
In 5 we've created a newmode in xrandr, now we need to add it to the virtual screen, as follows xrandr -addmode VIRTUAL1 "1368x768_60.00" (please remember to use your own newly created mode name, which is the value including quotes in 5). From the previous result copy what is in front of Modeline, (in this case "1384x768_60.00" 86.62 1384 1456 1600 1816 768 769 772 795 -HSync +Vsync) and use that as a parameter to xrandr -newmode command. As an example for gtf CLIENT_WIDTH CLIENT_HEIGHT 60 is In the server, run in a terminal gtf CLIENT_WIDTH CLIENT_HEIGHT 60 (replace the CLIENT_WIDTH and CLIENT_HEIGHT with the corresponding values).
You need to know client's screen size (we'll call them: CLIENT_WIDTH, CLIENT_HEIGHT) and the servers' screen size (we'll call them: SERVER_WIDTH, SERVER_HEIGHT).Required Software: On the server install x11vnc and gtf, on the client install a vnc client.
Definitions: The "server" will be the computer which screen you want to extend, the "client" will be the computer (or tablet, any device able to deploy a VNC client) that you want to use as a screen. Please be aware of the notes and considerations at the end of this answer. I had a couple of issues with this code but I managed to get it working as shown below. Xdmx didn't worked for me, however I was able to get it working using x11vnc by means of using my own code derived from the VNC Virtual Display Linker python code.