Warp wrote:
>   But I don't understand how an image file can have a PPI.
>   The computer probably doesn't even know the physical size of your
> monitor. How can it figure out its PPI?

In fact, it likely does!  DDC carries information about the physical 
dimensions in centimeters.  And the DDC extensions for digital displays 
provide the pixel width and height (separately, of course) of to 0.01 
millimeter precision.  You can even get information about the RGB pattern 
used by the display.  All displays with DVI interface are required to 
support DDC, and even most VGA monitors have supported DDC for many years. 
DDC is the feature that allows i.e. Windos or Mac OS to detect the valid 
display settings you may select.  It also allows you to modify some 
parameters of _some_ displays, i.e. brightness or contrast via your computer 
rather than pressing buttons (an optional feature, depends on display!).

To get an overview of all the information flowing between your monitor and 
your computer, check out <http://www.vesa.org/Public/EEDIDguideV1.pdf> (for 
most displays) and <http://www.vesa.org/Public/EDID%20EXTENSIONS/DIEXT.pdf> 
(for additional information supplied by digital displays).

	Thorsten

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