Extend non-HiDPI external display above HiDPI internal display

extend.sh

#!/bin/sh
# extend non-HiDPI external display on DP* above HiDPI internal display eDP*
# see also https://wiki.archlinux.org/index.php/HiDPI
# you may run into https://bugs.freedesktop.org/show_bug.cgi?id=39949
#                  https://bugs.launchpad.net/ubuntu/+source/xorg-server/+bug/883319

EXT=`xrandr --current | sed 's/^\(.*\) connected.*$/\1/p;d' | grep -v ^eDP | head -n 1`
INT=`xrandr --current | sed 's/^\(.*\) connected.*$/\1/p;d' | grep -v ^DP | head -n 1`

ext_w=`xrandr | sed 's/^'"${EXT}"' [^0-9]* \([0-9]\+\)x.*$/\1/p;d'`
ext_h=`xrandr | sed 's/^'"${EXT}"' [^0-9]* [0-9]\+x\([0-9]\+\).*$/\1/p;d'`
int_w=`xrandr | sed 's/^'"${INT}"' [^0-9]* \([0-9]\+\)x.*$/\1/p;d'`
off_w=`echo $(( ($int_w-$ext_w)/2 )) | sed 's/^-//'`

xrandr --output "${INT}" --auto --pos ${off_w}x${ext_h} --scale 1x1  --output "${EXT}" --auto --scale 2x2 --pos 0x0

This works well for my current set up where I have a 4K laptop (XPS 9560) beneath a FHD external display, but I have to run it twice: first time the external monitor has a big chunk of whatever's on the 4K monitor, and the second time it goes in fine.

On Unity, connect first the external monitor, and on display config make sure the scale factor for both screens is 2. Then run the script, perhaps twice.

The downside of this approach is however the FHD monitor is a scaled down version of 4K, so your computer is in fact driving 2x4K monitors (plus the overhead of resampling) and this is very resource intensive. The image quality on the FHD monitor is not great.

I see the same as above with gnome 3. Unfortunately sometimes it doesn't seem to actually work on the second run

Thanks for this script! I typically have to run it twice, as well, but I'll take that over the alternative!

It's wierd. I seem to have a similar setup as many, a Surface Pro with a res of 2736x1824 and an external display in FHD, yet, if I run it as it is, the external display windows look very small. I want its scale to be 1 and the internal display to look better to the eye (i.e. scale of 0.5), this seems to do the opposite?

If I try to set the correct scaling, it looks weird, I guess I am not getting the formula, maybe I have to multiply by 2 rather than divide in the offset calculation?

Thanks a lot for this script! To have the full external screen area usable, I had to add theses bits :

Get internal screen height,

int_h=`xrandr | sed 's/^'"${INT}"' [^0-9]* [0-9]\+x\([0-9]\+\).*$/\1/p;d'` 

Calculate total width calculation,

tot_w=$(( $ext_w+$int_w ))

And at last, append this option to the final xrandr command.

xrandr [...] --fb ${tot_w}x${int_h}

I have an easy to modify python script here with arguably more options: https://github.com/ashwinvis/xrandr-extend