My query is same/similar to Setting KDE to crop the laptop's screen to match external VGA device's ratio In my case my screen resolution is 1366 x 768 which is non-standard but still resolves to 16:9 whereas most old projectors have 4:3 with the result that the picture is cut. In such a case is something else feasible. I did see Viewing a part of the screen in a window on Linux which seems promising. I have not understood though the --pos 100x100 code which has been shared therein. Here's the output of my display via xrandr $ xrandr -qScreen 0: minimum 8 x 8, current 1366 x 768, maximum 32767 x 32767eDP1 connected primary 1366x768+0+0 (normal left inverted right x axis y axis) 309mm x 174mm   1366x768      60.00*+   1360x768      59.80    59.96     1024x768      60.00     800x600       60.32    56.25     640x480       59.94  DP1 disconnected (normal left inverted right x axis y axis)DP2 disconnected (normal left inverted right x axis y axis)HDMI1 disconnected (normal left inverted right x axis y axis)HDMI2 disconnected (normal left inverted right x axis y axis)VIRTUAL1 disconnected (normal left inverted right x axis y axis)Also here's my X version $ X -versionX.Org X Server 1.17.2Release Date: 2015-06-16X Protocol Version 11, Revision 0Build Operating System: Linux 4.1.0 x86_64 DebianCurrent Operating System: Linux think-debian 4.2.0-1-amd64 #1 SMP Debian 4.2.1-2 (2015-09-27) x86_64Kernel command line: BOOT_IMAGE=/boot/vmlinuz-4.2.0-1-amd64 root=UUID=aaafd0a1-62db-4216-80be-cf0dc016ac7c ro initrd=/install/initrd.gz ipv6.disable=1 init=/lib/systemd/systemdBuild Date: 11 August 2015  10:51:15AMxorg-server 2:1.17.2-1.1 (http://www.debian.org/support) Current version of pixman: 0.33.2    Before reporting problems, check http://wiki.x.org    to make sure that you have the latest version.As can be seen xorg is latest and similar is the case with xrandr. Have 8 GB of RAM so that should be enough for whatever is required herein. 
output cut on the projector because laptop resolution is 16:9 and projector is lower
x11;xorg;dual monitor;dualhead