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holes in cmyk separations

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  #1  
Old 01-10-2008, 01:15 AM
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Sveltepig Sveltepig is offline
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holes in cmyk separations

hi all
hope you can help me with this.
I work in the publishing industry, photo-retouching images for print.
I use an rgb workflow for all my image work, and then convert to cmyk to place an image in InDesign. The more I learn about working with photographs, the more I seem to be having this issue:
Printers and cmyk-workflow retouchers recommend that there be no holes (areas of missing information) in the cmyk separations in the final placed cmyk image file, in order to minimise banding (especially not when using gravure printing (which clips the possible image information as it is able only to reproduce tones of 7% or more)).
In other words there must be an even tone in all channels, including cyan (especially on skin-tones). But the techniques I am using more and more (pwl, dodge and burn) leave me with a glowy, glossy image in rgb, and an image full of holes (on the highlight areas) in cmyk.
When I fill in these holes (using a combination of apply image and information copying from other channels), the glowy, glossy skin effect is minimised.
How do I get around this?
What am I doing wrong?
please help.
Karin
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Old 01-10-2008, 05:13 AM
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Re: holes in cmyk separations

Hmm. Gravure printing should be able to hold at least 2% more than that.

Does the info exist in the original or does it disappear with your corrections?

Chris
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Old 01-10-2008, 05:25 AM
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Re: holes in cmyk separations

info exists in some of the files, but many of the digital photographs I receive are very warm on the skin tones, and I think it's the high red values that encourage loss of info in the eventual cyan separations.
k
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Old 01-10-2008, 05:49 AM
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Re: holes in cmyk separations

At the moment try using the output portion of levels to bring some of the data back in. It will limit how much gets clipped out with the conversion to cmyk. I suggest doing it above the base layer if possible. This MIGHT work, although I believe your original approach with apply image is a better way to go. hard to say without seeing a good version of the file.

In the future you might want to get the raw digital files and do the conversions yourself if possible. Make sure you get a flatter out put to leave a bit of lateral movement on each end for corrections.

Chris
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Old 01-10-2008, 06:19 AM
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Re: holes in cmyk separations

so what you're saying is that the holes shouldn't appear if the original file and the subsequent work I do on it is done correctly?
Maybe I am overdoing the dodging - this is where the problem often occurs...

What you are saying about adjusting the output in levels makes me wonder if a cmyk profile that clips the colours to the printing limits would help this issue.

Am I wrong in thinking that there is only one way to convert mode from rgb to cmyk?

I use proof colours to estimate the gamut loss and potential holes when working in rgb, which helps to know where I will need to apply image later (once image is in cmyk mode), but I still find myself wondering if this is arse-about-face and counterproductive.

If I attempt to correct the problem in rgb mode, I find myself lowering the red curve more than I would have expected to in order to balance out the m and y channels. Is it just that my expectation is inaccurate? My monitor is callibrated, and the final printed product is very close to the cmyk screen version (albeit less luminous).

I will post some samples tomorrow to illustrate.
thanks for your time
Karin
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Old 01-10-2008, 07:08 AM
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Re: holes in cmyk separations

Karin

If the files are coming in to you already processed to their respective RGB space than more then likely it is nothing you are doing wrong. But it does sound like at the least the balance is off from the beginning.

Dodging to much will cause a kind of banding itself in the highlights. There can be a tendency to create a "vignette" effect that will be problematic at press.

A CMYK Profile should not have to clip to the limits set in RGB mode. It should and does mostly recreate the look faithfully. Setting it in RGB should suffice. Sometimes tweaking a CMYK profile will create some havoc elsewhere, most likely seen at the clip points on each end.

There are two ways to convert. Using convert of course is the most obvious. Color mode RGB to cmyk will utilize whatever your profiles are in your color settings panel. They will utilize whatever intent was built into the CMYK profile. So theoretically, they will look the same whichever method you choose if you do not change the intent in Convert....

Proof colors is great for reading gamut etc. It does not however let you know where hilight and shadow clipping occur in an image unless you use point samples. I talked with a Adobe engineer about building in the clipping feature a couple months ago. Keep your fingers crossed that she can do it. I would really welcome that feature.

I don't know how you are converting or what the build is for your cmyk profile. There CAN be a tendency to see some colors diminish in saturation. Luminance should not change though. Are you using a custom CMYK profile?
What is the ucr for the profile? Is it a GCR profile?

Chris
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  #7  
Old 01-10-2008, 08:09 AM
KR1156 KR1156 is offline
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Re: holes in cmyk separations

I compose 3-d cigarette boxes here using Illustrator and photoshop. they are usually white boxes, like a marlboro box, our press house allows no less than 3% dot.

so, as a little sort of guide, or warning to myself when i get carried away with highlights, i make another image window and i'll have let's say the red channel shown, so this way as i'm dodging or what not, I can keep a little bit more of an eye on the tone should it start clipping.

after you're staring at an image for hours, or are in a groove, you tend to lose sight of these things.

don't know if that's any help. But having a clipping feature like chris said would be tremendous help.
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Old 01-10-2008, 11:51 AM
pixelzombie pixelzombie is offline
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Re: holes in cmyk separations

that's also a problem with flexo files as they aslo have MIN dot issues, we would make custom curves to check images for such issues before a proof was ever made and we could easily see ares of the file that needed additional work, idon't know if that helps or not...
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Old 01-11-2008, 06:11 AM
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Re: holes in cmyk separations

hi all
here is a small sample of what I am talking about.
It is not a finished piece - I would adjust colour some more and dodge and burn more too, but will do for an example.
first image shows face with cyan separation.
second image shows face with apply image from magenta channel in holes on cyan channel and new cyan separation.
lastly, a close-up of the holes.
k
Attached Images
File Type: jpg holes.jpg (90.2 KB, 34 views)
File Type: jpg no-holes.jpg (99.9 KB, 33 views)
File Type: jpg cyan.jpg (99.6 KB, 31 views)
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  #10  
Old 01-11-2008, 06:31 AM
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2 ways to convert?

sorry, guys, am a bit brain-dead today
many of your comments a bit above my head at present..
please bear with me if I am being stupid or asking the same questions more than once..

It was the gravure printers that drew my attention to this issue, but I imagine holes are also a drawback in other forms of offset printing, so I would like to learn how to approach this problem. The pictures I uploaded in my last post illustrate the problem.

My main question is: do any of you regularly have this issue, or has it something to do with my methods? I would hate to think that I am approaching my images in a way that exacerbates the problem. Do you think that clipping the levels and trying to keep the red channel darker than a certain point is a good idea? Or is it maybe better to just fill in the holes after the fact (once it is cmyk)?

Chris, you say:

"There are two ways to convert. Using convert of course is the most obvious. Color mode RGB to cmyk will utilize whatever your profiles are in your color settings panel. They will utilize whatever intent was built into the CMYK profile. So theoretically, they will look the same whichever method you choose if you do not change the intent in Convert...."

what is the other method? is it convert to profile? isn't that the same as mode>cmyk if you want the profile that is set as your working space?

when I refer to the lower luminosity in print I just mean it is on paper and has no light shining through it, so the colour looks very much as it did on screen, but it is flatter somehow as there is no light backing it up. Is luminance a bad way of describing this? What would you say?

KR1156, how do you have more than one image window?
is it like image>duplicate? how does it update as you work?

so many questions.
am going home now for a weekend and will try not to obsess about this stuff. Maybe it'll come to me in a blinding flash when I arrive at work on monday morning.
thanks again for the input.
k
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