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Can the print size AND the resolution both be changed by using the Image|Scale image option? Apparently one can change both by using this command.
It can, the trick is set the pixels-per-inch (ppi) before touching the width / height setting.
30 second animation of that
https://i.imgur.com/fh1DNUc.mp4 and Gimp will do the calculations as well.
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Also, there are multiple Interpolation options like Linear, Cubic, NoHalo...
Use the most modern option NoHalo.
...but up-scaling to 24x36 @ 300 ppi is about 230% and for poster sizes photographic quality 300 ppi is not always necessary since viewing distance comes into play. see:
http://resources.printhandbook.com/page ... inting.phpThere is an equation for ppi(dpi) taking a viewing distance 36" DPI = (1 / ((36*.0.000291) / 2))
Just looking at a similar size print you would not normally be much closer and that gives a ppi of about 190 say 200 ppi. A scale factor of 150% a bit more reasonable.
The other thing to consider if sending to a commercial printer, let them do the scaling, their printer scaling algorithms every bit as good as Gimps.