RGB to CMYK conversion

Processing
1

hello, happy new year
I need to convert from RGB to CMYK Isocoatedv2eci. For many reasons it will be very hard.

Maybe someone converted a tif to CMYK and had it printed successfully, i.e. acceptable?

I wanted to use Krita/Cyan, but by chance, comparing the jpg with the tif output, I discovered that the jpg file and the tif file are totally different.
I did over 100 tests on many files, and checked with:
Image Compare Tool Online
Compare Images Online – Free Image Comparison Tool
https://www.imagetools.org/compare https://www.diffchecker.com/

The difference between the jpg and the tif is around 90% or up to 98%
Please believe me, this is a special situation, and testing for differences inside the files cannot be done with an image manipulation program, which is for viewing first of all, editing, improving an image. So a program like Krita will first “change the images”, and the 2 “images” will appear almost identical, for Krita.

A specialized program, engineered to explore the actual pixels, if not the single bits, will instead strive to “preserve” and analyze only the differences between the 2 files, if differences exist. And will provide an approximate mathematical result, and display basic
images to show approximately the “visible” differences.

These programs only try to check the 2 files, no intervention, no editing, nothing else.

I am totally sure of this issue. Btw, if you create a jpg and a tif file from the same image, even using Krita, and you compare the 2 files with the mentioned programs, they will show zero or very little difference. As obviously logical, and largely expected.

Boring, sorry, but this is why in principle, I am now discarding Krita/Cyan: maybe the tif CMYK output is ok, but who knows what’s going on, in the conversion engine?

This thought might change if somebody, using Krita, produced a CMYK tif and printed it with acceptable quality.

Anyway this question is more general, not limited to using Krita.

I am not eager at all, also at my age, to install and learn new stuff, especially Photoshop.
But if there are no choices, well…

Btw I use Gimp, but now I need some advice or assurance about this RGB to CMYK
(Isocoatedv2Eci) conversion for printing.
I hope someone comes in with some information. My best
enri

1 Like
2

Hello!
Welcome to the forum :slight_smile:

I think I don’t get your point entirely, but… you do realise you will NEVER get the same files after colour conversions?

The whole point of converting from RGB to CMYK is to translate data from light registering / emitting devices to a paper + ink combo. Most of the time you cannot achieve exactly the same colours when printing. A camera, scanner, computer screen capture or display much more and much brighter, vivid colours than you can typically print. It’s always a tradeoff and such a file is inherently different than the RGB original.

1 Like
4

Or do you even need to be converting?? :slight_smile:

2 Likes
5

In GIMP, how do I “convert” an RGB image to a CMYK image? I can’t find where it does that.

6

I wondered about that … I thought that was done by the print driver.

Besides which, my printer has five color cartridges: C,M,Y,Gray,Black.

1 Like
7

Hello,
currently you can’t do it, except integrating Cyan, which can also work on its own. Perhaps the next 3. Gimp version could do it.
I would be curious how Gimp 3. works, since if it uses again the same engine as
Cyan, we have the same issue I described in my post

1 Like
8

I totally agree. My bad, the publisher wants a CMYK file

9

hello and thanks,
yes that’s why I am terribly worried and I am discarding Krita (Littlecms really, I think)
It’s not my choice

10

Thank you.

As to your original question and having tried one of your links, I see no reason why a TIFF should not be ~90% “different” to a JPEG on a pixel-by-pixel basis because JPEG has compression and chroma sub-sampling but TIFF does not. Try comparing a TIFF with a PNG … try comparing different quality/sub-sample JPEGs.

1 Like
11

if you re read you have the solution: no, a tif and a jpg from same image are identical or almost so . Pure logic confirmed by the programs I listed. Because you are not doing any edit whatsoever on the image. Only if you will do edits the tif and jpg will differ maybe much

12

Not obvious to me.

Obviously.

Now, if I convert a 2.5Mb JPEG to a TIFF, the TIFF is 28.5 Mb. How do your links handle that?

Although the images “look” the same on your screen, I doubt that a byte-by-byte comparison would come close but I’ll give it try …

1 Like
13

I personally think your best bet is to convert your image to cmyk, then edit it again to get it closer to how you want it to look. Sad but true. Or start over with your edit and use a CMYK profile as your softproof from the beginning.

1 Like
14

And, sure enough, 99.99% different!:

difference
difference1092×901 130 KB

15

Hi Enrico, can we see the file you are about to convert from?

1 Like
16

enclosed an example : original file 3.2 MB output: jpg 8.159 MB, tif 106.666 MB but still
IDENTICAL!!!
My bad, I should have stated that, obviously, you are saving/exporting from the same “input” file" and outputting a jpg and a tif with maximum quality.
As I did in all my 100 tests on conversion to CMYK.

Screenshot (61)
Screenshot (61)1920×1080 752 KB

17

Maybe it’s time to wrap things up.

As far as I understand, you’ve got your photos, as usual, in RGB colour space.
They are going to be printed in e.g. a book or magazine, so your publisher or printing house wants them in CMYK colour space.

Why do you think or want they should be identical in terms of pure IT stuff?
Isn’t it enough if they appear identical to the eye?

2 Likes
18

thanks but no idea how to do it. My aim is to get an acceptable conversion. Perhaps with some softproofing. Then how can I edit in CMYK first. And second, how can we be sure of any result from editing, before actual printing. I would be content with an acceptable conversion, which is the main issue

19

Well, it should be.

However, the required ISO Coated v2 ECI describes behaviour of typical four colour offset press + standard inks + coated paper. It’s for large volume commercial printing conditions.

But to be honest, a good printing house should accept tagged RGB photos in PDF/X-4 file, so that the RIP could handle the conversion in the best possible way.

2 Likes
20

Ask the printing house for a contract proof. It should be printed with UGRA / Fogra Media Wedge (or another standard in your area) and validated with spectrophotometer.

2 Likes
21

2 updates:
first: the developer of the conversion engine replied to me, so , very good
second: I wanted to do another “odd” test, not really believing too much…
right now I did it with , at least for me, amazing results which I could only suspect

I took a CMYK file as “input” and exported a jpg (max quality obvious) and a tif
wow, the 2 output files differ by some 93%!!

So perhaps something very odd, to me at least, happens already at the CMYK level
Now, further tests , plus above all, some talking with the developer