CMYK color mode Gimp 2.10

[rich2005]

(07-18-2025, 10:44 AM)Jānis Wrote: Thanks, putting it in "~/.config/Cyan/icc/" worked!

Only, when I open the converted file in GIMP again, "the out of gamut" marker still paints about the same areas as out of gamut than before the conversion. Shouldn't converting to CMYK get rid of all unprintable colours?

The Gimp 2.10 softproofing is a preview of what to expect when printed using the specified profile. Using Cyan plugin just creates a color-separated CMYK file.  You might see some change if you open the original then open as layers the cmyk file. Although Gimp is 100% RGB so the cmyk image is converted.

If you want to lose the out-of-gamut values then before using cyan, use soft-proofing and adjust the color curve until the out-of-gamut marker vanishes. This more like the actual printed result.

This example a bit extreme, usually a small adjustment on the bottom left makes a big difference.

   

I would just leave as-is. Generally the printer does the correction. and accept that bright colours become dulled.

[Jānis]

I don't think I would even notice colours becoming a bit more dull...

We printed a book, 27 copies, mostly just for our relatives/friends cycle. The cover turned out to be awful. It's so dark, it's hard to distinguish any details or colours. I don't think it even contains those neon and bright colours everybody is warning about.

Here is the most part of the artwork of the cover, how it looked originally, in the monitor, and how it approximately looked when printed:
   

Technically, I got the second image by passing in the first image to rgb2cmyk.org, saving it as TIFF and then opening in my regular image viewer. (If opened in GIMP, it is displayed lighter.) And then putting the first and second images together and making a screenshot of both.

But, anyway, this is approximately the feeling I get when I view the original artwork on screen and the actual book in real life side by side.

Small adjustments don't seem to make a big difference. If I raise the bottom left and lower the top right in the levels, until the out-of-gamut overlay disappears, I get a very washed out image. I don't even understand why darkness is a problem, when the actual book is so much darker than it should have been. Apparently, they can print dark image just fine...

This is the original part of the cover art:
   

Quote:Using Cyan plugin just creates a color-separated CMYK file.

I apparently don't understand this really.

Let's assume, there are 6 colours my screen can display – A,B,C,D,E,F, and there are only 3 colours the printer can print – a,c,e.
A is very similar to (is the same as) a, C to c, E to e.
B, D, F is out of gamut.
But B is the most similar to a, D to c, F to e.

So, if my image is a flag of three bands in colours A, C, and F (F being out of gamut),
when the printer tries to print it, it should print the colours a, c, and e – if it's smart enough.

That's why I expected that converting the RGB image to the CMYK image with the printer-specific profile would convert the image so that
colour A is stored as a, C as c and F as e (let's say that matching F to e depends on the algorithm being used, perceptual, relative colorimetric + black compensation or not etc.).

And now, when I import it back into GIMP, which converts it to RGB, the colours should be converted back to a => A, c => C, e => E.
Meaning, the colour F could now be colour E, which I would see as a bit changed image, but at least now the whole RGB image should be within the CMYK gamut.

At least, that's what I thought or hoped for. Are you saying that that's not what actually happens?

[rich2005]

Looking at your images, not much there that should be out-of-gamut but of course it does depend on the icc.

The old advice still holds:

Some RGB colors that you can see on your monitor (in particular, blue, green and all bright vibrant colors) cannot be printed and/or replicated with standard CMYK inks. Keep in mind that, when creating a file for print, you should always make the original file in CMYK color mode before starting to work on it.

So next time maybe use krita which has a cmyk mode.

However do your own test. With pure RGB patches, (1) write a cmyk image using cyan and open it back into Gimp (converted back to sRGB) (2) Still lots out-of-gamut (3) that benefit from a correction and a re-export using Cyan.
...or you can leave it. The printer will reduce anything out to in.