Determining map_view Pixel DPI

The other day I was trying to figure out the pixel DPI on my map_view so that I could size a raster_image on the map. I had always thought that !window_system!.screen_resolution() would do that for me. It seemed to work as long as I put in some additional empirical fudge factor. I discovered later that if I am working with map_view, I can use map_view.pixel_size to calculate DPI. map_view.pixel_size returns the size of a pixel in millimetres so to convert that to DPI...

_method map_view.if!dpi
      ## if!dpi : integer
      ##
      ## returns _self's DPI

      _return (1/_self.pixel_size) * 25.4
_endmethod