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CMY minus K of CMYK channels be looking like RGB?

Forum|Forum|4 years ago
May 26, 2021
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Is there a way to make CMYK channels with unchecked black channel be looking like as it would be regular RGB instead of CMY ? So kind of CMYminus K appearing like RGB ? Without a gamma shift?

My guess it could be some curves tweak turned into LUT file for example on top of a stack but since I don't fully understand the math behing RGB >> CMYK convertion I am not sure how to make backward-conversion CMY>>>RGB. And I'd like it as a live adjustment layer.

I would appreciate any help. Thanks

How to

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解決に役立った回答 Bob_Hallam

then I assigned a CMYK profile for an inkjet printer using clear film wide gamut inks and white ink, reverse printed and profiled through the clear media. This "wider gamut" profile provided the best preview from the other CMYK profiles that I had installed.

By @Stephen Marsh
=

Oh, you mean in "Customise proof colors" and "proof colors" checked in, right? But I also struggle to find it in its list of profiles

This will be a bit of trial and error unless you know your way around the CIELab and how to convert to CIE XYZ color space well. The benefit of this method for your situation is that you can adjust the preview you receive until your happy with the look and save a custom CMYK color space for this project.

Use Photoshops custom CMYK in Color Settings (Scroll to the top while choosing the CMYK Profile and choose "Custom", then in the next dialog under ink colors choose Custom. Now you have the option to specify tour ink colors as XYZ values. Not the best possible interface for sure, but if you know how to convert from CIELab to CIEXYZ then you can enter correct values in this area to enter expanded gamut colors. The transform is simple so I'd start there and begin with brighter more saturated CIELab values and convert those to CIEXYZ.

The advantage of this method for your problem is to create brighter primary and secondary colors so your monitor can display closer to the original RGB appearances.

Stephen Marsh

Community Expert

Consider the following two screenshot images:

The upper gradient image is Adobe RGB.

The lower gradient is a CMY channel image in CMYK mode. The RGB file was converted to multichannel, then a new white alpha added, then image > mode CMYK, then I assigned a CMYK profile for an inkjet printer using clear film wide gamut inks and white ink, reverse printed and profiled through the clear media. This "wider gamut" profile provided the best preview from the other CMYK profiles that I had installed.

Perhaps something similar would be a "close enough" preview for you to manipulate the image by leveraging the K channel.

This CMY(K) version can be returned to multichannel > delete 4th channel > image/mode RGB, assign Adobe RGB and there is no change/difference to the original RGB channels.

K

kirkr5689作成者

Inspiring

then I assigned a CMYK profile for an inkjet printer using clear film wide gamut inks and white ink, reverse printed and profiled through the clear media. This "wider gamut" profile provided the best preview from the other CMYK profiles that I had installed.

By @Stephen Marsh

Thank you, it smells like a solution , But where can I get this 'wider gamut inkjet printer ICC using "clear film inks and white ink, reverse printed and profiled through the clear media" I see no of such thing in default drop down list

when hit "assign profile" for CMYK document. Can you share the exact ICC file please? And I don't know what folder I would have to put it in so it would be appearing in that drop down?

K

kirkr5689作成者

Inspiring

then I assigned a CMYK profile for an inkjet printer using clear film wide gamut inks and white ink, reverse printed and profiled through the clear media. This "wider gamut" profile provided the best preview from the other CMYK profiles that I had installed.

By @Stephen Marsh
=

Oh, you mean in "Customise proof colors" and "proof colors" checked in, right? But I also struggle to find it in its list of profiles

Norman Sanders

Legend

For the history buffs:

Back in the seventies (yes, fifty years ago) some specialty printing firms (one that I remember was Dexter Press, located in West Nyack, NY) came up with a three color process, eliminating black. The fourth cylinder may have been used for varnish, but I am not sure.

At any rate, the method was used primarily for very short run jobs: picure postcards. I think the clients were primarily motels, restaurats and similar establishments. The work was gang run -- a large sheet that included the orders of many customers. Some offset houses even specified that all incoming copy had be 35mm chromes. (I suspect most chromes were snapshots made by the client with his nifty new toy.)

The process was called Pleasing Color but I don't recall hearing from any professional who was pleased. Shadows in the images were often brown, and color fidelity could be bad enough to make a grown man cry. The process died.

K

kirkr5689作成者

Inspiring

Well, in my supposed process it's not that important if it looks perfect. Just a bit closer to what initially was RGB for preview reason only. On export I am going to copy those first 3 channels into normal RGB anyway and what was in K to companion grayscale file.

So I just need some view transform on top of the stack , not necessarily a perect one

TheDigitalDog

Inspiring

@kirkr5689 wrote:

Well, in my supposed process it's not that important if it looks perfect. Just a bit closer to what initially was RGB for preview reason only.

You'll have to assign profiles until one is 'close enough' to the original. You have some original values and Photoshop needs to know the scale of the values. R23/B67/G255 isn't the same color in sRGB vs. Adobe RGB (1998) and isn't even a real color in ProPhoto RGB. How can PS show you a color appearance without knowing this? It can't. It can assume and it does with untagged images. But the assumption may be a mile off. That's why the Assign Profile command exists. And isn't often used or needed to be used.

Author “Color Management for Photographers" & "Photoshop CC Color Management/pluralsight"

TheDigitalDog

Inspiring

You want to take a CMYK image and remove the K and end up with (you'll have to end up) RGB?

Use Split channels command. Delete K. Use Merge Channels command. You'll get an RGB document san's the K recipe.

EDIT: Be sure to select RGB from the option dialog below:

Be sure to Assign an RGB profile; this becomes 'untagged'.

Author “Color Management for Photographers" & "Photoshop CC Color Management/pluralsight"

K

kirkr5689作成者

Inspiring

I don't think it's working.

I basically want to make a copy of RGB document . Convert it to CMYK/16 or 32 . Then copy /paste all of original doc RGB channels one by one into CMY channels accordingly , unchek K checkbox in layer style dialog so only first 3 channels is working and K being a spare one. Than make this document look perfectly same as origial RGB one since it now has all prfectly same first 3 channels . And still having K channel as a spare one for totally different technical info purpose .

My gues it should be possible to make a kind of LUT transform adjusting layer or something that I could put on top of the layer stack that would made those 3 CMY channels look like RGB . Considering all 3 channels are copy/pasted from RGB.

.

K

kirkr5689作成者

Inspiring

The goal is to have extra technical info chanel for each RGBA layer. Alpha channels are not suited well since they relate to document ,not each layer individually. You can't dynamically blend alpha channels in same way you can do with layers in the stack until you produce a black&white layers from each channel and blend them as layers in a separate group.

And even if you have a separtate stack/group of such black&white layers it's an issue to keep them synched with related rgb layers .

Using CMYK 4 channels approach for each layer you can end up with twice less complicated layer stack , no necessety for chain linking or do smart object based instances and cliping groups.

If understood right I need to assing a profile that would interprete CYN channels as RGB ones , Have no idea how to make such profile and hoped it can be just LUT file I could load into LUT adjustment layer.

I tried to make a screen of my monitor with CMY and K disabled and then do curve adjustment to bring back blacks and contrast , then export it as a profile and load it in lookup table adjustment layer in my actual CMY document.

But 3D LUT is not working there and other "device" and "abstract" things I have no idea what to do with them and if they could do it.at all.

ps, I want to make a copy from my original RGB document , turn this new copy into CMYK , disable K in layer style dialog and somehow make those 3 CYM channels left look same as my original RGB document( after copy/pasting each of them from RGB document) while still staying in CYMK format.

c.pfaffenbichler

Community Expert

What is the actual goal here?

Depending on a CMYK Space’s GCR/UCR settings the darkest parts may become pretty light without black compared to the full composite, so I am not quite sure what good this would be.

Stephen Marsh

Community Expert

An "interesting" request : | What would be the application, how/why would you use it?

K

kirkr5689作成者

Inspiring

Having an exrtra channel besides typical RGBA in evry layer could make lots of applicatons possible. And could make the layer stack a lot less complicated and easy to read . Look at it as if evry layer have its own extra alpha channel beyond just transparency.

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