CMYK issue: Krita or Cyan - Printable Version

CMYK issue: Krita or Cyan - Printable Version

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CMYK issue: Krita or Cyan - enrimoore - 01-04-2025

Hello everybody and happy new year

I am asking here: Cyan is integrated with Gimp.
I need to convert RGB to CMYK for printing.
I wanted to use Cyan or Krita, they seem equivalent.
I found something very odd to me, so after a lot of tests, imho the issue is that
whatever the optional settings, with the exact same conversion to CMYK, tif and jpg :
a difference of about 95% exists between the jpg and tiff versions
(using a very consistent application)

The two CMYK versions, tif or jpg, are not even similar obviously. Even visually.

Many other tests suggest that this cannot be caused just by jpg loss of quality.
I would use the tif format, but this strange total difference seems worrying to me.

No idea if somebody way more knowledgeable, can suggest something.
I emailed the developer of Littlecms, but I don't know if they will clarify.
Maybe somebody can just confirm that the tiff version should be correct enough?
Btw, the only format producing an output equivalent to tif, is Photoshop, psd format.
Also, the result is the same with Krita or Cyan. I think they use Littlecms.

Thanks for any help or suggestion and best
enri

RE: CMYK issue: Krita or Cyan - rich2005 - 01-04-2025

Interesting question. I do know of differences between applications (Gimp / Krita / PS / even Imagemagick) using the same source RGB image. There is another one now, Gimp 3.0 can export a CMYK jpeg or tiff and that does differ from Gimp 2.10 +cyan plugin.

However I do not find any noticeable difference between a jpeg or tiff from the same application, maybe I am not critical enough.

Not easy to give worthwhile examples ,
FWIW using Krita as a viewer jumping between Gimp 2.10 cyan jpeg then tiff followed by Gimp 3.0 jpeg then tiff. I have found Krita will impose its default icc for tiffs - all these using ISOcoated.
https://i.imgur.com/TWrwLE1.mp4
Much the same but with a RGB - Gimp 2.10 - Gimp 3.0 - PS7 cmyk (- not much difference to Gimp 2.10 cyan but it is noticeable). https://i.imgur.com/90lubOX.mp4

If you are sending off CMYK to a printer, then really you work from the start with Krita. The other thing is, printers these days are better at accepting RGB images (the last one I used only wanted good quality RGB)

I do remember an issue with an earlier Development Gimp 2.99 where cmyk jpegs and tiffs were visibly different, some problem with the jpeg library but I believe that fixed.

One of the Gimp developers does sometimes visit and if so, will give good advice, keep looking in.

RE: CMYK issue: Krita or Cyan - enrimoore - 01-04-2025

Many thanks,
yes RGB is much better, but I "must" use CMYK for the printer.

All my almost 100 files are well over 40 Mb, so I did a very very small test which I can attach.
Origin : a srgb file. Conversion to Isocoated v2 ECI with Krita and Cyan.
Difference between jpg and tiff is about 93% (lower than in all my tests, but not much)

Well, no, I now see that even with a very small file, the output is almost 3 Mb, so unable to attach, sorry.

Anyway, Krita and Cyan give exactly the same output. Surely they both use same conversion tool.
I need an acceptable CMYK conversion tool, without buying Photoshop, which I don't need at all.

I am using Nx Studio on raw exporting a tiff, then Gimp for assembling 3 photos and completing a book cover.

I don't know if other options are available. And if they are at least acceptable, too.
Advice welcome
enri

RE: CMYK issue: Krita or Cyan - rich2005 - 01-04-2025

(01-04-2025, 02:50 PM)enrimoore Wrote: yes RGB is much better, but I "must" use CMYK for the printer.

Just checking Wink

I hope you do not expect your RGB image to be the same in CMYK colorspace

Because: quote 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.

Quote:Difference between jpg and tiff is about 93% (lower than in all my tests, but not much)

Just out of interest, how are you measuring that. Here using ImageMagick

Code:

magick compare -metric RMSE cmyk.jpg cmyk.tiff null:

I get about a 2% difference between the same file from cyan exported to jpeg then tiff. I put the difference down to jpeg compression artefacts. I get the same 2% converting from rgb to cmyk using my ancient PS7

Quote:Anyway, Krita and Cyan give exactly the same output. Surely they both use same conversion tool.
I need an acceptable CMYK conversion tool, without buying Photoshop, which I don't need at all.

I think you are going to be stuck. Gimp / Krita / Scribus (another option) originally come from linux and will use the same library files.

Quote:I am using Nx Studio on raw exporting a tiff, then Gimp for assembling 3 photos and completing a book cover.

I don't know if other options are available. And if they are at least acceptable, too.
Advice welcome
enri

Nikon images ? You could ask the question on https://discuss.pixls.us/

RE: CMYK issue: Krita or Cyan - enrimoore - 01-04-2025

thanks again
Nikon is ok, but no help with CMYK there. And for a book cover.
Really for me Gimp is quite good, what I need is the CMYK thing.

I don't have imagemagick, but very strange your result.
Pity, I can't attach some example.

But the results are really weird in jpg
What I use is online https://www.textcompare.org/image/#google_vignette
it's very consistent I did hundreds of tests for many types of conversion,
zero issues except in this particular case

Your finding makes me wonder if I am doing a mistake...
but I simply open a file , in Krita or Cyan, check the color space, then convert usually to
isocoatedECI, then I export (in Cyan save) to jpg and then to tiff.
So the image in workarea should be identical, as I don't change anything between the 2 exports.
I even tried all possible settings in Krita for jpg and tiff.

I could try to export once and then close the original file. Then open it again and do the second export.
But if it works, I might go really crazy

Well could you try with this very simple file? the result is weird (93% ) and apparent

enri

RE: CMYK issue: Krita or Cyan - rich2005 - 01-04-2025

(01-04-2025, 07:22 PM)enrimoore Wrote: .....big snip......
I could try to export once and then close the original file. Then open it again and do the second export.
But if it works, I might go really crazy

Well could you try with this very simple file? the result is weird (93% ) and apparent

enri

If your image is open in Gimp (or Krita) no need to close and reopen. Using Cyan export as different formats without closing Cyan.
---------------
I still do not understand your "change" value but it does not matter.

Using your image Shoes-sRGB.jpg and a converted from Gimp cyan shoes-cmyk.jpg I compare the out-of-gamut colours. Out-of-gamut are colours that cannot be printed using cmyk inks That is why you get a duller, less vivid print than looking at the image on your computer monitor.

It is easiest to show this using Krita, you can use Gimp but the settings are scattered about.

Another 40 second animation https://i.imgur.com/XiY9cNF.mp4

The shoes-cmyk.jpg is much larger, 2.4 MB , so here it is on my storage
https://u.pcloud.link/publink/show?code=XZo4LJ5Z9UfTSlnb1FzhRIAxd6qTbmhXLO6y

so bottom line, still no solution to your question, whatever you use going from RGB to CMYK will introduce changes to the colours.

RE: CMYK issue: Krita or Cyan - enrimoore - 01-05-2025

thanks but I was unable to clarify my whole point: only CMYK conversion issue. My bad

Actually I finished the biggest job, using Gimp only for path selection & special curves, and combining 3 layers /photos
Before sending a tiff to a print service, I used 305glossyfujisupreme icc only for softproofing. The printed pics are way way
better than I expected, since the softproof was very similar but worse than the isocoatedv2eci softproof. Which is strange and suggests doubts
about softproofing as well. Btw, a bug in 2.10.38: it remains stuck with the last softproof. So you must edit/preferences etc.Close Gimp. Restart Gimp.
So all ok? yes but with a good service, and with a RGB printer.
The publisher wants a CMYK Fogra39 file. AND they use tens of print services, with different machines: digital printers.
AND Fogra39 is obsolete and meant for offset printing only.
So, no I never expected a good result from a RGB to CMYK conversion. Quite on the contrary, I was awfully worried. Now much more.

I trust https://www.textcompare.org/image/ after a huge number of tests
But, Rich2005 made me think of a further test with Gimp, layers, and ...I was getting crazy: files almost identical! Then, however Gimp converts to srgb!!
So I did many further tests with https://www.diffchecker.com/ https://www.img2go.com/compare-image https://www.imagetools.org/compare
Well the results are not 100% identical among all these tools, but maybe one says 83% another 95%, or one 98% vs 94% : difference between the files to compare.
(btw, Rich2005, you might want to dig into imagemagik: could it be a member of the company? no idea)

Let's divide RGBs in 2 groups:
Monitor RGBs: Adobe1998, NikonAdobe 4...,Nikon srgb 4..., IEC srgb..., srgb elle v2 etc etc.
Printer RGBs: e.g. 305glossyfujisupreme and a bunch of Saaldigital icc s, from basiccolor Germany
My findings, per type of conversion:
1 across monitor RGBs: zero or less than 1% difference. But ... yes icc tag changed!
2 from monitor to printer RGBs: maybe 20% difference
3 from monitor RGBs to CMYKs : Conversion done yes. But between tif and jpg output: difference up to 99%. Id est: TOTALLY different.

At 80, an obsolete logic:
1 maybe all the fuss on color spaces, conversion can mess colors etc.etc. is "much ado about nothing". Or, maybe, in nice words, the conversion
by krita/cyan/lcms is broken. In rude words, pieces of junk. for conversion
2 I have no printer to test
3 UFO thing. Quality unknown: tif ok, jpg junk? OR jpg ok, tif junk? OR jpg and tif 50% junk?
Plus, serious doubts on softproofing as well. But its another piece of cake.
I am curious yes, but right now I think that a full explanation is anyway unlikely

For me somebody who had a file converted to CMYK (possibly isocoatedv2eci) and printed successfully, would be a real help
But this is Gimp. There is another forum more inclined to colour conversion, including CMYK ?

(Photoshop: (sure this is a hard job: can it be trusted?) install and learning new stuff, which I'll never use again )

Well, maybe the whole point is a bit more clear now. Or is it a... mess
enri

RE: CMYK issue: Krita or Cyan - rich2005 - 01-05-2025

Well I am sure it is clear to you.

Imagemagick is FOSS https://imagemagick.org/index.php

Ask about printing on https://discuss.pixls.us/

RE: CMYK issue: Krita or Cyan - enrimoore - 01-05-2025

First, thanks a lot for your hint

But, still totally unclear? English is not my language

If a conversion from, say Adobe1998 to Gimp srgb or to IECsrgb or to Nikon srgb4.... outputs files identical to the Adobe1998 file, all is perfect?

If, converting from any (eg. sRGB) RGB to CMYK you export in jpg and after a second you export the same CMYK converted
image in tiff, you now have one jpg file and one tiff file, right? Now, really you don't expect that these two CMYK files should be very similar?
If you don't agree on this, I am unable to clarify. If you agree, then discovering that those two files are totally different, is irrelevant?
For me it is a big surprise.

Sure, one can say, like myself initially: I could use the tiff, since it should be better.
But another person might say that it's like tossing a coin. Or anyway who knows what's happening? So how can it be reliable?

Or maybe I am just totally out of neurons, plus this work is terrible.
Or hopefully maybe I am still almost ok. But totally unable to explain something (odd) to somebody else

Thanks again anyway (if you happen to hear from someone who converted to CMYK and printed successfully, very grateful)
enri

RE: CMYK issue: Krita or Cyan - rich2005 - 01-05-2025

(01-05-2025, 02:24 PM)enrimoore Wrote: -----snip-----
If, converting from any (eg. sRGB) RGB to CMYK you export in jpg and after a second you export the same CMYK converted
image in tiff, you now have one jpg file and one tiff file, right? Now, really you don't expect that these two CMYK files should be very similar?
-----snip---

Well, I do disagree, as stated earlier, Using Gimp 2.10 and Cyan plugin exporting in one format then again in another format, I can find no real difference. There will probably be some jpeg compression artefacts but that is the nature of jpeg.

Again using Krita for the CMYK images, stacking as layers and in difference mode there is no observable difference.

[attachment=12874]

You can check yourself, the two images https://u.pcloud.link/publink/show?code=XZES4J5ZPAoKff4G8p48xjLoKvaqgB5E3sRV

There is also two txt files with the image data obtained using Imagemagick. Again no real difference.

Go get other opinions from pixls.us