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I want to run dual monitors and find that I can do so if I select the resolution to be 800 x 600 for each, since the sum of dimensions when not mirrored is 1600 x 600 both landscape or 1200 x 800 both portrait.

The trouble is that the natural resolution of my monitors is much more than 800 x 600, in fact it is 1440 x 900. Summing in either direction exceeds the 1600 x 1600 virtual space. Is this a limitation I can improve by using a different video card (and matching driver)?

Edit: I am using a Radeon HD 5450 and the latest (Dec 2011) Catalyst driver for x86-64 architecture on Ubuntu 11.10. I did not write a custom xorg.conf.

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video card , driver, and any options you use in /etc/X11/xorg.conf all play a role. What video card are you using, what driver, and did you write a custom xorg.conf ? – bodhi.zazen Jan 6 '12 at 16:22
IMO, it should not take a custom xorg.conf to get dual monitors working. The 1600 by 1600 limit is annoying and unfriendly towards new users. – William Jan 7 '12 at 4:00
up vote 1 down vote accepted

The values of the X System virtual screen can be configured by System > Preferences > Monitors (in Ubuntu 10.10, I did not try 11.10). The configuration is automatically made to /etc/X11/xorg.conf when the physical screen selections and screen rotation selections are made via the System > Preferences > Monitors tool. Still, rotation might not be possible for all resolutions since there is a limit set when the X System starts.

To overcome this I did not need to change the video card. Instead I manually edited /etc/X11/xorg.conf to set the virtual resolution in both dimensions to double the longest dimension of my monitors. This made rotation possible. This is not to say that this solution will work if graphics card memory is limited but in my case it worked.

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