Dots Per Inch

• Horizontal and Vertical DPI isn't the same
• DPI requires knowing the screen size and the resolution

• But one can scale around by adjusting DPI without knowing what the natural DPI is

• Screen DPI is different from the System DPI, which is what we are usually interested in

• Even if EDID (see Adjusting Monitor Brightness in Linux) helps in knowing the physical dimensions, this does not work for a projector (see Colour Systems).