the-fallen:
2: if it is higher than 1920x1080 (I only use the X-resolution) then calculate the factor
the current resolution is higher than 1920x1080.
That decision might not be so popular with people who use 27 inch and 30 inch monitors, which display 2560x1440 and 2560x1600 resolution. They have similar same pixel density as the smaller, more common consumer screens using 1920x1080 and 1366x768 and other low resolutions.
It will take about 100 lines of code and it will work at least on Linux and Windows
Really, only 100 lines?
You might be right, but you might also be underestimating the number of places in the Arduino IDE that have fixed pixel coordinates.
Maybe you ought to jump into this and write some of those 100 lines? I'll be really impressed if you manage to pull it off with so little code.