autoscale X/Wayland applications based on current DPI settings

autoscaled.sh

#!/usr/bin/env bash

program="${1}"
append=


# -----------------------------------------------------------------------------
# HiDPI decision

primary_found="no"
connected_displays=0
xdpi=1000
ydpi=1000
calc_dpi() {
    local screen="${1}" role="${2// /}" wpx="${3}" hpx="${4}" wmm="${5}" hmm="${6}"

    [ -z "${role}" ] && role="unknown"
    x=$(( ((wpx * 10 / ((wmm * 10) / 254)) + 5) / 10 ))
    y=$(( ((hpx * 10 / ((hmm * 10) / 254)) + 5) / 10 ))

    connected_displays=$((connected_displays + 1))
    if [ "${role}" = "primary" ]
        then
        xdpi=${x}
        ydpi=${y}
        primary_found="yes"
    elif [ "${primary_found}" != "yes" ]
        then
        [ ${x} -lt ${xdpi} ] && xdpi=${x}
        [ ${y} -lt ${ydpi} ] && ydpi=${y}
    fi

    echo >&2 "Screen: ${screen}, role ${role}, resolution ${wpx}x${hpx} px, dimensions ${wmm}x${hmm} mm, density ${x}x${y} dpi"

    return 0
}

eval "$(xrandr |\
    grep " connected " |\
    sed "s|^\(.*\) connected\(.*\) \([0-9]\+\)\+x\([0-9]\+\).* \([0-9]\+\)mm x \([0-9]\+\)mm.*|calc_dpi '\1' '\2' '\3' '\4' '\5' '\6';|g" |\
    grep ^calc_dpi
    )"

echo >&2 "System minimum screen density ${xdpi}x${ydpi} dpi"

#find_min_dpi() {
#	local t x y
#	for t in $(xdpyinfo | grep "dots per inch" | sed "s|^.* \([0-9]\+x[0-9]\+\) .*$|\1|g")
#	do
#		x="${t/x*/}"
#		y="${t/*x/}"
#		echo >&2 "Found DPI: ${t}, parsed as ${x} x ${y}"
#
#		[ -z "${x}" -o $((x + 0)) -eq 0 ] && x=96
#		[ -z "${y}" -o $((y + 0)) -eq 0 ] && y=96
#
#		[ ${x} -lt ${xdpi} ] && xdpi=${x}
#		[ ${y} -lt ${ydpi} ] && ydpi=${y}
#	done
#}
#find_min_dpi
#echo >&2 "Minimum screen DPI: ${xdpi} x ${ydpi}"
#
#connected_displays=$(xrandr | grep " connected" | wc -l)
#[ $((connected_displays)) -eq 0 ] && connected_displays=1
#echo >&2 "Connected displays: ${connected_displays}"

# decide if we need double scaling or not
HiDPI="yes"
[ ${xdpi} -le 168 -a ${ydpi} -le 168 ] && HiDPI="no"
# [ ${connected_displays} -gt 1 ] && HiDPI="yes"


# -----------------------------------------------------------------------------
# per application settings

if [ "${HiDPI}" = "yes" ]
then
    DEFAULT_SCALE="2"    # GTK/Qt applications - integer only
    CHROME_SCALE="2.1"   # google chrome/chromium settings
    FIREFOX_SCALE="2.1"  # firefox settings
    FIREFOX_FONT="16"    # in px
else
    DEFAULT_SCALE="1"    # GTK/Qt applications - integer only
    CHROME_SCALE="1.3"   # google chrome/chromium settings
    FIREFOX_SCALE="1.3"  # firefox settings
    FIREFOX_FONT="14"    # in px
fi

# -----------------------------------------------------------------------------
# helper functions

single_app_scale() {
    local s="${1}"

    export GDK_SCALE="${s}"
    export CLUTTER_SCALE="${s}"
    export QT_AUTO_SCREEN_SCALE_FACTOR="${s}"
}

firefox_change_setting() {
    local setting="${1}" value="${2}"

    for x in $(find ${HOME}/.mozilla/firefox -name prefs.js)
    do
        echo "firefox settings found: ${x}"
        dir=$(dirname "${x}")
        if [ ! -L "${dir}/lock" ]
        then
            if ! grep -q "^user_pref(\"${setting}\"," "${x}"
                then
                echo "Adding firefox setting ${settings} to ${value}"
                echo "user_pref(\"${setting}\", \"${value}\");" >>"${x}"
            fi
            
            echo >&2 "changing firefox setting ${settings} to ${value}"
            sed -i "s|^user_pref(\"${setting}\", \".*\");$|user_pref(\"${setting}\", \"${value}\");|g" "${x}"
        else
            echo >&2 "firefox is locked"
        fi
    done
}

firefox_set_ui_font_size() {
    local size="${1}"
    
    for x in $(find ${HOME}/.mozilla/firefox -name prefs.js)
    do
        echo "firefox settings found: ${x}"
        dir=$(dirname "${x}")
        if [ ! -L "${dir}/lock" ]
        then
            [ ! -d "${dir}/chrome" ] && mkdir -p "${dir}/chrome"
            if [ ! -f "${dir}/chrome/userChrome.css" ]
                then
                echo >&2 "Adding file: ${dir}/chrome/userChrome.css"
                cat >"${dir}/chrome/userChrome.css" <<EOF
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* set default namespace to XUL */
* { font-size: ${size}px !important; }
EOF
            fi
            
            echo >&2 "changing firefox UI font size to ${size}px"
            sed -i "s|^\* { font-size: .* \!important; }$|* { font-size: ${size}px !important; }|g" "${dir}/chrome/userChrome.css"
        else
            echo >&2 "firefox is locked"
        fi
    done
}

if [[ "${program}" =~ .*chrom.* ]]
then
    append="--force-device-scale-factor=${CHROME_SCALE}"
    
elif [[ "${program}" =~ .*firefox.* ]]
then
    firefox_change_setting "layout.css.devPixelsPerPx" "${FIREFOX_SCALE}"
    firefox_set_ui_font_size "${FIREFOX_FONT}"
fi

logger -p info "AUTOSCALE: ${*} ${append}"
exec "${@}" ${append}

Run any X11/Wayland application like this:

/path/to/autoscaled.sh application [app params]

The script is to be used on 4K screens that have different DPIs. So, on the high density laptop screen you get 2.x scaling, while on a desktop screen you get 1.x scaling.