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I am trying to to add a resolution mode for X with the xorg-dummy driver to be able to fit the x11vnc server's output to my screen resolution. I am running

x11vnc -xdummy -xrandr

with the x11vnc Xdummy script in my path. If I don't add a xorg.conf file in /etc/X11 then I get only a bunch of lower resolutions.

Therefore I added this xorg.conf file https://www.xpra.org/xorg.conf with which I also have a number of higher resolutions. Among them is 1920x1200 which I need, but rotated by 90 degrees.

Using http://www.arachnoid.com/modelines/ , I therefore computed the Modeline

Modeline "1200x1920_10.00" 27.20 1200 1192 1304 1408 1920 1921 1924 1932 -HSync +Vsync

And added it to the xorg.conf file together with the corresponding Mode. After restarting, however, I still don't have this mode. In my /var/log/Xorg.log I see thigs like

Not using mode "1200x1920" (no mode of this name)

but also

Not using mode "1920x1200" (no mode of this name)

The later works in fact. The former does not. I also tried adding it with xrandr, but I get

Failed to get size of gamma for output default

Furthermore I tried supplying the mode using the argument -geom 1200x1920 to the Xdummy script. This does not add the resolution either.

Does xorg-dummy only support the modes already specified in the sample xorg.conf or am I making a mistake? The goal of of the exercise is to be able to switch the vnc-server resolution dynamically. I could use other vnc-servers but most of these seem to have issues related to the use of KDE. KDE's own krfb is unfortunately much slower.

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  • +1 for pointing me to xpra. Sorry for not being able to help.
    – Rmano
    Jan 24 '14 at 19:59
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Official guide to compute modeline (with cvt) is here:

Resolution: Adding undetected resolutions

I think will be adequately to quote this part here:

Due to buggy hardware or drivers, your monitor's correct resolutions may not always be detected. For example, the EDID data block queried from your monitor may be incorrect.

If the mode already exists, but just isn't associated for the particular output, you can add it like this:

$ xrandr --addmode S-video 800x600

If the mode doesn't yet exist, you'll need to create it first by specifying a modeline:

$ xrandr --newmode <Mode``Line>

You may create a modeline using the gtf or cvt utility. For example, if you want to add a mode with resolution 800x600 at 60 Hz, you can enter the following command: (The output is shown following.)

$ cvt 800 600 60
# 800x600 59.86 Hz (CVT 0.48M3) hsync: 37.35 kHz; pclk: 38.25 MHz
Modeline "800x600_60.00"   38.25  800 832 912 1024  600 603 607 624 -hsync +vsync

Then copy the information after the word "Modeline" into the xrandr command:

$ xrandr --newmode "800x600_60.00"   38.25  800 832 912 1024  600 603 607 624 -hsync +vsync

After the mode is entered, it needs to be added to the output using the --addmode command as explained above.

Be careful and pay attention for last sentence.
Either, look your modeline for the words -hsync +vsync that typed with small letters. Use official guides. Enjoy.

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