vincentbernat.i3wm-configuration/bin/xsettingsd-setup

#!/bin/sh

showdpi() {
    output=$1
    pixels=$2
    mm=$3

    # Compute DPI
    dpi=$((pixels * 254 / 10 / mm))

    # For laptop screens, we need to apply a correction factor as the
    # screen is nearer
    case $output in
        eDP*) corrected=$((dpi * 96/144)) ;;
        *) corrected=$dpi ;;
    esac

    # Authorized factors: 1, 1.25, 1.5, 2, 3, 4, ...
    rounded=$(((corrected + 12) / 24 * 24))
    [ $rounded -gt 168 ] && rounded=$(((corrected + 48) / 96 * 96))
    [ $rounded -lt 96 ] && rounded=96

    echo "$output: ${dpi}dpi (corrected to ${corrected}dpi, rounded to ${rounded}dpi)" >&2
    echo "$rounded"
}

# Examples:
# showdpi HDMI-A-0 3840 527
# HDMI-A-0: 185dpi (corrected to 185dpi, rounded to 192dpi)
# showdpi DisplayPort-3 3840 880
# DisplayPort-3: 110dpi (corrected to 110dpi, rounded to 120dpi)
# showdpi eDP-1 2560 310
# eDP-1: 209dpi (corrected to 139dpi, rounded to 144dpi)

# Compute DPI of each screens
dpis=$(xrandr --current \
    | sed -En 's/^([^ ]+)* connected.* ([0-9]+)x.* ([0-9]+)mm x .*/\1 \2 \3/p' \
    | while read output pixels mm; do
    showdpi $output $pixels $mm
 done \
    | tr '\n' ' ')

# Use first screen DPI
dpi=${dpis%% *}
dpi=${dpi:-96}

echo "using ${dpi}dpi" >&2

# Xrdb update
{
    echo Xft.dpi: $dpi
    echo Xft.rgba: $( [ $dpi -gt 144 ] && echo none || echo rgb )
} | xrdb -merge

# X server update (+ trigger update)
xrandr --dpi $((dpi-1))
xrandr --dpi $dpi

# Build xsettingsd
mkdir -p $XDG_RUNTIME_DIR/i3
{
    cat  ~/.config/i3/dotfiles/xsettingsd
    echo Xft/DPI $(( $dpi*1024 ))
    echo Xft/RGBA \"$( [ $dpi -gt 144 ] && echo none || echo rgb )\"
    echo Gdk/WindowScalingFactor $(( $dpi/96 ))
    echo Gdk/UnscaledDPI $(( $dpi*1024/($dpi/96) ))
} > $XDG_RUNTIME_DIR/i3/xsettingsd.conf