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Author Topic: RGB to CMYK conversion question?  (Read 4850 times)

Coloreason

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RGB to CMYK conversion question?
« on: May 17, 2012, 03:34:03 AM »

Is there a standard RGB color space that have values which when converted will match precisely these CMYK values in U.S. Web Coated (SWAP)v2:

C100, M0, Y0, K0
C100, M100, Y0, K0
C100, M0, Y100, K0
C0, M100, Y0, K0
C0, M100, Y100, K0
C0, M0, Y100, K0



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Czornyj

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Re: RGB to CMYK conversion question?
« Reply #1 on: May 17, 2012, 05:26:39 AM »

No, it's impossible - it just doesn't work like that

Coloreason

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Re: RGB to CMYK conversion question?
« Reply #2 on: May 17, 2012, 11:28:52 AM »

Thanks,
I thought that using a wider RGB space when converted to CMYK  will make the most saturated colors possible in the CMYK but based on my tries I get the opposite result. So, what is the best strategy if creating an artwork intended for use with RGB and CMYK output? Recreate the artwork again for each space? I thought that color management was designed to make things like this easier,  work in one wider space and get the closest match in another. If using colors outside the CMYK color space I thought the most saturated colors in RGB will match the most saturated colors in the CMYK. Then what's the purpose of the color management conversions. What do I need to understand?
« Last Edit: May 17, 2012, 11:30:37 AM by Coloreason »
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Coloreason

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Re: RGB to CMYK conversion question?
« Reply #3 on: May 17, 2012, 11:43:46 AM »

Another question that will hopefully help me understand.
Is it possible to find exact color match of the example values from my first post if they are used separately one by one? Like an image with one flat color only.
I thought that a wider color space encompassing a narrower color space can match all colors of the narrower color space. Is this not true?

edit: If creating an artwork with unknown destination but knowing that it will be printed on different CMYK printers which are not yet specified in the job, is it better to create in RGB or CMYK color space if the goal is creating design that will contain the most saturated colors possible in each eventual CMYK color space?
« Last Edit: May 17, 2012, 11:53:47 AM by Coloreason »
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Scott Martin

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Re: RGB to CMYK conversion question?
« Reply #4 on: May 17, 2012, 12:00:37 PM »

I think the best things you can do are:

1) Work in ProPhotoRGB so these gamut extremities aren't clipped
2) Assign colors in your ProPhotoRGB file by using the CMYK values in the color picker.
3) Look at the CMYK values in your info palette while working in your RGB file.
4) Use the newer "WebCoatedSWOP2006Grade3.icc" profile instead of the older "USWebCoatedSWOP.icc" for your RGB to CMYK conversions. They are much better.
5) Make sure your Color Settings are setup using your preferred CMYK space and conversion options as these are used not only for the conversion but also for the CMYK values presented in the Info palette while working on an RGB file.

This will help minimize, but not eliminate, the discrepancies you are seeing.

Hope this helps!

Scott Martin

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Re: RGB to CMYK conversion question?
« Reply #5 on: May 17, 2012, 12:04:16 PM »

edit: If creating an artwork with unknown destination but knowing that it will be printed on different CMYK printers which are not yet specified in the job, is it better to create in RGB or CMYK color space if the goal is creating design that will contain the most saturated colors possible in each eventual CMYK color space?

If your talking about creating photographic imagery for offset reproduction then you are necessarily starting in RGB and must intelligently mange the CMYK conversion.

If you are talking about creating type, layout, vector objects and any non-photogrpahic imagery for offset reproduction then you might as well create it in CMYK.

How much effort you want to spend optimizing your files for different CMYK spaces is up to you.

digitaldog

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Re: RGB to CMYK conversion question?
« Reply #6 on: May 17, 2012, 12:30:36 PM »

Another question that will hopefully help me understand.
Is it possible to find exact color match of the example values from my first post if they are used separately one by one? Like an image with one flat color only.
I thought that a wider color space encompassing a narrower color space can match all colors of the narrower color space. Is this not true?

Essentially there is no way to get the toothpaste (CMYK) back into the tube to generate the original RGB values. You can generate Lab values from the CMYK using the profile itself but they do not correlate in any way to RGB. It is a bit like that old game “Telephone” where by the time you get to the last person in the chain, what was initially said is long gone and unknown.

Quote
edit: If creating an artwork with unknown destination but knowing that it will be printed on different CMYK printers which are not yet specified in the job, is it better to create in RGB or CMYK color space if the goal is creating design that will contain the most saturated colors possible in each eventual CMYK color space?

You can’t create CMYK from RGB until you know what the parameters of the CMYK process is. So you start with RGB and you stay there until you have those specific parameters (defined by an ICC profile). IOW, get the print job specifics before you even think about the world of CMYK.
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Coloreason

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Re: RGB to CMYK conversion question?
« Reply #7 on: May 18, 2012, 09:24:13 PM »

I think the best things you can do are:

1) Work in ProPhotoRGB so these gamut extremities aren't clipped
2) Assign colors in your ProPhotoRGB file by using the CMYK values in the color picker.
3) Look at the CMYK values in your info palette while working in your RGB file.
4) Use the newer "WebCoatedSWOP2006Grade3.icc" profile instead of the older "USWebCoatedSWOP.icc" for your RGB to CMYK conversions. They are much better.
5) Make sure your Color Settings are setup using your preferred CMYK space and conversion options as these are used not only for the conversion but also for the CMYK values presented in the Info palette while working on an RGB file.

This will help minimize, but not eliminate, the discrepancies you are seeing.

Hope this helps!

Thanks,
I made some tests in Photoshop by creating new images in different RGB color spaces and using the CMYK sliders set to C0 M100 Y100 K0 I filled each image with one red flat color, and then I converted the RGB images to CMYK which by default converts to U.S. Web Coated (SWOP)v2 with Relative Colorimetric intent. Here's the results that I believe everyone will get with this test.

sRGB   to   U.S.Web Coated (SWOP)v2 = C0 M99 Y97 K0
AdobeRGB   to   U.S.Web Coated (SWOP)v2 = C0 M96 Y85 K0
ProPhoto   to   U.S.Web Coated (SWOP)v2 = C0 M100 Y58 K0

and here's a snapshot, the documents on the left column are the RGB spaces and on the right column when converted to CMYK



The test show that the wider the color space the worse the color match. I don't know what to think about this but using wider RGB color spaces for a CMYK destination doesn't seem very attractive.

I'm not familiar with WebCoatedSWOP2006Grade3.icc. All I know that each CMYK profile is for a certain printer and paper and my work is most likely to end using offset printing in Canada which I believe is with the U.S.Web Coated (SWOP)v2 standard.

« Last Edit: May 18, 2012, 09:26:07 PM by Coloreason »
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Schewe

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Re: RGB to CMYK conversion question?
« Reply #8 on: May 18, 2012, 10:00:23 PM »

The test show that the wider the color space the worse the color match. I don't know what to think about this but using wider RGB color spaces for a CMYK destination doesn't seem very attractive.

I don't think you understood what you did nor understand the results. You specified a specific CMYK file while the image was in RGB, right? All that does is try to map the CMYK settings to the various color spaces and none of the color spaces will create the same color when you spec the CMYK colors. As you can see, the resulting RGB colors are all different–because all three RGB color spaces are different. Go back and reread the second correct post by Czornyj. What you are asking for isn't possible.

What you think you want you can't have. There is no RGB color space where you can specify a CMYK color and expect to end up with that exact CMYK when you do an RGB>CMYK transform.

When you spec a CMYK color it has 4 channels. To get that into RGB requires a color transform, understand? The conversion from CMYK to RGB color is not going to be accurate. If you then take whatever RGB color you have you have to go through an RGB>CMYK conversion. It replaces gray components of equal amounts of CMY with black. As a result, the final CMYK color CAN'T by definition, match a CMYK color specified in RGB.

The ONLY thing you can do is to NOT use RGB as a working space when you need to create accurate CMYK color numbers.

And, this has NOTHING to do with creating CMYK separations of RGB photographic images...since specific RGB number will never hit CMYK numbers exactly because of the addition of the 4th black generation, all you really are concerned about is getting good CMYK color from RGB originals.
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Coloreason

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Re: RGB to CMYK conversion question?
« Reply #9 on: May 18, 2012, 11:46:40 PM »

Schewe, I followed the instructions from Onsight, as quoted in my message but I may have made some mistake in making my tests properly or the color settings in Photoshop were acting weirdly because now I can't recreate the results I posted earlier and I'm wondering what went wrong. Now when I do the same, the colors are very close and match as expected. Of course not exactly because color management uses algorithms and not color by color lookup list, but the color match and values are now much more reasonable.

For now, never mind my posts. I'll keep trying things, and If I find what happened, or have issues, I'll post back. Thanks everyone for you help and sorry for the confusion.
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Scott Martin

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Re: RGB to CMYK conversion question?
« Reply #10 on: May 20, 2012, 02:14:04 AM »

It looks like you've assigned R=255 G=0 B=0 as your color to the ProPhotoRGB file. That would explain your result. If you type in C=0 M=100 Y=100 K=0 you see it calculates a different RGB formula altogether and translates very closely to C=0 M=100 Y=100 K=0 when converted. I think you'll find my earlier suggestions to be practical.

I'm afraid you might be getting caught up in the technical nitty gritty here. The bigger question is "What are you doing?" What's leading you to these questions? Are you working with photographic imagery or non-photographic content? What's your workflow? We need to see the forest - not just the trees. I suspect your "ah ha" moment lies elsewhere.

Schewe

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Re: RGB to CMYK conversion question?
« Reply #11 on: May 20, 2012, 04:12:45 PM »

I'm afraid you might be getting caught up in the technical nitty gritty here. The bigger question is "What are you doing?" What's leading you to these questions? Are you working with photographic imagery or non-photographic content?

Exactly correct...if you are trying to match a client spec'ed color for halftone repro you're gonna have a hard tie doing that exactly when working in RGB. That would be the time to convert the photo you may be working on to CMYK and then doing the color fill or adjustment to hit specific CMYK numbers. Just note that, for example with PMS colors, a continuous tone photo will likely never exactly match a specified color because of lighting and gradations. If you are trying to hit specific number I would suggest averaging them out and try to only get close.

But we really need to know what it is you are doing. If all you are doing is photo seps from RGB to CMYK then you are going down a rabbit hole that won't get you what you think you want.
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