calculate screen DPI

dpi.py

#!/usr/bin/env python

# calculate screen DPI
# supports both python 2 and 3

import re
import argparse
from math import sqrt
from subprocess import check_output

__xlib = False
try:
    # note: Xlib is buggy and prints messages in the stdout
    from Xlib import display
    __xlib = True
except ImportError:
    #print("Cannot load Xlib, falling back to xrandr")
    pass


def cmd_parse():
    """Parse command line arguments"""
    parser = argparse.ArgumentParser(
        description="DPI calculator")

    parser.add_argument("-r", "--resolution", action="store",
        help="Screen resolution in WxH format")

    parser.add_argument("-a", "--ratio", action="store", default="16:9",
        help="Screen aspect ratio (default: %(default)s)")

    parser.add_argument("-d", "--dimension", action="store", type=float,
        required=True, help="Screen diagonal size (in inches)")

    parser.add_argument("-n", "--normalize", action="store_true", default=False,
        help=("Normalize DPI to reduce scaling artifacts to GUI that use bitmaps"
            " (default: %(default)s)"))

    parser.add_argument("--verbose", action="store_true", default=False,
        help="Show a more detailed result")

    return parser.parse_args()


def _parse_resolution(resolution):
    """Return width and height from a string formatted
       as 'WidthxHeight'"""
    return resolution.split('x')


def _parse_ratio(ratio):
    """Return x and y units from a string formatted as 'X:Y'"""
    x, y = ratio.split(':')
    return float(x), float(y)


def get_resolution_from_X():
    s = display.Display().screen()
    return (s.width_in_pixels, s.height_in_pixels)


def get_resolution_from_xrandr():
    output = check_output('xrandr', universal_newlines=True)
    is_resolution_line = re.compile(r"\*").search
    for line in iter(output.splitlines()):
        if is_resolution_line(line):
            resolution = line.split()[0]
            break

    return _parse_resolution(resolution)


def get_physical_dimensions(diagonal, w, h):
    ratio = w/h
    height = sqrt((diagonal * diagonal) / (1 + ratio * ratio))
    width = height * ratio
    return round(width, 2), round(height, 2)


def calculate_dpi(width, height, p_width, p_height):
    dpi_w = width / p_width
    dpi_h = height / p_height
    return dpi_w, dpi_h


def normalize_dpi(dpi):
    dpi_values = (96, 120, 144, 168, 192)

    new_dpi = dpi
    minimum = 1000
    for dval in dpi_values:
        difference = abs(dval - dpi)
        if difference < minimum:
            minimum = difference
            new_dpi = dval

    return new_dpi


if __xlib:
    get_resolution = get_resolution_from_X
else:
    get_resolution = get_resolution_from_xrandr


if __name__ == "__main__":
    args = cmd_parse()
    if args.resolution:
        width, height = _parse_resolution(args.resolution)
    else:
        width, height = get_resolution()

    x, y = _parse_ratio(args.ratio)
    p_width, p_height = get_physical_dimensions(args.dimension, x, y)


    dpi_w, dpi_h = calculate_dpi(int(width), int(height), p_width, p_height)
    dpi = int(round(dpi_w))

    if args.normalize:
        dpi = normalize_dpi(dpi)

    if args.verbose:
        print("Width: {} Height: {} DPI: {}".format(width, height, dpi))
    else:
        print(dpi)

Hello Tasos,

I've come across this script googling the following question:

I've recently replaced my old 19" 1360x768 monitor with a 23.8" 2560x1440 one.
The recommended DPI is 100% in the smaller monitor and 125% in the other.

With 125% DPI, the icons in various applications are re-scaled and I don't like it.
Setting the DPI to 100%, everything is too small.

Is it possible to get a decent display and keep the original images size?

I've also realized that a 19" monitor would be more convenient to me.
So my main question is: how can I be sure that the recommended DPI in a potential monitor should be 100%?

I'd be grateful for your help.

Yaron

Hi @Yaron10,

It has been a long time since I used this (around 2013), so to be honest I am not entirely sure if this is possible. Since then I am guessing that Desktop Environments have gone a long way to property handle dpi 🤔

You can check the Archlinux Wiki at least this is what I used back then. It seems that if you use Gnome you can scale text individually as seen here. But again this depends on the DE you are using.

Hello Tasos,

Thank you for the detailed reply. I do appreciate it. 👍

Best regards.