I have a monitor with resolution 1440x900, 19 inch, whose PPI is 89.37.

I can set the screen DPI with command:

# xrandr --dpi 100

But the problem is my monitor's PPI is only 89.37, how can xrandr command sets the monitor's DPI bigger than PPI? (From my understanding, PPI is monitor's property, and it cannot be changed, but DPI is something that can be tuned to get a better display, am I right?)

So, my problem is:

  1. What happens beneath OS (or how OS handles this?) if the DPI is bigger than PPI?
  2. What happens if the DPI is bigger than PPI?

The “DPI” setting that you can set with xrandr is purely an indication to applications, it doesn't configure the hardware. Normally the monitor reports to the computer what its resolution and pixel density is. You can replace the reported value with a fake one if you want, or set a value if the monitor doesn't report any. The OS doesn't care as such, the DPI settings are only used by applications that want to draw something at a certain size, as a multiplier to convert from a distance unit to pixels. For example, if an application wants to typeset text that's 1/6 in tall, then it would multiply 1/6 by the DPI setting to find that it should use a 15px font with the monitor's default setting and a 17px font with a 100 DPI setting.

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