X: 25 years and still lacking vsync'ed double buffering [LWN.net]

X: 25 years and still lacking vsync'ed double buffering

Posted Oct 2, 2009 17:39 UTC (Fri) by mjg59 (subscriber, #23239)
In reply to: X: 25 years and still lacking vsync'ed double buffering by dlang
Parent article: LPC: 25 years of X

LCDs aren't (in the relevant cases) pixel addressable devices. The framebuffer is still scanned out at the vertical refresh rate and the entire image sent to the LCD each time - starting at the top left pixel and finishing at the bottom right. There's even an interval between the end of a frame and the beginning of the next one.

Whatever the original meaning of the word "tearing" as relating to video, its use to describe the effect caused by framebuffer updates not being synchronised with scanout and the associated discontinuity in the image is well established.

to post comments

X: 25 years and still lacking vsync'ed double buffering

Posted Oct 2, 2009 17:59 UTC (Fri) by dlang (guest, #313) [Link] (2 responses)

if a LCD is connected via a VGA interface include the vertical retrace delay, ones connected via digital interfaces do not.

having an image change halfway through an update from one frame to the next with no other distortion of the screen (and cleaned up on the next frame refresh) is a _very_ minor issue compared to loosing sync which scrambles the screen for a significant amount of time (several seconds on a LCD)

X: 25 years and still lacking vsync'ed double buffering

Posted Oct 2, 2009 18:02 UTC (Fri) by mjg59 (subscriber, #23239) [Link] (1 responses)

"if a LCD is connected via a VGA interface include the vertical retrace delay, ones connected via digital interfaces do not."

They do. A *lot* depends on having that interval available.

"having an image change halfway through an update from one frame to the next with no other distortion of the screen (and cleaned up on the next frame refresh) is a _very_ minor issue compared to loosing sync which scrambles the screen for a significant amount of time (several seconds on a LCD)"

The common case is watching videos or playing games, where it's not cleared up on the next frame refresh because another update has happened in the meantime. It manifests itself as a consistent discontinuity in the image. It's greatly irritating.

X: 25 years and still lacking vsync'ed double buffering

Posted Oct 11, 2009 22:24 UTC (Sun) by Ross (guest, #4065) [Link]

> They do. A *lot* depends on having that interval available.

Yes, but the delay between updates can be very small in comparison to what are needed for CRTs. This can even be taken advantage of with normal analog VGA connections to get a higher refresh rate and/or resolution with the same pixel clock (see reduced blanking modes).

> The common case is watching videos or playing games, where it's not cleared up on the next frame refresh because another update has happened in the meantime. It manifests itself as a consistent discontinuity in the image. It's greatly irritating.

Or it may slowly move up or down the screen. It can be very distracting in the right situation.

-Ross