Editing X video extension (section)

{{Short description|Video output mechanism for the X Window System}}
{{for|the website with a similar name|XVideos}}

{{primary sources|date=March 2013}}
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|name                       = X video extension
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|author                     = [[X.Org Foundation]]
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|latest release version     = 2.0
|latest release date        = {{Start date and age|1991|7|25}}
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The '''X video extension''', often abbreviated as '''XVideo''' or '''Xv''', is a video output mechanism for the [[X Window System]]. The protocol was designed by David Carver; the specification for version 2 of the protocol was written in July 1991.<ref>[http://www.x.org/releases/X11R7.5/doc/videoproto/xv-protocol-v2.txt Official XVideo specification, version 2.0]</ref> It is mainly used today to resize video content in the video controller hardware in order to enlarge a given video or to watch it in full screen mode. Without XVideo, X would have to do this scaling on the main CPU.  That requires a considerable amount of processing power, which could slow down or degrade the video stream; video controllers are specifically designed for this kind of computation, so can do it much more cheaply. Similarly, the X video extension can have the video controller perform [[color space]] conversions, and change the contrast, brightness, and hue of a displayed video stream.

In order for this to work, three things have to come together:

* The video controller has to provide the required functions.
* The [[device driver]] software for the video controller and the X [[display server]] program have to implement the XVideo interface.
* The video playback software has to make use of this interface.

Most modern video controllers provide the functions required for XVideo; this feature is known as ''hardware scaling and [[YUV]] acceleration'' or sometimes as ''2D hardware acceleration''. The [[XFree86]] X display server has implemented XVideo since version 4.0.2. To check whether a given X display server supports XVideo, one can use the utility <code>xdpyinfo</code>. To check whether the video controller provides the required functions and whether the X device driver implements XVideo for any of them, one can use the <code>xvinfo</code> program.

Video playback programs that run under the X Window system, such as [[MPlayer]], [[MythTV]] or [[xine]], typically have an option to enable XVideo output. It is very advisable to switch on this option if the system [[graphics processing unit|GPU]] video-hardware and device drivers supports XVideo and more modern rendering systems such as OpenGL and VDPAU are unavailable – the speedup is very noticeable even on a fast [[Central processing unit|CPU]].

While the protocol itself has features for reading and writing of video streams from and to video adapters, in practice today only the functions <code>XvPutImage</code> and <code>XvShmPutImage</code> are used: the client program repeatedly prepares images and passes them on to the graphics hardware to be scaled, converted and displayed.