vincentbernat.i3wm-configuration/bin/xsettingsd-setup

#!/bin/sh

DISPLAY=${DISPLAY%.0}

# For information
xrandr --current \
    | sed -n 's/^\([^ ]\)* connected.* \([0-9]*\)x.* \([0-9]*\)mm x .*/\1 \2 \3/p' \
    | while read output pixels mm; do
    dpi=$(($pixels * 25.4 / $mm))
    echo "$output: $dpi DPI"
done

# Don't try to guess DPI. For a laptop, we don't want the same DPI as
# for an external screen. Just hardcode stuff...
case $(hostname),$(autorandr --current) in
    zoro,default)       dpi=144 ;;
    guybrush,default)   dpi=144 ;;
    neo,*)              dpi=192 ;;
    *,*)                dpi=96 ;;
esac

change() {
    # Build xsettingsd.local
    {
        cat  ~/.config/awesome/xsettingsd
        echo Xft/DPI $(( $1*1024 ))
        echo Xft/RGBA \"$( [ $1 -gt 144 ] && echo none || echo rgb )\"
        echo Gdk/WindowScalingFactor $(( $1/96 ))
        echo Gdk/UnscaledDPI $(( $1*1024/($1/96) ))
    } > ~/.xsettingsd

    # Signal xsettingsd
    systemctl --user reload xsettingsd@$(systemd-escape -- "$DISPLAY").service

    # Also use xrdb for very old stuff (you know, LibreOffice)
    echo Xft.dpi: $dpi | xrdb -merge
}

change $dpi