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RGB to Lab/CMYK and back to RGB

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  #21  
Old 07-29-2011, 05:44 AM
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Der_W Der_W is offline
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Re: RGB to Lab/CMYK and back to RGB

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Originally Posted by DWThomp View Post
Thanks for those links. I installed the action, but didn't have the hsb/hsl plugin installed. I went thru the action to see what it was doing and then recorded my own to just add the CMYK & Lab channels.
It seems you misunderstood the purpose of the action :-).
If you use this action here from the linked thread at MM: http://www.twicebakedphoto.com/download/RGBtoCMYK.atn and use the "RGB to CMYK Layers (c) 2010 Sean R Baker" action on any of your images, this will create all four CMYK channels in layers in your RGB file in a way that won't lose any data (because the image is not really converted to CMYK, it's more of a recalculation and redistribution of data).
With these layers you can then work with the advantages of CMYK without it's disadvantages and while staying in RGB.
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  #22  
Old 07-29-2011, 09:55 AM
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D Thompson D Thompson is offline
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Re: RGB to Lab/CMYK and back to RGB

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Originally Posted by Der_W View Post
It seems you misunderstood the purpose of the action :-).
Thank you and you're exactly right - I did . I installed the one with the hsb/hsl plugin required (I guess I could load the plugin to make it work) and got the errors when it would get to those points. I've got the actions in your link and see the difference. Thanks again.

For the action I made - Duped the original, convert to CMYK, copied the channels and moved to the original, closed copy. Duped original, converted to Lab, copied the channels and moved to original, closed copy.
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  #23  
Old 08-10-2011, 04:35 AM
bakerser bakerser is offline
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Re: RGB to Lab/CMYK and back to RGB

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Originally Posted by DWThomp View Post
For the action I made - Duped the original, convert to CMYK, copied the channels and moved to the original, closed copy. Duped original, converted to Lab, copied the channels and moved to original, closed copy.
The one Jonas linked you to does something way cooler, and won't clip color spaces on you .
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  #24  
Old 08-11-2011, 02:42 AM
kav kav is offline
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Re: RGB to Lab/CMYK and back to RGB

I still don't see the point of making a round trip. If I have to hand over an RGB file I do try to ensure it's cmyk friendly via a generic profile similar to the device they'll be using. I don't like to risk single channel clipping/detail loss when they make a conversion. If it's a crucial product color they have options but on most stuff it'll just end up as a mess.
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  #25  
Old 08-11-2011, 02:56 AM
RobertAsh RobertAsh is offline
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Re: RGB to Lab/CMYK and back to RGB

If you do Edit > Convert To Profile instead of Image > Mode > CMYK you shouldn't run into any problems like that. The point of converting to CMYK is either to work with skin tones in CMYK color space or to extract shadow detail with the convenient long shadow portion of the K curve.

It's like anything else in Photoshop, if you don't feel you need to do it, no need to bother. But if there are any times you need it or want to use it, it's there.
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  #26  
Old 08-11-2011, 03:32 AM
kav kav is offline
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Re: RGB to Lab/CMYK and back to RGB

There's a decent workaround for the blacks. You can duplicate an image, convert to cmyk, and duplicate the K channel back as an alpha channel. The reason you're finding skintones easier to correct there is because it doesn't produce as many irritating hue shifts. This is still an incredibly clunky solution if your file is supposed to end as an rgb file.
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  #27  
Old 08-11-2011, 07:32 PM
RobertAsh RobertAsh is offline
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Re: RGB to Lab/CMYK and back to RGB

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Originally Posted by kav View Post
There's a decent workaround for the blacks. You can duplicate an image, convert to cmyk, and duplicate the K channel back as an alpha channel. The reason you're finding skintones easier to correct there is because it doesn't produce as many irritating hue shifts. This is still an incredibly clunky solution if your file is supposed to end as an rgb file.
Wouldn't duplicating the K channel as an alpha channel and copying that into the RGB file subject you to the same short shadows curve that RGB has already?

What works well for me is:
  • Image > Duplicate
  • In the duplicate file, Edit > Convert to <your favorite CMYK>
  • Do your manipulations in the duplicate file
  • Edit > Convert to <your working space RGB>
  • Copy the flattened file back into the RGB file
Of course it's clunky Until and unless Adobe figures out how to do color space conversions without discarding adjustment layers. That's why you only do this when you need to.

The reason for fewer irritating hue shifts in CMYK is because human skin has lots of magenta and yellow, and little or no blue and green. When you manipulate the Red curve in RGB you're manipulating both the Magenta and Yellow CMYK curves simultaneously -- with no option to manipulate Y and M individually, which is sometimes needed for good results.
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  #28  
Old 08-11-2011, 08:14 PM
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John Wheeler John Wheeler is offline
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Re: RGB to Lab/CMYK and back to RGB

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Originally Posted by RobertAsh View Post
....What works well for me is:
  • Image > Duplicate
  • In the duplicate file, Edit > Convert to <your favorite CMYK>
  • Do your manipulations in the duplicate file
  • Edit > Convert to <your working space RGB>
  • Copy the flattened file back into the RGB file
Hi Robert
I am not one that advocates jumping to a lower gamut color space and back yet I know this works for some including yourself.

Here is an alternate workflow to consider that I believe gives you the same result without producing a separate document.
  • Set up your Color Settings so that the RGB working space is what you want to work in and the conversion options are also set the way you want (e.g. relative colorimetric, black point compensation etc). Set them the same you would when using Convert to Profile.
  • If you have multiple Layers already in your RGB stack then create a stamped layer
  • Turn that stamped layer into a Smart Object
  • Open that Smart Object (if you set you Color Settings right the RGB Color Space will stay the same)
  • Within the Smart Object do Edit > Convert to Profile > your favorite CMYK space
  • Edit as desired in CMYK color space
  • Save the Smart Object (optionally close it as well)
  • Go back to the Main RGB document. The CMYK of the Smart Object will automatically be converted back to your RGB Working Space using the conversion options set in Color Settings (Identical to using Edit > Convert to Profile)
  • Proceed with you Photoshop edits on top of the Smart Object

I believe this should give you exactly the same result as your step by step above yet keep everything in one document. Also given that the CMYK is in Smart Object, you can reopen the Smart Object at any point and tweak the CMYK stack as you please, save, and jump back to your Main Document with the new edits propagated into your RGB Layer Stack. Let me know if this does not produce identical results for you because it does for me.
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  #29  
Old 08-11-2011, 08:49 PM
kav kav is offline
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Re: RGB to Lab/CMYK and back to RGB

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Originally Posted by RobertAsh View Post

The reason for fewer irritating hue shifts in CMYK is because human skin has lots of magenta and yellow, and little or no blue and green.
This is incorrect. The skin doesn't actually contain red, green, or blue pigments. It reflects much more red than green or blue light, which really isn't ideal for digital camera sensors sadly We all work around this problem. Some of the high saturation values present in RGB are out of gamut for pretty much any cmyk profile. Since the saturation is so much more controlled it won't necessarily exacerbate hue shifts in things like skin as much. Basically the changes in saturation are more constrained.

The shadow curve thing is just that you're thinking of a gamma 1.8 based profile (at least that's how photoshop represents it) although cmyk profiles vary quite a lot. Newsprint for example has a really weird curve.

By the way what cmyk profile are you using in your tests?
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  #30  
Old 08-11-2011, 10:17 PM
edgework edgework is offline
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Re: RGB to Lab/CMYK and back to RGB

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Originally Posted by RobertAsh View Post
When you manipulate the Red curve in RGB you're manipulating both the Magenta and Yellow CMYK curves simultaneously -- with no option to manipulate Y and M individually, which is sometimes needed for good results.
Not true at all. You don't manipulate the red curve for the magenta /yellow values, you use it for the cyan. Test it out yourself: in an RGB file set two sample points right next to each other so that they give the same numerical readout. Then set one of them to give CMYK values. You can easily see the results with curves in each channel. Darken the green curve (lower the intensity of the light) and you will see the magenta values rise accordingly. Likewise for the blue channel, which increases the yellow values. Red works on it's compliment, cyan.
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