Configuring mixed DPI monitors with xrandr

I've recently started using a high DPI laptop together with a regular DPI monitor in a dual monitor setup. While there are GUI tools to configure dual monitors, I wanted to do it through the CLI. The usual way to configure monitor settings via the CLI in Linux is with a program called xrandr. As it took me a bit of trial and error to get everything working well, I thought it would be useful to write about the commands I settled on and how they work.

The setup

The laptop sits on the left of the monitor. The tops of both line up.

Usually I want to use both monitors at the same time. However I've found it necessary to disable one of them, usually the laptop, when I want to share my screen. This means it is important that I can easily switch back and forth between dual and single monitors.

It is also important that I retain the ability to use the laptop without the external monitor.

The tricky parts

There were two aspects I found problematic:

First, the fact that the two monitors have differing DPI means each monitor needs different settings to account for that. xrandr allows setting the DPI via --dpi, but this applies globally to all monitors. I dealt with this by setting the global DPI to that of the highest DPI monitor, and then used --scale to adjust the lower DPI monitor.

Second, switching back and forth between dual monitors to one monitor lead to interesting behaviour. For example, I kept losing the scale setting on the lower DPI monitor.

I had a semi-working setup with little trouble, but it involved restarting X when I wanted to switch between dual and single monitors. This worked, but obviously was inconvenient. I wanted a solution that would work without restarts.

The solution

I came up with two xrandr commands. One configures to use both monitors, and the other configures to use only the external monitor.

Dual monitors

When I want to use both monitors, this is the command I run:

xrandr --dpi 276 --fb 7040x3960 \
    --output eDP-1 --mode 3200x1800 \
    --output DP-1-2 --scale 2x2 --pos 3200x0 --panning 3840x2160+3200+0

Here's an explanation of the options:

• Global options:
  • --dpi 276 sets the DPI to 276.
  • --fb 7040x3960 creates one screen with resolution 7040x3960. This is the combined resolution of the two monitors. The high DPI monitor has 3200x1800 resolution. The lower DPI monitor has 1920x1080 resolution, but I double it as I scale it by 2 (see below). Combine these like so: 3200+1920*2 x 1800+1080*2 = 7040x3960. Both monitors share this screen.
• High DPI monitor options (--output eDP-1):
  • --mode 3200x1800 says to use resolution 3200x1800. This is the default, but specifying it is necessary if the monitor is disabled (as it is when using the external monitor by itself) as it enables the monitor.
• Lower DPI monitor options (--output DP-1-2):
  • --scale 2x2 is similar to zooming out. This is necessary as we set the DPI globally to that of the high DPI monitor.
  • --pos positions it to the right of the laptop monitor.
  • --panning allows our pointer to access the whole area.

Single monitor (external)

If I want to use only the lower DPI monitor, then I run this command:

xrandr --dpi 276 --fb 3840x2160 \
    --output eDP-1 --off \
    --output DP-1-2 --scale 2x2 --panning 3840x2160

Most of these options are similar to the first command's. The main difference is disabling the laptop monitor with --output eDP-1 --off.

Note 3840x2160 comes from 1920*2 x 1080*2.

You might wonder why I set high DPI and scale the lower DPI monitor here. Wouldn't it be simpler to set a lower DPI and use the monitor's native resolution? That's possible, but since I've configured other things (such as my X terminal fonts) to a size suitable for a high DPI screen, I'd have to adjust settings elsewhere. Keeping the DPI the same and continuing to scale allows me to avoid that. Since I'm scaling, I need to configure a large screen and set panning for the same reasons as before.

Single monitor (laptop)

This is the default state prior to running any xrandr commands. In theory I could have another command for this, but so far I haven't needed to switch to this.

A final note: --mode vs --auto

I initially tried using --output eDP-1 --auto when I wanted to enable the laptop monitor after disabling it. However, I found this lost the --scale setting, so I had to either restart X or run the same xrandr command twice. Using --mode does not have this problem.

Resources

I did a bunch of searching figuring out this setup. The two best resources I found were these:

• The xrandr(1) man page. It breaks down xrandr's options by those that apply globally vs. those that apply per output which I found helpful.
• The Arch Linux wiki HiDPI article. This article has a lot of good high DPI related information, including many xrandr examples.

Tags: Linux, cli, xrandr, monitors, laptop

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