Wednesday, March 9, 2011

Vertical Synchronization: How to avoid tearing effect in high FPS display

When displaying graphics in high FPS, Tearing Effect can be avoided by enabling Vertical Sync.
Screen tearing is a visual artifact in video where information from two or more different frames is shown in a display device in a single screen draw.

The artifact occurs when the video feed sent to the device isn't in sync with the display's refresh, be it due to non-matching refresh rates, or simply lack of sync between the two. During video motion, screen tearing creates a torn look as edges of objects (such as a wall or a tree) fail to line up.
Tearing can occur with most common display technologies and video cards, and is most noticeable on situations where horizontally-moving visuals are commonly found, such as a indicator moving horizontally in the screen.

Vertical Synchronization, also called Vertical Sync, or simply VSync for short, was primarily required because of the physical limitations of CRT monitors. A CRT monitor has to constantly light up the phosphors on the screen many times per second to maintain an image, and can only do this a certain number of times per second based on how fast the electron gun in the monitor can move. Each time it has to redraw the entire screen again, it moves the electron gun inside the monitor from the bottom of the screen to point to the top left of the screen, ready to 'repaint' all the lines on the screen from top left to bottom right, and back again for the next refresh. The period during which the electron gun moves to the top of the screen for a new refresh is called the Vertical Blanking Interval (VBI).

Enabling VSync tells your graphics card to synchronize its actions with your monitor. That means the graphics card is only allowed to swap its frame buffer and send a new frame to the monitor when the monitor says it is ready to repaint a new screen - i.e. during the VBI[Vertical Blanking Interval]. Your graphics card and monitor do not have to be in sync; they can still operate properly when VSync is disabled, however when VSync is disabled, you can experience a phenomenon called Tearing in periods when your graphics card and monitor go out of sync, precisely because the graphics card and monitor are acting without regard for each other's limitations.


It is an unfortunate fact that if you disable VSync, your graphics card and monitor will inevitably go out of synch. Whenever your FPS exceeds the refresh rate (e.g. 120 FPS on a 60Hz screen), or in general at any point during which your graphics card is working faster than your monitor, the graphics card produces more frames in the frame buffer than the monitor can actually display at any one time. The end result is that when the monitor goes to get a new frame from the primary buffer of the graphics card during VBI, the frame may be made up of two or more different frames overlapping each other. This results in the onscreen image appearing to be slightly out of alignment or 'torn' in parts whenever there is any movement - and thus it is referred to as Tearing.

In OpenGL, WGL_EXT_swap_control can be used to turn on or off VSync.


Posted By : Santhosh G.

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