Hmm this is probably it...
$ xdpyinfo ... screen #0: dimensions: 768x1222 pixels (0x0 millimeters) resolution: 2147483647x2147483647 dots per inch
Hmm this is probably it...
$ xdpyinfo 483647 dots per inch
...
screen #0:
dimensions: 768x1222 pixels (0x0 millimeters)
resolution: 2147483647x2147