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RGB to Lab/CMYK and back to RGB

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  #11  
Old 07-28-2011, 05:30 AM
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Der_W Der_W is offline
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Re: RGB to Lab/CMYK and back to RGB

Did you know you can work in CMYK without leaving RGB?

It's possible to create the CMYK channels in RGB without needing to convert and without great loss (actually in most cases no loss will occur at all).
Just try this action by Sean Baker: http://sean-blog.twicebakedphoto.com...yk-in-rgb.html
And for an explanation of the steps: http://sean-blog.twicebakedphoto.com...explained.html

Hope that helps some of you :-)!
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  #12  
Old 07-28-2011, 06:26 AM
edgework edgework is offline
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Re: RGB to Lab/CMYK and back to RGB

Quote:
Originally Posted by DWThomp View Post
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.Thank you for the help and tips.
Use a curve adjustment layer in luminosity mode. Works wonders when the tones are in a decent range but the overall shape is flat. Any further color tweaks can be done with another curve in color mode. I rely on a good skin mask to just bring the skin back to RGB. If the image is destined for print, I'd do whatever moves I need in RGB, then wait until moving into CMYK to work on skin and shadows.

As Andrew said, there's no reason to push the entire image into CMYK and lose the advantage of RGB unless it's going to press, in which case you'll lose it anyway, at some point.

One thing to keep in mind about the reduced gamut of CMYK: most items in the real world fit into it fairly well. Landscape shots, face shots and most interiors will suffer very little from the conversion. Clothing can take a heavy hit. Find an image of a dress with an intricate design built from subtle shades of greens and blues, then weep at the mud that results when you move to CMYK. At that point, you have to accept that you'll never match the color, and you try to preserve detail instead.

Also, virtually every color image you've ever seen in a book or magazine that impressed you with it's color and impact did the job with poor old CMYK inks, shoddy as they may be compared to the light from a screen. So all is not lost in the translation.

Last edited by edgework; 07-28-2011 at 06:48 AM.
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  #13  
Old 07-28-2011, 08:12 AM
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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 edgework View Post
One thing to keep in mind about the reduced gamut of CMYK: most items in the real world fit into it fairly well. Landscape shots, face shots and most interiors will suffer very little from the conversion. Clothing can take a heavy hit. Find an image of a dress with an intricate design built from subtle shades of greens and blues, then weep at the mud that results when you move to CMYK. At that point, you have to accept that you'll never match the color, and you try to preserve detail instead.

Also, virtually every color image you've ever seen in a book or magazine that impressed you with it's color and impact did the job with poor old CMYK inks, shoddy as they may be compared to the light from a screen. So all is not lost in the translation.
I agree that the color of many photographs would fit within a CMYK color space such as US Web Coated (SWOP) v2 yet once you take that round-trip to a limited gamut space, the colors are gone. There are many color printers (inkjet) that have a much wider gamut than prepress CMYK as per the comparison below. The inner gamut is US Web Coated (SWOP) v2 CMYK space and the out gamut is for an ESPON 2400:

Screen shot 2011-07-28 at 6.31.54 AM.jpg

I also have a friend (landscape photographer) who prints exclusively on inkjet printers. He wants the extra punch from post processing and part of that is just tonality yet part of that is a wider color gamut. Makes a difference on his sales.

So before jumping in to truncating your color space with a round trip to CMYK, be sure you know what you lose and if that still meets your needs.
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  #14  
Old 07-28-2011, 09:17 AM
edgework edgework is offline
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Re: RGB to Lab/CMYK and back to RGB

You can get brighter colors from a laser printer. I used to go crazy when clients would send in these ratty 11 x 17 comps with their day-glo toner colors and want to know why the Epson proof looked so dull. Even worse were the clowns who'd show up with a printed piece and say "I need to match these spot colors." I'd try to patiently explain that's why spot colors were invented... BECAUSE YOU CAN'T MATCH THEM!!! but it never did any good.

No question, keep an RGB master .psd file. But people probably should acquaint themselves with the pitfalls of CMYK; it's been around for a long time and chances are everyone will bump up against it sooner or later.
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  #15  
Old 07-28-2011, 09:54 AM
RobertAsh RobertAsh is offline
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Re: RGB to Lab/CMYK and back to RGB

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Originally Posted by edgework View Post
....Even worse were the clowns who'd show up with a printed piece and say "I need to match these spot colors." I'd try to patiently explain that's why spot colors were invented... BECAUSE YOU CAN'T MATCH THEM!!! but it never did any good....
What do you mean by 'that's why those colors were invented? Please explain more, that's an interesting data point.
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  #16  
Old 07-28-2011, 10:44 AM
edgework edgework is offline
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Re: RGB to Lab/CMYK and back to RGB

The Pantone Color System. Mostly used for solid colors that can't be achieved in CMYK. Color logos for magazine covers, for example. Anything that's a bright orange, brilliant red or exotic purple, blue or green or pastel is printed with a fifth ink, specially mixed according the to Pantone formulas. Even when you can get something close with CMYK, you'd end up with halftone dots that look cheesy, whereas the unique ink is applied at 100% and so presents a smooth, continuous surface.

Sometimes you'll see a fifth ink used in photos to provide a color boost for special items: cars, clothing, etc. Usually combined with black to keep detail and shape.
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  #17  
Old 07-28-2011, 06:49 PM
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Re: RGB to Lab/CMYK and back to RGB

Quote:
Originally Posted by Der_W View Post
It's possible to create the CMYK channels in RGB without needing to convert and without great loss (actually in most cases no loss will occur at all).

Hope that helps some of you :-)!
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.

Quote:
Originally Posted by edgework View Post
Use a curve adjustment layer in luminosity mode. Works wonders when the tones are in a decent range but the overall shape is flat. Any further color tweaks can be done with another curve in color mode. I rely on a good skin mask to just bring the skin back to RGB. If the image is destined for print, I'd do whatever moves I need in RGB, then wait until moving into CMYK to work on skin and shadows.
I've played around with your suggestions, but am having a time getting the changes made in the CMYK copy back into the RGB original. I'm still playing around with it, but so far I flatten the copy and bring it into the original.

I'll need to keep the end result in RGB as the lab I use requires files to be sRGB. I haven't been converting to CMYK and then converting back, but a post by Robert in the other thread gave me some questions. It seems a lot of the tutorials, etc use CMYK values and got me wondering.

Thanks again for all the help so far.
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  #18  
Old 07-28-2011, 10:51 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 DWThomp View Post
I've played around with your suggestions, but am having a time getting the changes made in the CMYK copy back into the RGB original. I'm still playing around with it, but so far I flatten the copy and bring it into the original.

I'll need to keep the end result in RGB as the lab I use requires files to be sRGB. I haven't been converting to CMYK and then converting back, but a post by Robert in the other thread gave me some questions. It seems a lot of the tutorials, etc use CMYK values and got me wondering.

Thanks again for all the help so far.
You can usually use the red channel to make a good skin mask. When you dupe your file and take it into CMYK your red copy will travel with it. Use that to select the skin and just drag it back to the RGB file.
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  #19  
Old 07-29-2011, 12:44 AM
RobertAsh RobertAsh is offline
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Re: RGB to Lab/CMYK and back to RGB

Quote:
Originally Posted by edgework View Post
The Pantone Color System. Mostly used for solid colors that can't be achieved in CMYK. Color logos for magazine covers, for example. Anything that's a bright orange, brilliant red or exotic purple, blue or green or pastel is printed with a fifth ink, specially mixed according the to Pantone formulas. Even when you can get something close with CMYK, you'd end up with halftone dots that look cheesy, whereas the unique ink is applied at 100% and so presents a smooth, continuous surface.

Sometimes you'll see a fifth ink used in photos to provide a color boost for special items: cars, clothing, etc. Usually combined with black to keep detail and shape.
Thanks. I know what Pantone is but didn't know its origin or original purpose. Very interesting!
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  #20  
Old 07-29-2011, 12:48 AM
RobertAsh RobertAsh is offline
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Re: RGB to Lab/CMYK and back to RGB

Quote:
Originally Posted by edgework View Post
You can usually use the red channel to make a good skin mask. When you dupe your file and take it into CMYK your red copy will travel with it. Use that to select the skin and just drag it back to the RGB file.
This is efficient because in CMYK you usually want to either work on skin or prepare a file for printing presses.


If the skin is hard to select the way Edgework suggests, then you can
  1. duplicate the image,
  2. convert to profile CMYK,
  3. work on the skin,
  4. convert to profile <whichever flavor of > RGB,
  5. drag the flattened image back into your original image and
  6. mask out the parts you don't need in the original.
Edgework's idea is a lot cleaner and easier whenever you can do it his way.
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