Today was a lot of fiddling with xrandr in order to get my laptop screen and external monitor to play nice together.

While it was a bit confusing at first, the article on ArchWiki about HiDPI was very useful. There wasn’t a command on the page that was exactly what I needed, but between the wiki and the man pages, I was able to piece together what I needed.

Here’s how I figured out the numbers I needed to create a HiDPI display and a not HiDPI display that sit next to each other. My exact setup is as follows:

• My primary display is named eDP-1, has a resolution of 2560x1600, and a DPI of 227 pixels per inch
• My other display is named DP-1, has a resolution of 1920x1080, and a DPI of 166 pixels per inch
• I want my other display to the right-of my primary one

To get the DPI of my displays, I used this site, and just opened it in Chrome on each monitor.

To get the resolution of my displays, xrandr will tell you that if you just type xrandr into the terminal. It displays a little * next to the resolution. This is also how I got the name of each display too.

From the ArchWiki, I grabbed the following formula:

xrandr --output eDP-1 --auto --output HDMI-1 --auto --panning [C*E]x[D*F]+[A]+0
--scale [E]x[F] --right-of eDP-1

I had to make the following modifications though:

xrandr --dpi 227 --output eDP-1 --auto --output DP-1 --auto --panning
3840x2160+2560+0 --scale 2x2 --right-of eDP-1

Here’s what those changes mean:

• --dpi 227 is the higher DPI value between the two monitors I am using
• --output DP-1 was changed from --output HDMI-1 because that’s what my display is named
• --panning 3840x2160+2560+0 uses the following values for A,C,D,E,F:
  • A: 2560 (width of the primary display)
  • C: 1920 (width of the other display)
  • D: 1080 (height of the other display)
  • E: 2 (width scale factor)
  • F: 2 (height scale factor)
• --right-of means DP-1 should go on the right of eDP-1

The --panning was super helpful because otherwise, there’s a bug in xorg that makes it so you can’t reach half of the other display. It’s going to be fixed in xorg 1.20, which is coming with Ubuntu 18.10 I think.

In the mean time, --panning works great. Also, the scaling was a bit arbitrary I think, but --scale 2x2 was a result of trial and error and it looks great. To my understanding you can do decimal values, which I didn’t experiment with.

The last thing to do with all of htis is to throw my modified command into a shell script and thell i3 to run it on startup.

Oh and this whole xrandr thing has messed up the wallpaper of my other display. I think it’s definitely the scale option, but I’ll either fix it another day by tweaking the i3 settings or I’ll find a tiliing wallpaper.

With that, I am almost done tweaking this Ubuntu install! Things are coming along pretty nice.