Dan Margulis Applied Color Theory

When CMYK is Over the Ink Limit

RGB>CMYK>RGB when CMYK is over ink limit
Posted by: "williamtheis"
Sun Dec 31, 2006 4:38 am (PST)

Reading Dan's Professional Photoshop book, I really like the technique of building contrast in the K channel of CMYK for the horsewoman in Chapter 5. I have RGB images with large areas of neutral dark needing just this same detail enhancement.

My first question is that when I go RGB to CMYK I am a bit concerned about Dan's warning of the "big danger" of going in and out of CMYK due to its reduced gamut. Is there anything like "wide gamut CMYK" like there is for RGB to miminize the OOG problems for those of us that do the RGB>CMYK>RGB conversions? I understand that these arise from the ink limitations in the CMYK definiton but if CMYK is only a waypoint on the journey, it would seem there should be some work around that reduces this "big danger" but allows the benefit of CMYK

I also have tried to skip the step of reducing the ink limit (which for the horsewoman is the application of the Selective Color to the K of removing 27%C, 10%M and 10%Y)... seems like just converting right back to RGB without the step works just fine, or am I missing something?

I am printing using an Epson inkjet so the print driver is taking my final RGB right back into CMYK... so if there was a CMYK Epson print driver, I could do RGB>CMYK in the printer's gamut and the world might be in more control without this final dance of conversions

Bill Theis
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Re: RGB>CMYK>RGB when CMYK is over ink limit
Posted by: "Gene Palmiter"  
Sun Dec 31, 2006 5:22 am (PST)

http: //mike.russell-home.net/DigPhoto/WideGamutCMYK/Default.htm
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Re: RGB>CMYK>RGB when CMYK is over ink limit
Posted by: "Mike Russell"
Sun Dec 31, 2006 5:53 am (PST)

Thanks Gene,

I would add that a copy of the wide gamut CMYK profile is included on the book CD, and there is an updated version of the page Gene refers to here:
http: //www.curvemeister.com/tutorials/widegamutcmyk/index.htm

Mike Russell
www.curvemeister.com/forum/
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Re: RGB>CMYK>RGB when CMYK is over ink limit
Posted by: "williamtheis"
Sun Dec 31, 2006 1:37 pm (PST)

Thanks! I had not seen this (but suspected it existed. I will definitely give it a try...

so what about my other questions: is it OK to skip Dan's reduction of ink laydown if you are going right back to RGB?

Bill Theis
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Re: RGB>CMYK>RGB when CMYK is over ink limit
Posted by: "williamtheis"
Sun Dec 31, 2006 1:47 pm (PST)

an immediate problem that I foresee with the wide gamut CMYK is the heaviness of the black plate: Dan's technique relies upon increasing its contrast significantly but because wide gamut CMYK includes much of the midtones, they are affected as well. Not desireable... no need to go to CMYK if I can't get this benefit

on the other hand, Dan's CMYK seperation has Black Generation as "Light" and an 85% Black Ink Limit to confine the K to the darkest areas of the image.

Can there be a similar wide gamut CMYK made just for purposes of doing Dan's method discussed in Chapter 5?

Bill Theis
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Re: RGB>CMYK>RGB when CMYK is over ink limit
Posted by: "Howard Smith"
Sun Dec 31, 2006 1:47 pm (PST)

Bill, thanks to your query I checked out Mike Russell's excellent tutorial on wide gamut CMYK. Been meaning to do this for a long time and just never did. Trying the wide gamut CMYK profile with a very bright, saturated photo of a bouquet with a deep red background did not give me the results for which I had hoped. The blues and purples came through very well, needing only a little reduction in Cyan to come very close to the original RGB. Red was another problem, and one for which I could find no solution. The reds tended to be more pronounced in the yellow blossoms, but dimmer and darker in the deep red background. Still, it is a big improvement over a standard CMYK profile (please, let's not get into a long discussion about 'standard' CMYK which does not exist; just used here to illustrate my point). Or looking at it another way, it's almost certainly a big improvement over other CMYK profiles which do tend to lose their color intensity in a RGB>CMYK conversion even if you start with sRGB.

But, so much for that. I'm not much on theory, so here's a little solution that is probably theoretically faulty but one that gets the job done very nicely and in very short order.

Create a duplicate of your RGB image. Convert it to your choice of CMYK. Create a Curves adjustment layer and do whatever you like with the Black channel. When you are satisfied with your blacks, go back to the RGB image and add a blank layer above the background layer. Use Image/Apply Image to copy the black channel from the CMYK file and paste it into the blank layer of your RGB file. Looks awful, doesn't it?

Now change the blending mode of the pasted layer to Darken and adjust the opacity. There are your blacks with very little trouble, and you see that you can still adjust their intensity even though you're no longer in CMYK. Try some of the other blending modes just out of curiosity, but you'll probably come to back to Darken which will affect only the blacks in your image (or, more correctly, only the dark neutrals in your image).

Someone may notice that the pasted layer, while close, is not identical to the original CMYK black channel. That, of course is not a problem. For the purists who worry about such things, use a Clipping Group composed of a Curves adjustment layer (set to Luminosity) and the pasted black channel. With the RGB image in full color, adjust the Luminosity Curve until the darks in your RGB image match those in the CMYK image. That, of course, presumes that you were happy with the appearance of the CMYK image. Another nice thing about this method is that you can still edit only on the blacks, even after the conversion back to RGB.

There are other ways of doing the same thing, but probably none that are simpler or easier to understand. Always on the search for better ways, I hope someone can prove me wrong here.

Howard Smith
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Re: RGB>CMYK>RGB when CMYK is over ink limit
Posted by: "williamtheis"
Mon Jan 1, 2007 6:27 am (PST)

thanks Howard for pointing this out ... plus the limitations of the wide gamut CMYK...

actually Dan has very much the same approach that you suggest in Chapter 7 of Pro Photoshop when he was converting an RGB of the tomb of Machiavelli to B&W: he multiplied an enhanced copy of the K with the Red channel before conversion...

I agree that your approach should work but I am still thinking more in terms of getting a lighter K channel with that wider CMYK. I had been playing with just applying curves through an inverted luminosity mask on a seperate layer, then changing to luminosity blending but so far Dan's approach is far better in my initial tests. I'll also put your horse into the race...

I guess I need to understand how to produce a custom CMYK colorspace....

Bill Theis