Skip to main content
D
De09
Known Participant
October 18, 2020
Answered

Convert multichannel image to CMYK while preserving appearance of multichannel colours

  • October 18, 2020
  • 2 replies
  • 22996 views

Dear all

 

I have a Photoshop image in which I've replaced the original C, M, Y, and K channels with spot colours, so it's now an image in multichannel mode. I now want to make the image CMYK again while preserving the altered appearance of the separate channels. Is there no way of doing this? Any help appreciated.

 

De09

Correct answer Stephen Marsh

In your 4 channel MC mode file, add four new white "placeholder" blank channels at the top in slots 1-4.

 

Then use Image > Mode > CMYK

 

Then shift highlight/select your four "required" spot channels (the ones with actual content). 

 

Using the channels panel option menu, select the "merge spot channels" command.

 

You should now have a CMYK "equivalent" of the MC spot colour file. Of course, the resulting tonal values and overprint colours will probably be "wrong" and the gamut will be reduced to CMYK.

 

You could try doing similar with only three channels for RGB (gamut limitations will still apply and will vary with different RGB spaces).

2 replies

Stephen Marsh
Community Expert
Stephen MarshCommunity ExpertCorrect answer
Community Expert
October 19, 2020

In your 4 channel MC mode file, add four new white "placeholder" blank channels at the top in slots 1-4.

 

Then use Image > Mode > CMYK

 

Then shift highlight/select your four "required" spot channels (the ones with actual content). 

 

Using the channels panel option menu, select the "merge spot channels" command.

 

You should now have a CMYK "equivalent" of the MC spot colour file. Of course, the resulting tonal values and overprint colours will probably be "wrong" and the gamut will be reduced to CMYK.

 

You could try doing similar with only three channels for RGB (gamut limitations will still apply and will vary with different RGB spaces).

    D
    De09Author
    Known Participant
    October 19, 2020

    Amazing – thank you very much. This method works. As you say, the colours come out "wrong", but they're a close enough approximation to work with.

     

    How does this method work? Does the process of creating four new channels somehow fool Photoshop into thinking that it's a CMYK file to start with?

      Norman Sanders
      Norman Sanders
      Legend
      October 18, 2020

      When you consider that each channel of multichannel (or RGB or CMYK, for that matter) carries with it not only the color information but also a tone distribution record, altering the color by switching among those modes will change the tone values and tonal scale of each channel, so that preserving the appearance (tonal scale) of the separate channels while changing modes is not in the cards. You may choose to experment Multichannel and Channels > Split Channels and replacing the contents of the CMYK channels after the changing modes, but I leave that to you.   

       

      On the other hand, a quick and easy way to match the multichannel image in CMYK mode might be to make a screen shot of the it (which will produce an RGB file) and then change the Mode from RGB to CMYK. The particular CMYK profile and the limits of its color gamut must be considered, as well.     

        D
        De09Author
        Known Participant
        October 18, 2020

        Many thanks for this. What you've written in your first paragraph is what I'd feared. I'm surprised, though, that there's no facility in Photoshop for seeing the multichannels as if they were separate pictures which could then be blended together within a CMYK or RGB mode. There are examples of pre-desktop publishing graphic design wherein someone might have plates made according to C, M, Y and K, and then substitute other inks for use with each of these plates when they were actually on the press. That's all I'm trying to do, and it seems odd to me that Photoshop can't cope with something that could be done easily enough before computers were even used for design.

         

        Re. the screenshot option: I'd wondered about this, but surely this would only enable a low-resolution result?

         

        Is there any workaround whereby the separate multichannels can be exported as though each was a single-colour layer instead of a channel, then recombined in Photoshop to get the looks-like-multichannel result?

        D
        De09Author
        Known Participant
        October 18, 2020

        That's a surprise. Could you post the 4 b&w files that Split Channels produced. Please give each a file name that corresponds to  its Multichannel color. Your Multichannel consists of Yellow, Cyan, Green and Gray. Can I assume you want to place:

        Yellow in the Yellow channel

        Cyan in the Cyan channel

        Green in the Magenta channel

        Gray in the Black channel

        ... or have you something else in mind? 

        *************

         

        The sample above is a 4 color Multichannel Mode image consisting of C, M, Y, K

        The four black and white images are Split Channel files from the Multichannel file..

        The bottom left image is CMYK mode image with B placed in B, M placed in M, etc.

         

        And this is an RGB file with Split Channels applied and then intentionally mismatched so that, for examle, the R record was placed in the G channel, etc.

                  

         

         


        Thanks again for the detailed reply. I should have mentioned that the effect of the channels looking spatially out-of-sync with one another in the image I posted was intentional (in case that's what you meant by the image looking "a surprise").

         

        "... or have you something else in mind?"

        Yes. Originally I took a colour image, split the channels, and recoloured them as four Pantones:

        cyan was replaced with another blue: Pantone 310

        magenta was replaced with a green: Pantone 377

        yellow was replaced with another yellow: Pantone 128

        black was replaced with a grey: Pantone 445

         

        What I then wanted to do was to convert the resulting four-Pantone image into a CMYK image, not literally replacing one channel with another (eg. "Yellow in the Yellow channel" as per your list) but so that Photoshop would just take the multichannel colour values and reassign and redistribute them as closely as possible into CMYK – much in the way that red, green, and blue are redistributed across four channels (cyan, magenta, yellow, black) when you convert an RGB image to CMYK. I still don't really understand why Photoshop has no facility for this.

         

        The closest I've got so far is to save each channel of the multichannel image separately, then import them as individual tiffs in InDesign, switch the colour mode to "multiply", and then re-colour them in InDesign. This more or less works, but I'm surprised that it can't be done entirely in Photoshop, without needing a separate application.

        Terms & ConditionsAccessibility statement
        Using the community Terms of use Privacy Cookie preferences Do not sell or share my personal information ad choicesAdChoices