CMYK to RGB back to CMYK

M
Posted By
machalleen
Apr 27, 2004
Views
371
Replies
10
Status
Closed
I’m working in a prepress setting where our files sometimes need to move from CMYK to RGB and then back to CMYK color spaces. Yeah, I know… I’m asking for trouble, but that’s the way it is.

I have a color-critical piece that I’m currently working on. There is a blue (C100/M72/Y5/K20) that comes back after all is said and done as: C100/M80/Y20/K5.

My question is why? CMYK has a smaller gamut that should fit right into the RGB work space. The conversion back, it would seem, should result in colors at least closer to the original than I’m getting.

I’m using Photoshop CS. Color settings are Working Spaces: Adobe RGB/U.S. Web Coated (SWOP) v2; Color Management Policies: Off for RGB/CMYK/Gray; Conversion Options: Adobe ACE Engine, Relative Colorimetric Intent with Black Point Compensation and Dither for 8-bit/channel images.
Related Tags
back cmyk rgb

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J
jonf
Apr 27, 2004
"CMYK has a smaller gamut that should fit right into the RGB work space."

That’s your problem. It fits fine into the RGB workspace. The problem is the translation back to cmyk. Since the RGB gamut is a better definition, the cmyk translation has to be edited back down, and it doesn’t actually "remember" the original definition. Or in other words, a lot of cmyk colors can be defined by the same precise rgb definition. Photoshop doesn’t remember the original, so it picks just one of many possibilities.

Think of it as a dictionary definition. If you take the definition of a word from the pocket Websters and put it into the full Oxford it fits fine, but the Oxford editors are going to add a lot of additional info to make the definition more precise. But if you try to take that Oxford entry and fit it back into your Pocket Websters you’ll need to cut out a lot of verbage. You’d need to take some sort of manual steps to preserve the original definition so the translation back can get back to the original.

Moral of the story is that if you must convert color modes back and forth, you need to be careful to preserve your original color definitions.
JS
John_Slate
Apr 27, 2004
You will do much better to convert to Lab and back when it comes to maintaining CMYK numbers in saturated colors, or minimizing the addition of unwanted color.

If you are doing this to change total ink you might try Intellihance Pro from Extensis, a Photoshop plugin that will alter the total ink without changing ANY pixels that exceed the limit you put in…

Costs some $ but if you do this all the time it is worth it.
MO
Mike_Ornellas
Apr 27, 2004
16 bit conversions.
T
Todie
Apr 27, 2004
CMYK can have colors outside of some RGB spaces.

(I agree that 80% Magenta in Blue is too much for most CMYK press/ink/paper situations)

Photoshop changes (first) the depth to 16 bit and the mode to Lab when it converts from CMYK to CMYK. (Thank you, Bruce!)
PF
Peter_Figen
Apr 27, 2004
Was this saturated blue generated in RGB and then converted or was it generated in CMYK?
GB
g_ballard
Apr 27, 2004
If the original file is not tagged (or if you ignore the tag), or you do not first Image> Mode> Assign the correct profile before the CMYK>RGB — you are hosing the color yourself the moment you Convert (unless the file’s sourcespace happens to equal your WorkingCMYK, i.e., Web Coated (SWOP) v2…

In fact "Color Management Policies: Off" could explain a lot of things regarding the problem:

<http://www.creativepro.com/story/feature/14331-3.html>

<http://www.gballard.net/psd/honormyembeddedprofile.html>
T
Todie
Apr 27, 2004
So far, color modes have no memory.
PF
Peter_Figen
Apr 27, 2004
Color Management Off shouldn’t affect anything here, as the profiles in question are already loaded as the working RGB and CMYK, which is what the assumed profiles are with CM off.
GB
g_ballard
Apr 27, 2004
Okay, as you say, how did you know the original source file is U.S. Web Coated (SWOP) v2?
PF
Peter_Figen
Apr 27, 2004
Because that’s what it says in the original post. Machalleen tells us that he/she wants to convert FROM cmyk and that the working CMYK space is U.S. Web Coated (SWOP)v2, so from that I have to assume that the file is in fact U.S. Web Coated (SWOP)v2. That’s also why I asked in a previous post whether that specific color had been generated in CMYK or if it started out in RGB. I tried my own conversion and came up with number very similar to Machalleen’s. After the double conversion I was unable to use either Selective Color or Hue/Sat to reconstitute the original color. That begged the question of color origination as it appears that this particular shade of blue may in fact be one that RGB doesn’t like (I tried it with Wide Gamut RGB with the same result). All that being said, I have clients unknowingly make a double conversion that only cause a 1-2 percent change overall. Of course there were no saturated blues either.

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