Vertical Sync (synchronization) is a rendering option for a video card. This option prevents the video card from changing the display memory until the monitor is done with its current refresh cycle.
When vertical sync is applied, the rendering engine would match the maximum refresh rate of the monitor if the frame rate being produced is higher than the refresh rate of the monitor. This effectively throttles the maximum generated frames per second of the graphics card.
Vertical sync synchronizes the output video of the graphics card to the refresh rate of the monitor. This can be good when applied to applications that have frame rates significantly higher than the monitor’s refresh rate as this relieves tearing of the image and results in smooth playback. While if the graphics card is outputting significantly lower frame rates than the monitor’s refresh rate, this can result to the video output having judder as the graphics card may miss the monitor’s refresh deadlines.
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