Version 3 of screen distances

These are the valid screen distances, according to the man page for Tk_GetPixelsFromObj, Tk_GetPixels, Tk_GetMMFromObj, Tk_GetScreenMM:

<none> The number specifies a distance in pixels.
c The number specifies a distance in centimeters on the screen.
i The number specifies a distance in inches on the screen.
m The number specifies a distance in millimeters on the screen.
p The number specifies a distance in printer's points (1/72 inch) on the screen.