"ImageResolution_dpi" defines the mapping of a length coordinate to the resolution of the output device. The resolution [dpi] is defined as "dots-per-inch" and therefore a length L_mm defined in [mm] is mapped to a length L_dot in dots (or pixel) with the formula:
L_dot = round(ImageResolution_dpi/25.4 * L_mm)
where function round(..) rounds to the nearest integer. Typical values are "96 dpi" (for screen) or "600 dpi" for printer. For example if an "ImageResolution = 96 dpi" shall be used for a screen, then 1 mm is mapped to 4 pixel.