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Convert multichannel image to CMYK while preserving appearance of multichannel colours

Explorer ,
Oct 18, 2020

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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

Adobe Community Professional
Correct answer by Stephen_A_Marsh | Adobe Community Professional

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).

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Convert multichannel image to CMYK while preserving appearance of multichannel colours

Explorer ,
Oct 18, 2020

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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

Adobe Community Professional
Correct answer by Stephen_A_Marsh | Adobe Community Professional

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).

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Oct 18, 2020 0
Adobe Community Professional ,
Oct 18, 2020

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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.     

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Oct 18, 2020 2
Explorer ,
Oct 18, 2020

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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?

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Oct 18, 2020 0
Adobe Community Professional ,
Oct 18, 2020

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While you were posting your reply, I was editing my own to include the Split Channel option...replacing the CMYK channels. If you choose to experiment, please share your results with us.  (See the upper right corner of the Channels panel)

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Oct 18, 2020 1
Explorer ,
Oct 18, 2020

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Thanks. I'll have a go at this, although I'm inexperienced with Photoshop channels. At present I can split the channels and re-merge them so that they end up a CMYK mess, but that's all. I've attached here a part of the image: the left side is the multichannel version in yellow, cyan, green, and grey, and the right side is the image when merged back into CMYK.

Excerpt.jpg

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Oct 18, 2020 0
Adobe Community Professional ,
Oct 18, 2020

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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? 

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

 

Screen Shot 2020-10-18 at 4.06.37 PM.png

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.

          inter_grid.jpg

 

 

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Oct 18, 2020 1
Explorer ,
Oct 18, 2020

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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.

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Oct 18, 2020 0
Adobe Community Professional ,
Oct 18, 2020

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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).

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Oct 18, 2020 2
Explorer ,
Oct 19, 2020

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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?

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Oct 19, 2020 1
Adobe Community Professional ,
Oct 19, 2020

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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.

 

Hi Stephen,  Very nice. It looks like the current Color Settings are used when you convert the multi-channel to CMYK—the merged colors are different depending on the settings. I was able to maintain the spot color overprint appearance by converting to RGB using Relative Colorimetric, Merging the Spots, and then Convert to Profile back to the desired CMYK space:

 

Screen Shot 1.png

 

Convert to RGB with my Color Settings set to Relative Colorimetric:

Screen Shot 2.png

 

The color appearance doesn’t change on the merge:

 

Screen Shot 3.png

 

A conversion back to GRACol Coated again using Relative Colorimetric:

 

Screen Shot 4.png

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Oct 19, 2020 2
Explorer ,
Oct 19, 2020

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Hi Rob and all,

 

"It looks like the current Color Settings are used when you convert the multi-channel to CMYK—the merged colors are different depending on the settings."

 

Does this mean that the conversion from multichannel to CMYK will be more accurate (ie. remain closer in appearance to the original spot colours) if it is performed as

 

multichannel –> RGB –> CMYK

 

as opposed to

 

multichannel –> (straight to) CMYK

 

?

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Oct 19, 2020 1
Adobe Community Professional ,
Oct 19, 2020

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Yes, going from MC -> RGB + Merge Spots -> CMYK would result in a "closer visual appearance" (but not perfect) to the original MC file onscreen preview.

 

The point I was making is that the preview of the original MC file is not "accurate" to begin with... It is not an accurate "proof" of what would happen on press if you were printing with 4 overprinting spot colours of different tints/solids...

 

However, if you don't care about that and you just want a CMYK version that looks pretty close, then yes, MC -> RGB + Merge Spots -> CMYK should yield the best results. You may wish to experiment with MC to different RGB intermediate spaces (sRGB, Adobe RGB, Pro Photo in your colour settings) before converting into your final CMYK.

 

You need to use Image -> Mode -> RGB for the step from multichannel to RGB... However, you can use the "Convert to Profile" command for the RGB -> CMYK conversion.

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Oct 19, 2020 1
Adobe Community Professional ,
Oct 20, 2020

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Right, there’s no accurate color management for spot overprints and tints. If the spots are the Lab based Pantone Solid colors the preview of the colors at 100% would be somewhat accurate because the Lab values are instrument read from printed solid color swatches.

 

There are Pantone Solid colors that are well outside of the sRGB gamut, so ProPhoto would make the most accurate conversion for out-of-gamut solid inks. The conversion of 100% Pantone Lab colors to ProPhoto RGB then back to Lab produces the original Pantone Lab values, while the conversion via sRGB does not.

 

Here are 4 colors that are outside of the sRGB gamut and you can see the conversion via ProPhoto is better:

 

Screen Shot 4.png

 

 

 

 

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Oct 20, 2020 2
Explorer ,
Oct 20, 2020

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This is all a little beyond my knowledge of colour handling – I deal mainly with CMYK and Coated and Uncoated Pantones, as I mainly do print-based work. But thanks for the replies – from these I can look further into the differences between different forms of RGB and so on.

 

I had not heard of "Merge Spots" before, and have not yet been able to get this to work, but will try further. Thanks again.

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Oct 20, 2020 0
Adobe Community Professional ,
Oct 20, 2020

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and have not yet been able to get this to work, but will try further.

 

Can you share a sample file via Dropbox or your CC account?

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Oct 20, 2020 1
Explorer ,
Oct 20, 2020

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I've just tried to do this via Dropbox, and it inverted the colours in the image, ie. blacks became whites and so on. No idea what's happening there.

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Oct 20, 2020 0
Adobe Community Professional ,
Oct 20, 2020

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Its probably just the web preview, post the link so we can download the .psd

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Oct 20, 2020 1
Adobe Community Professional ,
Oct 20, 2020

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It should be as simple as this image to get it to work:

 

merge-spots.png

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Oct 20, 2020 1
Adobe Community Professional ,
Oct 21, 2020

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Also, notice that Stephen has the RGB channels hidden. When you make the conversion from Multi Channel to RGB, the RGB preview will initially be wrong until you Merge Channels.

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Oct 21, 2020 1
Explorer ,
Oct 21, 2020

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Hopefully this should work.

 

Image excerpt

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Oct 21, 2020 0
Adobe Community Professional ,
Oct 21, 2020

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Well spotted Rob, I'm pretty sure it works the same whether the RGB channels are visible or not. The key is selecting the spots.

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Oct 21, 2020 1
De09 LATEST
Explorer ,
Oct 25, 2020

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Many thanks again for all the replies, everyone – much appreciated.

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Oct 25, 2020 0