Xinerama

Xinerama
Four Head Xinerama

Xinerama is an extension to the X Window System which enables multi-headed X applications and window managers to use two or more physical displays as one large virtual display.

It was originally developed by Madeline T. Asmus of the Unix X Server Engineering Group at Digital Equipment Corporation (under the name PanoramiX), who contributed it to The Open Group for X11 Release 6.4 (X11R6.4), where it was renamed Xinerama. From there it was incorporated into XFree86 for the XFree86 4.0 release, and Solaris in the Solaris 7 11/99 release. According to Rob Lembree, X Server project lead at the time, the name Xinerama was inspired from the Cinerama widescreen theatre process. "We were frustrated by having big Alpha machines with multiple displays, and being unable to move applications from one to another. It was developed as much out of frustration as competitive advantage."

It is also an advantage if the window manager, via Xinerama awareness, can pop up new windows on the active physical display. Another advantage of Xinerama awareness is to only maximize windows to the dimensions of the active physical display.

Contents

General theory of operation

The general mode of operation is that multiple X screens (as denoted normally by :0.0, :0.1, etc) can be unified into a single workspace via enabling Xinerama in the X server. This creates a single unified work area that allows windows to be transferred across X screens.

XINERAMA extension

The Xinerama extension is used to provide clients with information about the layout of viewports within the unified workspace. It contains offset and size information to allow clients to make intelligent decisions about window placement, window maximization and other user interaction events.

Use in non-XINERAMA environments

Due to the X server's client/server architecture, the server can expose Xinerama information to the client without the X server's Xinerama infrastructure being active.

RandR and NVidia's twinview utilize this to provide window managers and clients with useful information about the output layout relative to the framebuffer.

Future of XINERAMA

The X.Org Consortium had convened a standards committee to document the protocol and API as formal standards, but that effort has now ended.[1] Development of the Xinerama code is now hosted on freedesktop.org and managed by the X.Org Foundation.

The RANDR extension exports its CRTC geometry in the Xinerama protocol, as well as through its own protocol. This breaks down when attempting to use multiple GPUs, because the reference X server's Xinerama implementation also tries to provide the Xinerama geometry to the protocol. Work to fix this is underway. [2]

As of the 1.10 X server release, the Xinerama rendering multiplexer and Composite extensions no longer conflict.[3]

Known problems

Common color depth

Xinerama requires that the physical screens have the same bit depth — an 8-bit screen cannot be combined with a 16-bit screen, for example.

Hardware rendering

In most[which?] Open Source implementations, OpenGL (3D) direct-rendering only works on one of the screens. Windows that should show 3D graphics on other screens tend to just appear black. This is most commonly seen with 3D screen savers, which show on one of the screens and black on the others. The Solaris SPARC OpenGL implementation, ATI Proprietary Linux driver and nVidia Proprietary Linux driver all support hardware accelerated rendering to all screens in Xinerama mode.

Static configuration

There is currently no way to dynamically add or remove physical screens, and there is no way to change the resolution of a screen. (The RandR feature is disabled when Xinerama is used). This is a particular difficulty for laptop and tablet PC users (or any mobile computer), where you commonly want to sit at a desk and use an external physical display in addition to the computer's built-in screen, but only when at the desk. In these cases, use of RandR, ATI's or nVidia's single GPU method is recommended.

Because Xinerama does not support adding or removing screens, it causes several problems:

  • When the external screen is not connected, windows may still appear on it. You have to drag them to the built-in screen without being able to see them.
  • The laptop may still be sending video signals, which uses unnecessary power and may reduce the battery life.
  • When you want to use a different external display (for example when moving between locations that have one), there's a good chance it will need different settings like screen resolution and refresh rate. But you cannot change these settings when Xinerama is enabled.

These are all problems with the Xinerama implementation rather than the design, and they could be fixed with sufficient work.

Window manager support

xmonad's xinerama support: tiling on three screens simultaneously.

Some[4] window managers and desktop environments have limited awareness of the separate physical screens in Xinerama. They work, but the desktop is simply stretched over the physical screens. When a window opens, the window manager may place it on an unexpected screen, which is confusing and annoying if the displays are not arranged like a single large desktop.

Despite these problems, Xinerama offers an advantage over separate X screens: You can move windows between Xinerama screens.

Dead space

The physical displays do not need to be the same resolution. Some window managers assume that the virtual display area is rectangular, but this isn't necessarily the case if the component physical displays are not the same size. The excess space, which is needed to make the area rectangular, is called "dead space". The window manager needs to be aware of Xinerama to recognize dead space, and avoid placing new windows there.

See also

Resource

Asmus, Madeline T. (December 1995) "The PanoramiX Extension" THE X RESOURCE 16: A Practical Journal of the X Window System. pp.59-73 ISBN 1565921666.

References

External links


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