X video extension
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The X video extension, often abbreviated as XVideo or Xv, is a video output mechanism for the X Window System. Its main use today is to rescale video playback in hardware (namely in the hardware of the graphics card), in order to enlarge a given video or to watch it in full screen mode. Without XVideo, this scaling would have to be done in software, which is possible but requires a considerable amount of processing power, sometimes to the point of slowing down/degrading the video stream. Similarly, the X video extension has the graphics card perform color space conversions. It can also be used to change contrast, brightness and hue of a displayed video stream in hardware, again saving processing power.
In order for this to work, three things have to come together:
- The graphics card hardware has to provide the required functions.
- The device driver software for the graphics card and the X server program have to support the XVideo interface.
- The video playback software has to make use of this interface.
Video playback programs that run under the X Window system, such as MPlayer or xine, typically have an option to enable XVideo output. It is very advisable to switch on this option if the system supports XVideo - the speedup is very noticeable even on a fast CPU.
To check whether a given X server supports XVideo, one can use the utility xdpyinfo. The XVideo capabilities of the attached graphics cards can be queried with the xvinfo program.
While the protocol itself has features for reading and writing of video streams from and to video adapters, in practice today only the functions XvPutImage and XvShmPutImage are used: the client program repeatedly prepares images and passes them on to the graphics hardware to be scaled, converted and displayed.
The protocol was designed by David Carver; the specification for version 2 of the protocol was written in July 1991.
External links
- Official XVideo specification, version 2.0 (http://cvsweb.xfree86.org/cvsweb/xc/doc/specs/Xv/xv-protocol-v2.txt?rev=HEAD&content-type=text/vnd.viewcvs-markup)
- Additions in XVideo version 2.2 (http://cvsweb.xfree86.org/cvsweb/xc/doc/hardcopy/Xv/video?rev=HEAD&content-type=text/vnd.viewcvs-markup)
- Xv man pages (http://www.xfree86.org/4.2.1/Xv.3.html)
- testxv.c (http://bellet.info/XVideo/testxv.c), a simple test program for XVideo; compile with gcc -o testxv testxv.c -L/usr/X11R6/lib -lX11 -lXext -lXv