Wednesday, March 9, 2011

Triple Buffering: Third buffer for fast graphics display

Triple Buffering: Third buffer for fast graphics display.
There are actually two buffers on modern graphics cards, the Primary Buffer and the Secondary Buffer, also often called the Front Buffer and the Back Buffer. Both are storage areas on the Video RAM of the graphics card, and the process of using two buffers at one time is called Double Buffering. It was only relatively recently that graphics cards had enough VRAM to provide two buffers at all resolutions, since a single frame of high resolution detailed graphics can take up a great deal of video memory, much less two of them.

The graphics card uses the secondary buffer to compose a new frame while the primary buffer is sending an existing completed frame to the monitor. When these tasks are done, the buffers are essentially 'flipped' around so that the recently completed frame in the secondary buffer now becomes the primary buffer ready to send to the monitor, while a new frame begins composing in what was the primary buffer a moment ago. This is repeated over and over and thus the use of two buffers means that the graphics card is not constantly waiting for a single frame buffer to be cleared before getting on with rendering more frames to store there. It's like putting out a fire using two buckets of water instead of just one - one bucket can be filled with water while the contents of the other is being thrown on the fire, and then they're switched and the process repeated; much faster than just using a single bucket.

There is still a problem with double buffering, and that is when VSync is enabled, the graphics card can often fill both buffers and then have to stop working on any new frames until the monitor indicates it is ready for a new frame for its next refresh. Only then can the graphics card clear the primary buffer, switch buffers and begin rendering the next frame in the secondary buffer. This waiting is what causes a drop in FPS when VSync is enabled on many systems.

Wouldn't it then make sense to have more than two buffers? Why not three buffers for example - that would give the graphics card more room to render frames without having to worry about where to store them before they're sent to the monitor, even if VSync is enabled. Well there is an option which does just that, called Triple Buffering. And it generally does precisely what the name implies, it creates a third buffer in the VRAM, which we can call the Tertiary buffer.


Posted By : Santhosh G.

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