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I'm using KUbuntu 9, and two monitors: CRT+DVI. Nvidia's TwinView works fine, but enabling Xinerama extension disables compositing in KDE.

Is there a way to enable compositing (desktop effects) with Xinerama? Why does this happen?

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Compositing requires certain graphical extensions (most commonly GLX) to be available on your graphics card and enabled in your X server. However, few drivers have simultaneous support for the technologies that drive Xinerama and GLX - enabling one tends to disable the other.

Recent (GeForce 7 or so and newer) Nvidia cards are capable of enabling such functionality, if the GPUs are similar enough. If not, the official Nvidia driver will only disable GLX extensions on the card that doesn't support Xinerama+GLX, rather than disabling it across all screens. AFAIK, the open-source nv driver doesn't support Xinerama+GLX at all (or if it does, it's subject to the same limitations as the official nvidia driver).

If there's absolutely no way for you to get the two working nicely on your hardware, you might want to take a look at the mesa library - it's a software 3D rendering tool that replaces hardware GLX and allows compositing to happen, albeit with a pretty big performance hit.

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10x for your explanation! The mesa library is worse than no compositing at all.. – kolypto Jul 8 '09 at 17:32

You usually do not enable the usual Xorg Xinerama extension when using TwinView, since the nVidia driver already provides a Xinerama extension compatible with TwinView.

From the driver's README:

Will window managers be able to appropriately place windows (e.g., avoiding placing windows across both display devices, or in inaccessible regions of the virtual desktop)?

Yes. The NVIDIA X driver provides a Xinerama extension that X clients (such as window managers) can use to discover the current TwinView configuration. Note that the Xinerama protocol provides no way to notify clients when a configuration change occurs, so if you modeswitch to a different MetaMode, your window manager will still think you have the previous configuration. Using the Xinerama extension, in conjunction with the XF86VidMode extension to get modeswitch events, window managers should be able to determine the TwinView configuration at any given time.

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