If you let one finger to draw a line as thick on a different screen!

The thickness of each person's finger is (a period) unchanged, but may use a variety of screen resolution of the phone or tablet, in the above painting line, how do the drawn line is the same feeling in the visual thickness.

Assuming a horizontal line is drawn, the line height is required in different screens are the same, but they are not the same pixel, so the question should be how to deal with it?

Should first consider the physical height of the line, right? Such is 3mm, based on the pixel density of the screen (how many pixels per millimeter), after replacing the pixel value, and then set the pixel line width when drawing lines.

What kind of indicators can be used?
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