I've been asked to create an image of 25mm x 11mm, with 300 DPI. I'm far from an expert so I've simply tried to create a new image in GIMP with these specifications.
The program doesn't allow me to use exactly 25mm x 11mm, the values are automatically adjusted (see screenshot).
I assume this is some constraint based on pixel to mm conversion or something like that.
What exactly causes this constraint and is there no way to create a image with the previously mentioned specifications?
Because you are working with mm units, and the PPI uses imperial units.
Because if you have exact mm you need to have halved pixels, which you can not, so the number is rounded to its nearest pixel count.
Let me do some math for you.
2.5 / 2.54 = 0.984 inch.
0.984 inch x 300 ppi = 295.275 px.
Not quite an answer, and too big for a comment, but bear in mind that "300ppi" is simply a useful rule of thumb.
There is a thing called the Shannon-Nyquist Sampling Theorem which, at its most basic says you need to sample twice as frequently as the source frequency in order to accurately reproduce the source frequency from the data.
For printing, they use halftone screens which are (were) typically in the range of 100 to 150 line per inch, with 100 being "newspaper quality" and 150 being "fine art book quality." You will quickly notice that 300 is twice 150, and so 300 was considered "the most you will need."
So the 150lpi screen, in metric units is very close to 6 lines per millimeter, which means we want 12 pixels per mm or 120 pixels per centimeter.
So if you routinely work in metric units, then you might as well use 120px/cm as your new "rule of thumb."
If you are not working for print, then just specify exact pixel dimensions always.
