Need to find RGB-color that matches specific CMYK-color (via DeviceLink-profile)

[ulrikt8202587]

Hi,

I have a, judging from extensive googling, rather unusual task at hand: I have a CMYK-color, and a DeviceLink-file (RGB->CMYK, taking care of black-text-staying-black), and need to figure out what RGB-color corresponds (best) to the CMYK-color.

I am operating on PDF's, that are generated from scraping webpages (RGB) and the resulting PDF will be sent to a print shop, thus the requirement of ending up with CMYK-color codes.

I am using GhostScript and Adobe Photoshop, but can't see a way to let the computer solve the riddle, and trying the manual approach has not proven successful so far.

BR,

Ulrik

[Test Screen Name]

Can't you just use the profile with a CMM like Microsoft ICM2? Or is your profile one-way?

[Test Screen Name]

Also, it may go without saying but any modern print shop (that is, one which has moved with the times, many haven't) will accept and may prefer properly tagged RGB PDF files.

[D Fosse]

First, the obvious: numbers refer to specific color spaces. For the same color, numbers in sRGB and Adobe RGB are different, as they are different in US Web Coated SWOP and Coated FOGRA39. Just to name arbitrary examples.

So which RGB specifically, and which CMYK specifically? There is no such thing as "generic" RGB or CMYK.

With that established, it's a simple matter of using the eyedropper in Photoshop. Convert to the target profile, and use the eydropper again.

Any print shop that accepts undefined "CMYK" should be avoided, because they don't know what they're doing and cannot be relied upon for accurate or consistent results.

[G_Hoffmann]

A device link profile works only in one direction:

RGB ––> CMYK

CMYK1 ––> CMYK2

Of course D Fosse is right - for any application of a device link profile except printing one has to

know the source profile and the target profile (for printing these informations were used implicitly,

making the device link profile.)

Now let's assume AdobeRGB (aRGB) as source and ISOcoated-v2-eci as destination profile.

An image in aRGB can be converted to ISO eventually faster and more accurate than

by using the standard workflow aRGB ––> LAB ––> ISO.

In fact the creation of such a profile, as explained for the RIP Colorgate Productionserver

requires an optimization:

  DEVICELINK PROFILER Modul DLPFM

  Voraussetzung: Sie haben das Devicelink Profiler Modul (DLPFM) erworben,

  den Proof Workflow gewählt und ein Referenzprofil des zu simulierenden

  Drucksystems erzeugt.

  Der DEVICELINK PROFILER ist ein eigenständiges und optional erhältliches Modul.

  DeviceLink Profile enthalten eine Farbraumtransformation von einem Gerätefarbraum

  (Simulation) in einen zweiten Gerätefarbraum.

  Der DEVICELINK Profiler Assistent optimiert DeviceLink Profile für den Proof Workflow

  in iterativen Schritten, wobei das Messergebnis über einen weiteren iterativen Schritt entscheidet.

  Erst durch eine Optimierung des DeviceLink Profils erreichen Sie eine optimale Farbsimulation

  im Proofergebnis. Der DeviceLink Profiler kann auch Multi-Color-Profile optimieren.

  Hier sehen Sie eine Übersicht der Schritte im Assistenten: ...

The creation requires as well Rendering Intents, in the mentionened RIP either equal or different

for raster graphics and vector graphics. Furtheron some rules how to Preserve (or not) pure

colors, especially the reproduction of RGB-black by K-only, as mentioned in the original post.

Now let's assume, we have told Photoshop the RGB space (aRGB), the CMYK space (ISO) and

the Rendering Intent (only one, for raster and vector).

Then we can read, using the color picker, aRGB-values and ISO-values without any application

of "convert to profile". The problem seems to be solved: get aRGB numbers from CMYK numbers.

Unfortunately this task doesn't have a general solution. Some RGB colors might have been

reproduced in CMYK affected by gamut clipping, especially if the source data were delivered

by usually highly saturated web sites.

Even if we knew that gamut clipping happened for a certain color, the source color could not be

reconstructed.

So far about a strict solution of the problem.

If the conversion from aRGB to ISO had been executed professionally, e.g. for book printing, then

images in RGB were prepared by Soft Proofing so, that no loss due to gamut clipping happened

(RGB colors were somewhat desaturated, blues shifted towards cyan, for instance), and Rendering

Intent is always Relative Colorimetric, then the method as mentioned above should work satisfying.

The interpretation of vector graphics (lines, rectangles, text), which exist in PDFs as independent

elements besides raster graphics, will still be doubtful.

Best regards --Gernot Hoffmann