 116)
117
118
monitor refresh (displays frame 118)
119
120
monitor refresh (displays frame 120)

Total frames: 120 (frames displayed: 60)

So even though you render 120 frames, 60 are shown... but at the end you are still at a point 120 frames in.

This why the game appears to be snappier, because you are cutting the number of frames being shown.

If your FPS is proportionate to your refresh rate, you won't get tearing. Like if you had a refresh rate of 60Hz, if you had a constant FPS of 120, 180, 240, etc. You wont get tearing. Becauase instead of it overlaying part of a frame, it overlays the entire frame, so you don't even see the original frame. So if you had 3 frames rendered by the GPU in 1 monitor refresh, you would only see frame 3.

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[Info] Explanation of FPS vs. Refresh Rate