What are Double Buffering, vsync and Triple Buffering?

When a computer needs to display something on a monitor, it draws a picture of what the screen is supposed to look like and sends this picture (which we will call a buffer) out to the monitor. In the old days there was only one buffer and it was continually being both drawn to and sent to the monitor. There are some advantages to this approach, but there are also very large drawbacks. Most notably, when objects on the display were updated, they would often flicker.


The computer draws in as the contents are sent out.
All illustrations courtesy Laura Wilson.


In order to combat the issues with reading from while drawing to the same buffer, double buffering, at a minimum, is employed. The idea behind double buffering is that the computer only draws to one buffer (called the "back" buffer) and sends the other buffer (called the "front" buffer) to the screen. After the computer finishes drawing the back buffer, the program doing the drawing does something called a buffer "swap." This swap doesn't move anything: swap only changes the names of the two buffers: the front buffer becomes the back buffer and the back buffer becomes the front buffer.


Computer draws to the back, monitor is sent the front.


After a buffer swap, the software can start drawing to the new back buffer and the computer sends the new front buffer to the monitor until the next buffer swap happens. And all is well. Well, almost all anyway.

In this form of double buffering, a swap can happen anytime. That means that while the computer is sending data to the monitor, the swap can occur. When this happens, the rest of the screen is drawn according to what the new front buffer contains. If the new front buffer is different enough from the old front buffer, a visual artifact known as "tearing" can be seen. This type of problem can be seen often in high framerate FPS games when whipping around a corner as fast as possible. Because of the quick motion, every frame is very different, when a swap happens during drawing the discrepancy is large and can be distracting.

The most common approach to combat tearing is to wait to swap buffers until the monitor is ready for another image. The monitor is ready after it has fully drawn what was sent to it and the next vertical refresh cycle is about to start. Synchronizing buffer swaps with the Vertical refresh is called vsync.

While enabling vsync does fix tearing, it also sets the internal framerate of the game to, at most, the refresh rate of the monitor (typically 60Hz for most LCD panels). This can hurt performance even if the game doesn't run at 60 frames per second as there will still be artificial delays added to effect synchronization. Performance can be cut nearly in half cases where every frame takes just a little longer than 16.67 ms (1/60th of a second). In such a case, frame rate would drop to 30 FPS despite the fact that the game should run at just under 60 FPS. The elimination of tearing and consistency of framerate, however, do contribute to an added smoothness that double buffering without vsync just can't deliver.

Input lag also becomes more of an issue with vsync enabled. This is because the artificial delay introduced increases the difference between when something actually happened (when the frame was drawn) and when it gets displayed on screen. Input lag always exists (it is impossible to instantaneously draw what is currently happening to the screen), but the trick is to minimize it.

Our options with double buffering are a choice between possible visual problems like tearing without vsync and an artificial delay that can negatively effect both performance and can increase input lag with vsync enabled. But not to worry, there is an option that combines the best of both worlds with no sacrifice in quality or actual performance. That option is triple buffering.


Computer has two back buffers to bounce between while the monitor is sent the front buffer.


The name gives a lot away: triple buffering uses three buffers instead of two. This additional buffer gives the computer enough space to keep a buffer locked while it is being sent to the monitor (to avoid tearing) while also not preventing the software from drawing as fast as it possibly can (even with one locked buffer there are still two that the software can bounce back and forth between). The software draws back and forth between the two back buffers and (at best) once every refresh the front buffer is swapped for the back buffer containing the most recently completed fully rendered frame. This does take up some extra space in memory on the graphics card (about 15 to 25MB), but with modern graphics card dropping at least 512MB on board this extra space is no longer a real issue.

In other words, with triple buffering we get the same high actual performance and similar decreased input lag of a vsync disabled setup while achieving the visual quality and smoothness of leaving vsync enabled.

Now, it is important to note, that when you look at the "frame rate" of a triple buffered game, you will not see the actual "performance." This is because frame counters like FRAPS only count the number of times the front buffer (the one currently being sent to the monitor) is swapped out. In double buffering, this happens with every frame even if the next frames done after the monitor is finished receiving and drawing the current frame (meaning that it might not be displayed at all if another frame is completed before the next refresh). With triple buffering, front buffer swaps only happen at most once per vsync.

The software is still drawing the entire time behind the scenes on the two back buffers when triple buffering. This means that when the front buffer swap happens, unlike with double buffering and vsync, we don't have artificial delay. And unlike with double buffering without vsync, once we start sending a fully rendered frame to the monitor, we don't switch to another frame in the middle.

This last point does bring to bear the one issue with triple buffering. A frame that completes just a tiny bit after the refresh, when double buffering without vsync, will tear near the top and the rest of the frame would carry a bit less lag for most of that refresh than triple buffering which would have to finish drawing the frame it had already started. Even in this case, though, at least part of the frame will be the exact same between the double buffered and triple buffered output and the delay won't be significant, nor will it have any carryover impact on future frames like enabling vsync on double buffering does. And even if you count this as an advantage of double buffering without vsync, the advantage only appears below a potential tear.

Let's help bring the idea home with an example comparison of rendering using each of these three methods.

Index Digging Deeper: Galloping Horses Example
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  • DerekWilson - Wednesday, July 1, 2009 - link

    "skipping" doesn't occur in games like it does with a video -- there is not a set number of frames that must be rendered in a set amount of time. The action happens independently of the frames rendered in a game, while for a video you there is an exact framerate that needs to be maintained in order to see smooth motion as it was captured.

    in the old days, console games would tie the game timer to framerate which was always set to vsync. If frame rate dropped from 60 FPS to 30, the game would actually slow down (when too much action was going on on the screen). Modern PC games do not rely on framerate to time their game, in stead framerate is a snapshot of the game at a certain time.

    if you drop all but the most recently completed frame, then you are just doing triple buffering the way this article describes.
    Reply
  • ufon68 - Monday, March 28, 2016 - link

    This not completely accurate.

    While usually, and for a good reason, physical simulation indeed "ticks" independent of FPS, the actual gameplay logic is usually tied to the frames being displayed.
    You don't see the game slowing down or speeding up because the simulation takes into account the FPS speed, ie. it multiplies everything by the delta time which is the time it took to tick the last frame.
    For instnance to translate an object along a vector at a certain speed, you move it every frame by: Direction * Speed * DeltaTime (DeltaTime being the time it took the game to tick the last frame in seconds, given a constant fps[just for simplifacation purposes, the fps can move up and down and this will still work] of 10, the DeltaTime is 0.1 for this given frame/tick)

    So it's not correct that what you see are snapshots of what's going on in the game world, it just gives you that impression by doing this neat trick.
    Reply
  • VinnyV - Monday, June 29, 2009 - link

    I just wanted to say that I really appreciate this article. I think I just went from understanding about 10% of what is usually discussed on this site to about 11 or 12%. Thanks! Please post more articles like this! Reply
  • iwodo - Monday, June 29, 2009 - link

    I see this as an Direct X Problem only? May be we should call Microsoft to improve on it... ( Too Late for Direct X 11?? ) Reply
  • Dospac - Sunday, June 28, 2009 - link

    Derek, it would be interesting to get to the bottom of the multi-GPU input delay issue as well as devise a quantitative way to test the delay with various setups. It's confounding that this hasn't been investigated sooner and been sorted out. The potential PQ improvement is well worth your efforts. Thank you! Reply
  • DerekWilson - Sunday, June 28, 2009 - link

    getting to the bottom of why delay happens conceptually isn't that complex -- there are a lot of issues in interGPU communication and synchronization that can cause issues.

    quantitative testing is possible but pretty expensive ... i'll see if i can convince Anand to invest in the equipment :-)
    Reply
  • DerekWilson - Sunday, June 28, 2009 - link

    Added a note at the end of the article to try and help clear the air about the confusion over triple buffering as a page flipping method and flip queues (render ahead) with three buffers.

    I also wanted to note that this topic is not just confusing for gamers -- game developer do not always get their labeling right and sometimes refer to flip queues a "triple buffering" incorrectly.

    I do apologize for not addressing this issue at publication, but I hope this helps to clear the air.
    Reply
  • Touche - Sunday, June 28, 2009 - link

    Have you seen this?

    http://msdn.microsoft.com/en-us/library/ms796537.a...">http://msdn.microsoft.com/en-us/library/ms796537.a...
    http://msdn.microsoft.com/en-us/library/ms893104.a...">http://msdn.microsoft.com/en-us/library/ms893104.a...
    Reply
  • DerekWilson - Wednesday, July 1, 2009 - link

    What they are showing is 1 frame render ahead with vsync. In MS DX terms, this is a flip chain with 2 back buffers and a present interval of one.

    This is them calling it triple if uses three total buffers. This is still a flip queue and should be referred to as such to avoid confusion.
    Reply
  • mikeev - Sunday, June 28, 2009 - link

    Add me to the list of people who tried triple buffering but had to turn it OFF due to the input lag.

    I ran the test in L4D anyway. It was unbearable. I couldn't hit a thing. The input lag was actually noticeably less with double buffering + vsync ON.

    Maybe I'm doing something wrong, but my results do not jive with this article at all.
    Reply

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