GIMP Version: 2.8.10
Operating System: Windows

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I need to print out a A1 size poster for my school assignment and the printing company only accepts images in CMYK colours.

the poster template given by the school is in a PowerPoint .pptx format, and due to the limitations of Microsoft PowerPoint, if I use Microsoft PowerPoint to export the PowerPoint slide as a JPEG/TIFF file, the max DPI is only 93, and in RGB only.

so then I taught of exporting the PowerPoint slide as a PDF file, and make use of GIMP to convert it to a high DPI CMYK image.

but then I realized that GIMP does not have a CMYK option so now I'm stuck.

the printing is going to take a few days and my deadline is coming up soon so any help is greatly appreciated.

by the way, my poster has a number of photos so the colour accuracy is somewhat important.

First thoughts,

A1 is quite a big size - half a square metre - 594 x 841 mm

At 200 ppi that is an image 4678 x 6622 pix, at 300 ppi 7017 x 9937 pix

Assuming this is a poster (CMYK is the giveaway) rather than a (very expensive) photographic print then printing companies usually have a limit on dpi value and 200 ppi just be might be ok.

It is going to be a large-ish file in MB, whatever format, so a webpage on-line conversion is probably out.

Current state of conversion apps.
Try and find someone with a copy of PS.
or
Krita will open gimp images and other formats and change the mode from RGB to CMYK. Worth a try.



The colours will change on conversion from RGB to CMYK, nothing you can do about it.

i tried out one of those online converter with a lower dpi image and the results are horrible.

the entire poster is a tone lighter and the green looks yellowish.

thx... will give Krita a try and see how it goes.

i installed Krita and it looks promising but due to some Krita bug, i can't export the image as 16/32 bit CMYK.

exporting as 8 bit works but the colours are off.

damn this RGB to CMYK issue is trying me nuts!!!

OK, back from the dentist and I prepared some screenshots

For a poster print 8 bit is OK. Nothing gained by 16 bit since all your editing has be done in Powerpoint.

As before there is a colour change . CMYK has a smaller gamut than RGB . You will have to live with it. The correct way is to start and finish in CMYK mode, then WYSIWYG.

FWIW A test in Krita. I did manage to crash it once but I think this is the best sequence.

The Image menu contains both the resize and convert colour space options.

Convert the colour space first.

screenshots http://i.imgur.com/xd4fczv.jpg

Once that is run, then resize. Just go for the A1 size in millimetres and set a dpi in inches. For a print that size I do not think it is worth going above 200.

screenshots http://i.imgur.com/w8eDqAL.jpg

The result showing image properties in XnView.

Left side is a tif from Gimp I would think your tif from powerpoint will be about the same size.
Right side is the tif from Krita, image dimensions, dpi, colour mode, all ok, but it is a large file

screenshots http://i.imgur.com/BgeGjzV.jpg

Check out Scribus it's a page layout program, CMYK, ICC color management, PDF. It also has a handy Preflight Verifier.
In future look into color management options. The color management link contains info on, Gimp, Krita, digiKam/showFoto, and Cinepaint.