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RGB to Lab/CMYK and back to RGB

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  #1  
Old 07-26-2011, 11:04 PM
D Thompson's Avatar
D Thompson D Thompson is offline
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RGB to Lab/CMYK and back to RGB

I asked a question in this thread (http://www.retouchpro.com/forums/pho...s-lab-raw.html) about going from RGB>Lab/CMYK>RGB and got some good answers from Robert & John. I usually stay in RGB and will change the color sampler readout to CMYK when doing some skin color corrections, but at times I have trouble adjusting when I'm working in RGB.

Robert had a work-around using a duplicate image converted to Lab or CMYK, flatten, convert to RGB, and drag back into the original file. This allows you to keep the previous adjustment layers in the original.

John brought up something that I've heard which is you can get shifts by converting from one to another and back. He also suggested that there are ways to minimize those impacts which should be the topic of another thread. So here it is.

Is the process that Robert uses the way to do it or are there other ways to minimize the impact. The stuff I do is probably not critical work, but I still try to do the best I can. I'd always heard the downside to going back and forth and just wonder what ways are suggested to lessen any changes.

Thanks to Robert & John for your thoughts in the other thread.
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  #2  
Old 07-27-2011, 07:39 AM
edgework edgework is offline
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Re: RGB to Lab/CMYK and back to RGB

There's no reason to jump into CMYK if the image isn't exclusively meant to go to press. You lose all subtle detail in blues and greens and forget about pastels. If I know the image is targeted for print, I'll save some final moves for CMYK, when I know I'm not going back, (I keep an RGB layered file in any event). You can get some effective shadow moves with the black plate that aren't available in RGB, and skin tones respond well in CMYK. One reason is that many of the really hot values that RGB can come up with don't exist in CMYK and get crunched into a more realistic range. Another is that all shadow detail is in the black plate, so the moves you make with CMY curves have a more direct effect on the relationships between the different inks in skin tones.

You can still take advantage of CMYK even when you want to stay in RGB. Dupe the image, convert to CMYK and pull the black plate back to the RGB image as a selection mask. It's not quite as effective as a real black plate, but it allows you to make adjustments that RGB alone doesn't do well, separating shadows from color ranges. If you can create a good skin mask some nice refinements are available in CMYK, which you can then pull back to RGB. (I've never found skin that can't be improved by a luminosity contrast curve to the magenta channel, something that doesn't translate well in RGB).

When skin is too hot, copying the red plate, duping the image, converting to CMYK and swapping the original red for the new cyan gets around the dot gain compensation that Photoshop builds into CMYK conversions. The resulting skin is usually a bit too blue, but that can be cured with a quick curve, whereas you can try all day long to jack up the worthless cyan plate from which most information is bleached out. Then you can pull the skin back to RGB.

So there are ways to take advantage of the different spaces and what they accomplish, but moving an image wholesale into CMYK if it's not required isn't one of them.
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Old 07-27-2011, 07:46 AM
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Re: RGB to Lab/CMYK and back to RGB

If you want to stay in RGB rather than converting back and forward then there is a plug in that allows you to do this.

I have not tried it but Curvemeister is said to give you the ability to work in RGB and edit in LAB, CMYK without leaving your preferred working environment,
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Old 07-27-2011, 12:16 PM
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John Wheeler John Wheeler is offline
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Re: RGB to Lab/CMYK and back to RGB

I tend to agree with edgework yet I also know that there are those that just "think" in CMYK and thinking in RGB is difficult. Here are some pointers that may be of help about Wide Gamut CMYK to help avoid some of the issues:


http://mike.russell-home.net/DigPhot...YK/Default.htm

Here is the link into CurveMiesters Tutorial section with some more specific info (same site that Tony linked in his post).

http://www.curvemeister.com/tutorial...cmyk/index.htm

Just converting to CMYK and back has a large loss of gamut as has been mentioned. Here is the visual overlay of the gamut limits:

Screenshot2011-06-08at12130PM.png


Here is a 3D image overlaying WGCMYK with Adobe RGB:

Screenshot2011-06-09at82645AM.jpg

The WGCMYK looks like it has a good overlap Adobe RGB.

However,

I tried the conversions going back and forth to the Wide Gamut CMYK Space, and while it did greatly reduce the loss of Gamut and any color shift compared to other CMYK spaces, there still was a shift that was a non-trivial amount.

Starting with an sRGB Color Space and the red color 255,0 , 0 and using a conversion to CMYK US Web Coated (SWOP) v2 the results on a round trip conversion were 207, 44, 45

Starting with 255,0,0 and converting to a wide Gamut CMYK the results on a round trip conversion were 254, 42, 11.

Less of a shift yet you end up with a slight Hue shift and reduced Saturation. This is too big a shift so I cannot recommend this as a good general approach.

I cannot vouch for the CurveMeister tool if it does a better job or not. Their website refers to this Wide Gamut CMYK space so it makes me wonder if their tools has the same issues. I cannot advise. They may have a trial version of the software to try out. Good Luck

PS - this post was leveraged from a post I did a couple months ago on NAPP
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Old 07-27-2011, 02:14 PM
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andrewrodney andrewrodney is offline
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Re: RGB to Lab/CMYK and back to RGB

CMYK is a highly device dependent output color space. Its sole ‘role’ is to provide CMYK values to produce some kind of print. I do hear many users say “I grew up with CMYK, its easier for me than RGB” but I don’t necessary get it. I suppose if you work with a single CMYK recipe for years, you get to know the numbers pretty well. The issue is, most every CMYK output device is vastly different from another. Even to the same device, all you have to do is alter GCR, UCR, stuff like that, and the numbers will be vastly different (think about it, UCR or GCR alter the relationships between CMY and K, ouch).

RGB when defined in what Adobe calls a working space is pretty simple in that a neutral is always an equal mix of RGB. Not the case with all RGB color spaces by a long shot. But for editing color spaces, sRGB, Adobe RGB (1998), ColorMatch, ProPhoto etc, that rule you can bank on.

For me, there are only two cases I’ll convert to CMYK. First and foremost is to produce an specific output ready document for print. And the recipe is one I’ll build or one I know is something I can bank on (Gracol 7 going to a press I KNOW is printing that exacting behavior). The other time is on a dupe to pull some channel to bring back into the RGB original for some kind of work. And I’m finding that rendering a grayscale channel as I wish, using the raw converter provides a lot more control here than trying to convert with some flavor of CMYK and hoping one of the channels is what I need. Remember, you can build grayscale appearing data from the raw and use that turned to your needs from the raw, just as you could take a single channel of a CMYK document.
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Old 07-28-2011, 01:20 AM
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D Thompson D Thompson is offline
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Re: RGB to Lab/CMYK and back to RGB

Quote:
Originally Posted by edgework View Post
If you can create a good skin mask some nice refinements are available in CMYK, which you can then pull back to RGB. (I've never found skin that can't be improved by a luminosity contrast curve to the magenta channel, something that doesn't translate well in RGB).

When skin is too hot, copying the red plate, duping the image, converting to CMYK and swapping the original red for the new cyan gets around the dot gain compensation that Photoshop builds into CMYK conversions. The resulting skin is usually a bit too blue, but that can be cured with a quick curve, whereas you can try all day long to jack up the worthless cyan plate from which most information is bleached out. Then you can pull the skin back to RGB.
I've not really worked much in CMYK itself, mainly just using it to help create a mask. The luminosity contrast curve to the magenta channel sounds interesting. After duplicate the image and converting to CMYK do you ctrl-click the magenta channel to load it as a selection and apply the curve or apply a curve to magenta and change the blend mode to luminosity? After that I'm not sure how to get it back to the original image. Same with the red plate technique for the hot skin - I can get it to the point of how do I get it back to the original image.
Quote:
So there are ways to take advantage of the different spaces and what they accomplish, but moving an image wholesale into CMYK if it's not required isn't one of them.
Thank you for the help and tips.
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Old 07-28-2011, 01:22 AM
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Re: RGB to Lab/CMYK and back to RGB

Quote:
Originally Posted by andrewrodney View Post
Remember, you can build grayscale appearing data from the raw and use that turned to your needs from the raw, just as you could take a single channel of a CMYK document.
Thank you for the good info and I'd never thought of doing this. I'll have to give it a try.
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Old 07-28-2011, 01:23 AM
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D Thompson D Thompson is offline
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Re: RGB to Lab/CMYK and back to RGB

Quote:
Originally Posted by John Wheeler View Post
PS - this post was leveraged from a post I did a couple months ago on NAPP
Thanks John.
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Old 07-28-2011, 03:09 AM
kav kav 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
I asked a question in this thread (http://www.retouchpro.com/forums/pho...s-lab-raw.html) about going from RGB>Lab/CMYK>RGB and got some good answers from Robert & John. I usually stay in RGB and will change the color sampler readout to CMYK when doing some skin color corrections, but at times I have trouble adjusting when I'm working in RGB.
Be careful of your settings with this. When I process a raw file I check for detail on all channels before I do anything else. Say you start off with clipping in your red channel. Regardless of what your cmyk numbers say this will create a potential problem. The cmyk references are most useful for avoiding clipping if you're just using a generic profile. You can't really balance for while still in RGB, and it's pointless making a round trip.
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Old 07-28-2011, 04:33 AM
RobertAsh RobertAsh is offline
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Re: RGB to Lab/CMYK and back to RGB

Dennis,

Thanks for the kind feedback

There are two really good reasons I've personally found to go into CMYK (besides final targeting for print):
  1. Drawing out shadow detail with curves. the K channel in CMYK is mostly midtones and shadows, not much of its length is highlights. Dan Margulis' book mentioned this and one time I was stuck and had to try it. It worked. Nowadays I use Shadow/Highlights and Fill Light and get by pretty well. But that one time CMYK saved my bacon, but good (it was a wedding reception hall image shot with no flash).
  2. Correcting skin tones. In situations where white balance is off it can be a bear to correct skin tones. I was in such a situation not long ago and was referred to this article. It has a nifty CMYK value chart for different ethnicities' skin tones. I had to adjust the values to be different from the ones listed in the article. The article saved my shoot by giving me a good starting piont, but its author says he works in RGB to adjust CMYK values.
So I thought -> so why not just work in CMYK then? That worked much better for me. Human skin has a lot of yellow and magenta. It doesn't have much green or blue (normally!!). Adjusting red adjusts both magenta and yellow by lock-step amounts, and you usually want to adjust them separately. So why not just adjust them directly? Edgework's note is a good summary of some similar situations he's run into.

Device dependency is a valid point. As a practical matter, however, it's good to keep in mind that all color spaces are device dependent (perhaps some more than others). Try looking at RGB images on one monitor versus another. Or in Photoshop vs. Internet Explorer. CMYK may be more so but no color space is immune from that problem. So do what meets your needs best.

It's best to stay in one color space if you can. I bought a Curvemeister license for just that purpose recently, will see if it works!

But whatever you do, don't fall into the 'color space wars' trap. Learn the strengths and weaknesses of each, both from experts and through your own application and experience. Then use the one that's best for the job at hand. Dan Margulis' book has a good summary, there are other sources as well.

Last edited by RobertAsh; 07-28-2011 at 04:38 AM.
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