Nice! Excellent information.
So does Inkscape ever set a value for these or is it always 0 (unknown unit?)
Not that it really matters all that much... Any print company worth anything will know that a raster file of pixel dimensions 2480 pixels x 3508 pixels is A4 @ 300dpi (no bleed)
Yes, I've encountered printers who didn't, but it's pretty basic stuff, so I tend to avoid companies that can't get DPI right. :)
It's generally advisable to send a pdf, which includes the relevant size information, since it properly stores all that stuff, but imho, there's nothing wrong with popping out a png at the proper pixel dimensions and saying "I'd like this printed at A4 300 DPI". That should be more than enough information for any printer to get it the right size.
It's been on my to-do list for a while to make a video on DPI.
It's quite astonishing how many designers (and printers) just don't understand it.
Thanks for the info. It would be a good idea to make use of the png unit specifier, however I don't know how many pieces of software will read the value and use it, even if we include it.
-C