I have:
Qt Code: 
  1. chrisw@newton /tmp/test/m $ ./m
  2. Running in CONSOLE mode
  3. physicalDpi: x=75 y=75
  4. logicalDpi: x=75 y=75
  5. chrisw@newton /tmp/test/m $ ./m 1
  6. Running in GUI mode
  7. physicalDpi: x=93 y=95
  8. logicalDpi: x=93 y=95
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on my Linux box. I used
Qt Code: 
  1. image.setDotsPerMeterX(3937);
  2.     image.setDotsPerMeterY(3937);
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to set 100DPI and the resulting saved images had the same DPI figures installed (and hence the same 'print size'). The distinction is only really relevant if you are going to put the image pixel-for-pixel onto a device with a different DPI and expect the same physical dimensions on the image.