How change display scale from the command line in Ubuntu 18.04 (xorg)

I am trying to get in the command line the same effect as changing the display scale factor from the Gnome Control Center.

I have tried the following command, but it does not have any effect:

gsettings set org.gnome.desktop.interface scaling-factor 2

I need this command to build a sort of fractional scale factor with the combination of xrandx and the scaling factor in the Gnome Control Center. Since, the execution of xrandr resets the scaling factor in the Gnome Control Center, I would need a command to recover the value in the Gnome Control Center.

Please, any suggestion is welcome. Thanks in advance.

Solution 1:

Determine your output device (mine is DP-1) by running xrandr on its own, then use this:

gsettings set org.gnome.desktop.interface scaling-factor 2
gsettings set org.gnome.settings-daemon.plugins.xsettings overrides "{'Gdk/WindowScalingFactor': <2>}"
xrandr --output DP-1 --scale 1.5x1.5
xrandr --output DP-1 --panning 3840x2160

Solution 2:

Looking at the gnome-control-center source code, this turns out to get configured via dbus. Here’s a script to toggle between 100% and 200% in a single-monitor configuration.

#!/usr/bin/python3
# Note: use system python3, not /usr/bin/env, because whichever python3 is on
# $PATH may not have dbus, but the system python3 does.

"""Toggle display scaling between 100% and 200%.

Based on https://gist.github.com/strycore/ca11203fd63cafcac76d4b04235d8759

For data structure definitions, see
https://gitlab.gnome.org/GNOME/mutter/blob/master/src/org.gnome.Mutter.DisplayConfig.xml
"""

import dbus

namespace = "org.gnome.Mutter.DisplayConfig"
dbus_path = "/org/gnome/Mutter/DisplayConfig"

session_bus = dbus.SessionBus()
obj = session_bus.get_object(namespace, dbus_path)
interface = dbus.Interface(obj, dbus_interface=namespace)

current_state = interface.GetCurrentState()
serial = current_state[0]
connected_monitors = current_state[1]
logical_monitors = current_state[2]

# Multiple monitors are more complicated. For now, since I only use one monitor
# in Ubuntu, everything is hard-coded so that only info about the first monitor
# is used, and only it will be connected after running the script.
#
# If someday updating this script: a logical monitor may appear on mutiple
# connected monitors due to mirroring.
connector = connected_monitors[0][0][0]
current_mode = None
# ApplyMonitorsConfig() needs (connector name, mode ID) for each connected
# monitor of a logical monitor, but GetCurrentState() only returns the
# connector name for each connected monitor of a logical monitor. So iterate
# through the globally connected monitors to find the mode ID.
for mode in connected_monitors[0][1]:
    if mode[6].get("is-current", False):
        current_mode = mode[0]
updated_connected_monitors = [[connector, current_mode, {}]]

x, y, scale, transform, primary, monitors, props = logical_monitors[0]
scale = 2.0 if scale == 1.0 else 1.0

monitor_config = [[x, y, scale, transform, primary, updated_connected_monitors]]

# Change the 1 to a 2 if you want a "Revert Settings / Keep Changes" dialog
interface.ApplyMonitorsConfig(serial, 1, monitor_config, {})