RetouchPRO

Go Back   RetouchPRO > Technique > Photo Retouching
Register Blogs FAQ Site Nav Search Today's Posts Mark Forums Read


Photo Retouching "Improving" photos, post-production, correction, etc.

RGB to Lab/CMYK and back to RGB

Reply
 
Thread Tools
  #31  
Old 08-11-2011, 11:03 PM
RobertAsh RobertAsh is offline
Senior Member
Patron
 
Join Date: May 2011
Location: Oakland, CA area
Posts: 380
Re: RGB to Lab/CMYK and back to RGB

Hi Edgework,

Not to be contrary or anything, and certainly not claiming to be the world's expert here, but by increasing the Cyan in CMYK you're effectively decreasing the Magenta and Yellow together by fixed values, and vice versa.

I've found it useful to manipulate the Magenta and Yellow separately in skin tones, which made using CMYK more convenient.

Last edited by RobertAsh; 08-11-2011 at 11:23 PM.
Reply With Quote top
  #32  
Old 08-11-2011, 11:14 PM
RobertAsh RobertAsh is offline
Senior Member
Patron
 
Join Date: May 2011
Location: Oakland, CA area
Posts: 380
Re: RGB to Lab/CMYK and back to RGB

Quote:
Originally Posted by kav View Post
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?
Hi Kav,

Sorry for being imprecise. I wasn't saying that skin contains R,G or B pigments. What I meant to say is that you can manipulate Red to significantly affect skin tones. Or you can manipulate Yellow or Magenta. All 3 affect skin tones, especially Red and Yellow (particularly with non-Caucasian skin, which is what I photograph most).

Also, in some (less precise) color wheels, Red falls between Magenta and Yellow. So by controlling those two you get more precise control of the Red than you get by manipulating the Red channel alone. At least I do.

For CMYK I tend to use US Web Coated (SWOP) V2. Suggestions very welcome.

I've used this technique for printing images myself and it worked fine but have never printed anything that requires CMYK embedded profile for press applications. I tried once with Moo.com for business cards but don't know how it would have worked because they applied their standard color correction that my file didn't need and messed up the color. They asked me to send it again specifying 'no color correction' but I'm too busy to try them again right now. But I plan to do that at some point.
Reply With Quote top
  #33  
Old 08-11-2011, 11:21 PM
RobertAsh RobertAsh is offline
Senior Member
Patron
 
Join Date: May 2011
Location: Oakland, CA area
Posts: 380
Re: RGB to Lab/CMYK and back to RGB

Hi John,

Hex suggested that approach some time ago of selecting all the layers and converting the whole lot into a single smart object (for everyone else reading, Hexabuzz is the pseudonym of a guy on the NAPP forum, John and I are both members there).

It worked for me a couple of times, and was pretty useful (though still really clunky) -- but for some reason I never did figure out, it stopped working.

However it's happening, Photoshop is not preserving my smart objects in between color spaces. It's not respecting my adjustment layers inside the smart object itself and dulls out the colors. It's the darndest thing.

I've bought Curvemeister now, have it downloaded but not installed yet. Hopefully that can help.

Thankfully, I really don't need to switch to CMYK much, have just done it a couple of times and I've gotten pretty decent using Shadows/Highlights for the shadow detail control.
Reply With Quote top
  #34  
Old 08-11-2011, 11:25 PM
RobertAsh RobertAsh is offline
Senior Member
Patron
 
Join Date: May 2011
Location: Oakland, CA area
Posts: 380
Re: RGB to Lab/CMYK and back to RGB

Again, to be clear, it seems clear to me that everyone here understands that I'm not saying what I do in such occasions is pretty or convenient - it's neither. Nor is it the only way to accomplish what it does. But so far the couple of times I needed it, my approach worked best for me personally so far. I'm also willing to try (or re-try) other approaches in the future to do this.
Reply With Quote top
  #35  
Old 08-12-2011, 12:39 AM
kav kav is offline
Senior Member
 
Join Date: May 2009
Posts: 574
Re: RGB to Lab/CMYK and back to RGB

Quote:
Originally Posted by RobertAsh View Post
Hi Edgework,

Not to be contrary or anything, and certainly not claiming to be the world's expert here, but by increasing the Cyan in CMYK you're effectively decreasing the Magenta and Yellow together by fixed values, and vice versa.

I've found it useful to manipulate the Magenta and Yellow separately in skin tones, which made using CMYK more convenient.
Whenever you change values you are changing density. Even for your purposes adding cyan won't be the same as decreasing yellow and magenta.

You aren't exactly choosing a cmyk profile with a very wide density range either. That one has a 300% ink limit. If you have an example of where this technique was needed perhaps I can offer a better alternative method.
Reply With Quote top
  #36  
Old 08-12-2011, 01:06 AM
RobertAsh RobertAsh is offline
Senior Member
Patron
 
Join Date: May 2011
Location: Oakland, CA area
Posts: 380
Re: RGB to Lab/CMYK and back to RGB

Quote:
Originally Posted by kav View Post
Whenever you change values you are changing density. Even for your purposes adding cyan won't be the same as decreasing yellow and magenta.

You aren't exactly choosing a cmyk profile with a very wide density range either. That one has a 300% ink limit. If you have an example of where this technique was needed perhaps I can offer a better alternative method.
True, but that is true for any color in any color space, not just cyan in CMYK. Lab color minimizes that effect to some degree but any change you make to one color will tend to change other colors in a somewhat non-symmetric manner. For the purposes of addressing Edgework's point I think my statement is reasonably accurate..... But you're right, there's a lot of precision that gets lost in general statements.

Is there a better CMYK to use? Would be great to learn about it.

There are two times I've needed to do this. One was during a wedding shoot in a darker-than-expected reception hall shooting available light. Another was during a portrait session where I forgot to color balance, plus I inadvertently overexposed the images. After a lot of hacking around with the portraits I got the color balanced but in a 2nd pass at it using a different image from the same portrait session, it turned out simple curves did the trick almost as well (pulling the shadow end down in Lightroom). Lesson learned.
Reply With Quote top
  #37  
Old 08-12-2011, 01:33 AM
kav kav is offline
Senior Member
 
Join Date: May 2009
Posts: 574
Re: RGB to Lab/CMYK and back to RGB

Quote:
Originally Posted by RobertAsh View Post
T
Is there a better CMYK to use? Would be great to learn about it.

.
I've tried to think of a good way to respond, but I guess I simply don't like this method.

Are you shooting raw? Do you have any files that you are able to show that exhibit these problems? I could probably show you better ways of dealing with these things.
Reply With Quote top
  #38  
Old 08-12-2011, 02:53 AM
bakerser bakerser is offline
Member
 
Join Date: Jan 2008
Posts: 91
Re: RGB to Lab/CMYK and back to RGB

Quote:
Originally Posted by RobertAsh View Post
Is there a better CMYK to use? Would be great to learn about it.
Genuinely OOC, what about the action which Jonas linked to doesn't work in your workflow?
Reply With Quote top
  #39  
Old 08-12-2011, 05:44 AM
edgework edgework is offline
Senior Member
 
Join Date: Jul 2004
Location: Brooklyn
Posts: 494
Re: RGB to Lab/CMYK and back to RGB

Quote:
Originally Posted by RobertAsh View Post
Hi Edgework,

Not to be contrary or anything, and certainly not claiming to be the world's expert here, but by increasing the Cyan in CMYK you're effectively decreasing the Magenta and Yellow together by fixed values, and vice versa.

I've found it useful to manipulate the Magenta and Yellow separately in skin tones, which made using CMYK more convenient.
That is exactly correct. So why, if your desire is to work on magenta and yellow separately (in RGB) would you use the Red channel? It's the wrong channel to use, for the very reason you point out.

If you are going to do color work in RGB (either before moving into CMYK, or because you don't need to move into CMYK at all), the green and blue channels are analogues to magenta and yellow. RGB = CMY. Raise Red, lower cyan. Raise Green, lower magenta. Raise blue, lower yellow. Study the Color Balance adjustment dialogue for a couple of seconds, and all will become clear (though don't actually use it for color work: it's just a subset of curves and gives roughly the same flexibility as levels.)

Last edited by edgework; 08-12-2011 at 06:46 AM.
Reply With Quote top
  #40  
Old 08-12-2011, 07:27 PM
RobertAsh RobertAsh is offline
Senior Member
Patron
 
Join Date: May 2011
Location: Oakland, CA area
Posts: 380
Re: RGB to Lab/CMYK and back to RGB

Quote:
Originally Posted by edgework View Post
That is exactly correct. So why, if your desire is to work on magenta and yellow separately (in RGB) would you use the Red channel? It's the wrong channel to use, for the very reason you point out.
Hi,

That's not what I'm saying, apologies if I wasn't clear on that. What I'm saying is that Red is too blunt an instrument. It's better (for me, anyway) to work with Magenta and Yellow individually instead.

Actually, back when I was doing this I really didn't think too much about working with Blue and Green instead of Magenta and Yellow. Very recently I've gotten my head around that better after attending one of the RetouchProLIVE webcasts here. I'm really busy, like to do things the most straightforward way possible (even if it's clunky) and often when I learn or develop a technique I just work with it. But working with Blue/Yellow and Green/Magenta is fine, I can try that.

One question,though, is that on really detailed color wheels I've Googled up often CMY does not equal RGB, as CMY and RGB are not opposites along all 3 colors. Any insight on that? does that affect at all how the opposite curves work?
Reply With Quote top
Reply

  RetouchPRO > Technique > Photo Retouching


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
I'm Back Flotsam2 Salon 3 06-20-2011 02:37 PM
Back to the future palms Photography 4 02-18-2011 03:05 PM
Converting CMYK to RGB and back to CMYK. xxxmen Input/Output/Workflow 23 01-06-2011 11:56 AM
PS3 - going back to adjust a layer maverick911 Photo Restoration 4 12-15-2010 07:14 AM
RGB separation on 2 separate areas tatrader Hidden Power Support 1 03-09-2005 12:32 PM


All times are GMT -6. The time now is 12:23 AM.


Powered by vBulletin® Version 3.8.4
Copyright ©2000 - 2017, Jelsoft Enterprises Ltd.
SEO by vBSEO ©2011, Crawlability, Inc.
Copyright © 2016 Doug Nelson. All Rights Reserved