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Convert multichannel image to CMYK while preserving appearance of multichannel colours

Forum|Forum|5 years ago
October 18, 2020
2 replies
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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

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).

Stephen Marsh

Correct answer

Community Expert

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).

rob day

Community Expert

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:

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

The color appearance doesn’t change on the merge:

A conversion back to GRACol Coated again using Relative Colorimetric:

Stephen Marsh

Community Expert

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.

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

Norman Sanders

Legend

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

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?

Norman Sanders

Legend

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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