Sorry for barging in here like that but I've got a really hard headache and I hope someone in here may have had the same plus have survived and are feeling generous enough to share their secrets. A senior guru with a single-fix-for-all solution is fine too. But hey, I imagine this should be a very common issue, so this whole lengthy post with any contributions should be of great help to our future generations of gimps.
The goal is extremely simple: take a picture, any picture, and print a poster of it a.k.a blow it onto a chosen number of A4 papers.
I've searched for such a plugin or script but I'm pretty much sure there is none, so the closest one I came up with was slice-save.scm which is basically Guillotine that saves the created images into files rather than opens them as new.
For the sake of simplicity let's say I want to have a picture 8x8 A4s big, landscape. So that's 64 pages totalling up to 2.38 meters wide and 1.68 meters tall (in reality that'd be a bit less, but more on that at the end).
So here's my version of the final solution:
1) Create a new A4-sized image on 300dpi (I've assumed 300 should be enough for a mediocre human eye to not notice the pixels). You get an image that's 3508px x 2480px big. Do Image > Canvas Size and with aspect ratio preserved try and work out how many pixels you will need to add to your poster picture to make it match the aspect ratio of an A4 page. That is, enter the width of your poster picture into the width of the A4 canvas; if the new height is less that the height of your poster picture, you will need to enter the height and note down the width instead.
2) The scripts we're gonna be using is based on guides, so open up calculator, and find out where your guides will be situated at. For that you will need to take the width of an A4 paper, for a 300dpi one you take 3508, hit "+", type in 3508 and hit "=". Note the resulting number (7016) down, hit "=" again, note it down, hit "=" again, note it down, etc. Do the same for height (2480). If you've done it right, you should now have a similar list:
1= 3508 x 2480
2= 7016 x 4960
3= 10524 x 7440
4= 14032 x 9920
5= 17540 x 12400
6= 21048 x 14880
7= 24556 x 17360
8= 28064 x 19840
So all the widths here will be vertical guides, and all the heights will be horizontal guides. Obviously you don't need a guide for the last one, but don't lose that number since that will be the final size of your poster image (I know, it's a dozen gigs uncompressed or ~40 megs as a PNG)
3) Okay here's goes the "fun" bit: after you've added the necessary pixels for your poster picture to match the ratio of an A4 paper (step 1) you now do Image > Scale Image, choose Cubic for interpolation and enter 28064 into width. After you've chosen "Scale" you are free to leave your computer, go out into the real world meet people etc. Leave it for 10-30 minutes. On your return you will see your computer either: a) crashed b) working c) still processing.
4) Let's assume you're very lucky, so what do you do when you've gotten away with something really big? You push the limits of abuse further: Filters > Distorts > Newsprint. I found this effect makes the poster look "cool" and sends the enduser a subliminal message to quit looking for pixels and enjoy the art. Again you go out and do something useful with your time.
5) If you're still feeling lucky save the image as a PNG or XCF, because your luck is about to run out. Go for another walk.
6) Alright, time for guides: bring up your notes (step 2) and set up the 14 guides according to them: 7 vertical and 7 horizontal
7) Assuming you've already got the slice-save.scm script in your GIMP-2.x/share/gimp/x.x/scripts (if not do that and go to Filters > Script Fu > Refresh Scripts), start the Script-Fu > Slice and Save, choose PNG, choose a destination, and prepare for what should've came at you long time ago.
8) Heh, sorry but you are unlikely to get past step 7, because your computer will either have crashed due to a lack of memory or ran out of disk space and crashed into oblivion. I've got quite a modern core duo with 7 GIGS of free space on my drive C: and the quota was nearing 100MB with GIMP haven't even processed page 6 out of 64
Anyone's got any suggestions?
By the way, ideally there would be a step 9 where you'd add a single pixel border around each exported file - that would come useful when trying to figure out at what angle you need to cut each sheet, assuming your printer is just like any other common printer, unable to fetch the paper correctly, resulting in contents being printed with margins of varying angles.