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CMY minus K of CMYK channels be looking like RGB?

Explorer ,
May 26, 2021 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

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Adobe Community Professional , Jun 01, 2021 Jun 01, 2021
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 t...

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Explorer ,
May 26, 2021 May 26, 2021

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Yeah, I see.   Just hoped there is a way to deceive Photoshop in a manner  it could work like those 2d compositing  soft from movie industry    that could stack exr files with gazilion extra channels  one on top of another  and produce blending/math opperations  for each of those channels in sync   while having a "look transform"  for a final view.     

Just thought why it couldn't be working similar way with at least one extra channel of CMYK model.

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Explorer ,
May 26, 2021 May 26, 2021

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Here is an example of possible goal for example

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Explorer ,
May 26, 2021 May 26, 2021

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Using K  as depth and doing dynamic mask for a green cube  on the fly with pretty simple "mask" clipping group

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Adobe Community Professional ,
May 27, 2021 May 27, 2021

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So to try and boil down your request (please correct me if I miss something) You are asking which CMYK ICC Profile will provide the closest appearance match to an RGB profile. So that when you make the file changes you want the resulting CMYK image appears the same or similar to the original RGB.  

 

If I understood that correctly the next logical question would be "Which RGB profile is used in your original Image?

 

 

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Explorer ,
May 27, 2021 May 27, 2021

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quote

You are asking which CMYK ICC Profile will provide the closest appearance match to an RGB profile

By @Bob_Hallam

To be more explicit  I am asking  wich CMYC ICC profile  will povide closest  match for only CYM channels and K disabled (unchecked in "layer style")  to be appearing  as if document would have  sRGB profile?    My original is sRGB

 

 

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Adobe Community Professional ,
May 27, 2021 May 27, 2021

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@norman.sanders wrote:

For the history buffs:

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.  


 

Fascinating history lesson, Norman!

 

~ Jane

 

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Adobe Community Professional ,
May 26, 2021 May 26, 2021

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Consider the following two screenshot images:

 

RGB-CMY.png

 

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.

 

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Explorer ,
May 27, 2021 May 27, 2021

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kirkr5689_0-1622181181817.png

 

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_A_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?  

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Explorer ,
May 27, 2021 May 27, 2021

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quote
kirkr5689_0-1622181181817.png

 

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_A_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

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Adobe Community Professional ,
Jun 01, 2021 Jun 01, 2021

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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.  Screen Shot 2021-06-01 at 6.04.34 AM.png

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Explorer ,
Jun 02, 2021 Jun 02, 2021

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Thanks a lot Bob

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Adobe Community Professional ,
Jun 03, 2021 Jun 03, 2021

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Happy to have helped! 

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