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Unraveling the mystery of VSYNC

30 July 2000

Forward: I did not realize it when I first wrote this article, but it turns out this subject matter is a bit controversial. In fact, I had one company respond by saying the following

"I talked to some of our evangelists internally and they don't want to comment on this because it's such a sensitive issue to developers. They take the whole Direct3D, OpenGL discussion really seriously and so we're nervous about alienating developers on either side of the issue."

Who knew that a simple discussion of Vsync could bring out such a response.

Section 1 - The Basics

In the first part of this series, we took a look at What does 'Optimal' Refresh Rate really mean? While that discussion focused on the 2D side of things, refresh rate also plays an important part in the 3D the form of VSYNC. But what is VSYNC?

Well before we can answer that, a little discussion of how 3D works is required. In order to get a smoother transition between frames in 3D games, the video card puts the contents of the upcoming frame into its frame buffer. [The frame buffer is part of the local memory that resides on the video card itself] It then moves the contents of the frame buffer to the screen. When this is complete, the frame buffer gets the next frame. This process repeats its self over and over.

Now, what is VSYNC? Well, VSYNC is basically the synchronizing of buffer swaps with your monitor's refresh rate. With VSYNC enabled, frame rates will not exceed the monitor's current refresh rate for that particular resolution. For example, if your monitor is using a refresh rate of 85Hz at 800x600, with VSYNC enabled, you will theoretically never exceed 85fps. So the refresh rate creates an artificial barrier that limits the frame rate.

So what happens if you are playing on an older monitor that only supports a 60Hz refresh rate. Will you have to live with a maximum of 60fps (assuming that your system can generate more fps)? Not necessarily. Newer video cards give you the option of disabling VSYNC. What happens is that this allows the buffer swapping to occur without synchronizing with the monitor's refresh rate. If it really was the refresh rate limiting you, disabling VSYNC may allow you to obtain frame rates in excess of 60fps. This, unfortunately, can also cause what are called 'visual anomalies': image tearing and flashing polygons. Some games run fine with VSYNC enabled, while other games crumble when VSYNC is disabled.

If you look in Display Properties, you may see an option to disable VSYNC in OpenGL by not Direct3D. Why is this? Well, in order to receive Microsoft's WHQL certification, this option cannot be available. Microsoft does not want Direct3D games to be played without VSYNC.

In the next section, the difference between Direct3D and OpenGL is explored from a developers viewpoint.


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