The Sad Story of PNG Gamma “Correction”
hsivonen.iki.fi
hsivonen.iki.fi
DPI is only remotely meaningful when dealing with something that was once printed (set it on scans so you know the original size), or soon will be (know how big to print it). In practice there's so much completely bogus DPI metadata out there that little software will respect it on output. For further brokenness, DPI is cleverly stored as a Rational in most formats, but nearly all metadata libraries expose it only as a float!
There's a ton of software out there written by some twit that read on the internet somewhere that "Macs are 72dpi, everything else is 96dpi", and then goes on to set that in every file it outputs depending on the platform. Inkscape does this and it is incredibly obnoxious.
edit: oh! and speaking of tagging digital formats with physical scale information, it took me ages to understand why text processors measure font size in points, which here refers to absolute size when printing, but don't display the text at that size on the screen (which is where it matters more, imho). maybe things are different now, but the only thing I remember from the adventure was that I decided never to touch anything typographic.