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Question

RGB to CMYK : 2 different cases with 2 different results when printed

Forum|Forum|5 years ago
October 15, 2020
6 replies
6728 views

Hello,

I have an image (a drawing) in RGB.

For printing purposes, I have to convert it to CMYK.

So here is what I did:

Case n °1
I open the RGB file and go to Image > Mode > and switch to CMYK.
I print it.

Case n °2
I create a blank CMYK file and paste the layers of the RGB drawing into it.

And I print.

While the 2 documents are now in CMYK, I have huge differences when printed.

Converting directly to CMYK (case 1) darkens the design a lot when printed, whereas importing the RGB layers into a blank CMYK document (case 2) leads to nearly the same color as the printed RGB.

I'm attaching 2 images:
- 1st one is to see the CMYK differences when printed (case 1 and case 2)
- the 2nd one is to see that RBG and case 2 printed out are very similar.

Can someone help me to understand those differences, and what should I do have perfect printed colors ?

Many thanks !

[Typo in subject line corrected from "RGB to CMJN" to "RGB to CMYK" by moderator.]

How to

This topic has been closed for replies.

J

Jack03Author

Participating Frequently

Hello,

Does anyone know what I can do regarding this last problem:

To show you, I made this screenshot (before I placed a white background to all images to be able to compare):

Left: RGB
Middle: CMYK with merged layers before CMYK conversion
Right: CMYK with flatten image before CMYK conversion

If I flatten the file before CMYK color conversion, it looks great but it removes the transparency which I need to keep.

If I don't flatten it and just merge layers, color conversion will darken the design (even change a bit the color), but I will keep the transparency.

Many thanks

T

Test Screen Name

Legend

Transparency WILL be different between CMYK and RGB. What aspect of the transparency do you need to keep?

J

Jack03Author

Participating Frequently

Hello,

I need to convert RGB to CMYK.

If I flatten the file before CMYK color conversion, it looks great but it removes the transparency which I need to keep.
If I don't flatten it and just merge layers, color conversion will darken the design (even change a bit the color itself), but I will keep the transparency.

I would need the converted colors of the flatten image (which is very close from RGB color), but I also need transparency so I don't know what to do here.

J

Jack03Author

Participating Frequently

Hello,

Alright, however the printer asked for CMYK files.

So should I convert to Japan profile directly ?

I guess I should.

thanks

J

Jack03Author

Participating Frequently

Hello,

Here is an update.

When I send my files to the printer, he has an inhouse designer that will set an .AI file with all my artworks organised on it and send it to print.

I finally got his file this morning and I have checked the CMYK profile he used, it is "Japan Color 2001 Coated CMYK profile" (the printer is in Asia).

So I suppose I have to pick the same profile ?

I won't be able to have more information on it, as the designer don't speak english at all 😞

How does it sound ?

Many thanks.

rob day

Community Expert

So I suppose I have to pick the same profile ?

Yes. If the designer has assigned Japan Color Coated to her AI file, then you should not convert to a conflicting CMYK profile.

While keeping the file as RGB and delaying the conversion until export or output is a more flexible workflow, if it’s required you can still make the conversion to the final CMYK output space in Photoshop as long as you know the destination profile, which is apparently Japan Color 2001 Coated.

If you make the conversion via Image>Mode your current PS Color Settings are used to make the conversion. It might be better to use Edit>Convert to Profile where you can specify all the conversion options. Relative Colorimetric will attempt to maintain the color appearance when the color is inside of the CMYK gamut :

If I place the AdobeRGB version and the Japan Color 2001 version in a CMYK AI file with Japan Color 2001 as the profile assignment, the output values of the 2 versions will be identical:

D Fosse

Community Expert

The conversion you are showing is from Generic CMYK to Japan Color Coated (a CMYK-to-CMYK conversion).

Do you have the original RGB version of the artwork? The ideal would be to make one conversion—RGB-to-CMYK—rather than your RGB-to-CMYK1-to-CMYK2. A conversion directly from the original RGB space using Releative Colorimetric will maintain the original color, which is what I assume you are looking for.

You don’t have to flatten on the conversion, but if you are using certain color blending modes they will function differently if they are not flattened, and you might get an unwanted a color shift.

For example the Difference blend mode works differently in CMYK mode than it does in RGB:

And also any adjustment layers will change appearance, as the numbers get a new meaning in the new color space.

NB, colourmanagement

Community Expert

Using "generic CMYK" is a very poor way to work, its also risky to guess at Swop coated (Photoshop's default).

Printing press output depends on how the press is run - also on inks, substrate and the way the plates are made. These days there are ISO standard press conditions, described by ICC profiles such as those from GRACoL and from the ECI / FOGRA in Europe.

If you are working with a printer who cannot give you an ICC profile to describe the process then you are effectively throwing a dart at a dartboard in the dark and hoping to hit the bullseye.

Guessing what CMYK to use is like sending a letter only knowing the town & guessing the street address and zipcode.

You could ask the printer for their experience with received files., When files are received at the printers, they will predominantly have an embedded ICC CMYK profile.

Ask them this - looking at the files which were received and printed successfully, what was the ICC profile most commonly embedded.

It seems you ideally need a better more modern printer - or the printer really needs to catch up with the way good standardized printing is done these days.

It's amazing the "just use generic CMYK" guys are still in business.

I hope this helps 
thanks
neil barstow, colourmanagement.net :: adobe forum volunteer
[please do not use the reply button on a message within the thread, only use the blue reply button at the top of the page, this maintains the original thread title and chronological order of posts]

T

Test Screen Name

Legend

Darkening is correct. Paste is not colour managed so it cannot give accurate colour, even if by good luck it seems to be what you want. How is it printed, and what CMYK profile do you use?

J

Jack03Author

Participating Frequently

Hello,

Thank you for your reply.

OK so now I know I should use Image > Mode to change to CMYK.

It will be printed at a professional offset printing company, and I use generic CMYK profile.

Now I've tested a new case 🙂
1. I open the RGB file
2. I flatten all visible layers
3. then Image > Mode > and I pick CMYK

I print this file, and it is very very close to the printed RGB, not dark as it was before.

I've seen on adobe website that they recommand to flatten the layers before changing the color mode:

in most cases, it is best to flatten a file before converting it. However, this is not required and in some cases it is not even desirable (when the file contains vector text layers, for example).

Do you think it is good if I do like it ?

Many thanks.

J

Jack03Author

Participating Frequently

Here is the link on adobe:
https://helpx.adobe.com/photoshop/using/converting-color-modes.html

"Flatten the file before converting it. The interaction of colors between layer blending modes changes when the mode changes."

D Fosse

Community Expert

The short answer here is that CMYK is wrong. Don't do it. CMYK is for commercial offset printing only. Inkjet/desktop printers are RGB devices that expect RGB data.

Second, if you should ever need to work in CMYK, you need the actual CMYK profile. There is no such thing as a generic CMYK. Image > Mode just gives you whatever is set up as working CMYK, which most likely isn't the correct one. To work in CMYK, you really need to know what you're doing.

As for printer color management in general, you set this up in the Photoshop print dialog. Leave the document color space as it is. Pick the correct print profile for the printer and paper you're using. Then go into the printer driver and pick the correct media type there. Also turn printer color management off.

A simpler, but less accurate way, is to set "Printer manages color" instead of "Photoshop manages color". Then it's all handled in the printer driver, but you still have to pick the correct media type (paper).

J

Jack03Author

Participating Frequently

Hello,

Thank you for your reply.

Yes, I should have been more precise.

The file will be send to a professional printer that needs CMYK.

Jacqyes

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