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RGB to CMYK color transformation

Mar 1, 2013 5:32 PM

I know RGB to CMYK color transformation may not exact. But how could it have so much difference?

 

I have a RGB = (28,97,255) want to transform into CMYK.

Which number should I use?

 
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  • Currently Being Moderated
    Mar 1, 2013 6:24 PM   in reply to w345

    Color numbers are in terms of a color space.

     

    PS / Edit / Color Settings / Working Spaces:  RGB set what you want, CMYK set what you want

     

    PS / Window / Color or F6 to turn on the color picker if it’s not already, or do again, if you’ve just turned it off, click the Color tab

     

    Click the little arrow-4-bars menu icon at the right of where it says Color and Swatches in the color-picker and set RGB if not already

     

    Type in your three numbers at the end of the sliders

     

    Click the little arrow-4-bars menu icon again and set CMYK

     

    Read the converted numbers at the end of the sliders

     

    Set it back to RGB, again, and make sure the numbers are the same, otherwise something might have been lost in the translation

     
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  • Currently Being Moderated
    Mar 1, 2013 9:09 PM   in reply to w345

    also, read up on out of gamut colors & Soft Proofing in Photoshop, this image may help

    click on the thumbnail and drag the popup Granger Chart onto the desktop

     

    open it in Photoshop (use the embedded profile)

     

    Granger_Chart_RGB.jpg

     

    go to: View> Proof Setup> Working CMYK (this is "soft proofing" in Photoshop)

     

    Command+Y (Mac) to toggel it on/off

     

    notice those pretty saturated colors in RGB mode don't translate well to CMYK mode (they are out of gamut)...

     
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  • Currently Being Moderated
    Mar 5, 2013 7:21 AM   in reply to w345

    What are the RGB and CMYK workspaces you have set in Color Settings?

     

    If I choose the North America General Purpose 2 defaults of sRGB and US Web Coated (SWOP) V2 then entering RGB of 28,97,255 converts to CMYK of 100,32,3,1 with a 100 being a sign that I have maxed out the cyan color which may or may not be good.

     

    Furthermore, there is a little warning triangle-exclamation-point at the lower left of the sliders and hovering says “Warning: out of gamut for printing”

     

    If I click on that warning triangle then convert back to RGB the numbers have changed to a much duller blue:  74,109,177.  This means that the bright blue in RGB cannot be printed using CMYK inks, only the duller color blue.

     

    RGB colors are in terms of luminous phosphors of a CRT or light coming from LEDs in your monitor whereas CMYK colors can only ever be as bright as reflected light from white paper that is dimmer than the glowing of a computer monitor.

     

    The colors from your print will be more like the duller CMYK colors not the bright RGB colors.

     

    What is the overall process you are involved in where you’ve asked to convert one RGB color to another CMYK color?

     
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  • Currently Being Moderated
    Mar 5, 2013 7:30 AM   in reply to w345

    You should avoid any conversion from RGB to CMYK in Photoshop at all. Keep your images always in RGB. If they are printed, eg. in InDesign, place the images with their RGB Color Space in inDesign and let InDesign do the conversion job upon PDF export.

    There exists never an equal value from any RGB-Space to any CMYK-Space, what you get is only a closest by value under specific conditions.

    As long as you don't know theses conditions you should never make any conversions.

     

    It is important that the color is correct in the choosen color space. The color space is technicaly defined by the color mode (in your cas rgb) and the profile, and out of technical bounderies also by the environment where you see these colors.

     
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  • Currently Being Moderated
    Mar 5, 2013 9:06 AM   in reply to w345

    Your use of

    The RGB (28,97,255) is equal to CMYK (81,63,0,0)

    seems to indicate that you might assume that RGB and CMYK values in and of themselves have an absolute meaning.

    But they don’t unless the Color Space is declared (usually in the form of an ICC profile).

    They can indicate that a pixel is more red, darker, … than another pixel in an image but the RGB values themselves do not define an absolute color impression.

    If no profile is embedded in a file Photoshop assumes its Working Space for the respective Color Mode.

     
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  • Currently Being Moderated
    Mar 8, 2013 7:18 AM   in reply to w345

    The attached image may help. Note that the entire CMYK color space is only a small subset of the colors possible in RGB.

     

    gamut.png

     
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  • Currently Being Moderated
    Mar 8, 2013 7:32 AM   in reply to OldBob1957

    Which RGB space specifically is that supposed to represent and which CMYK space?

    Because those naturally can vary considerably.

     

    Edit: I am afraid graphics like that could lead people to the faulty conclusion that there is one RGB and one CMYK space.

     
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  • Currently Being Moderated
    Mar 8, 2013 8:09 AM   in reply to c.pfaffenbichler

    c.pfaffenbichler wrote:

     

     

    Edit: I am afraid graphics like that could lead people to the faulty conclusion that there is one RGB and one CMYK space.

    You know, that is a very good point. I had never considered that pitfall. I shall be more specific with my graphics from now on.

     

    In any case; the above is actually from a series merely introducing the concept of "gamut" for those who may not have heard / do not understands the term, and is meant only to introduce the reality that all color spaces are not equal.

     

    That said, I believe I shall consign that particular graphic to the trash can.

     

    Thanks.

     

    --OB

     
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    Mar 9, 2013 6:56 AM   in reply to OldBob1957

    I hope I didn’t come off as purely negative.

    It’s just that Color Management seems to be a bit confusing for people who confront the issue for the first time and the clearer the concepts and differences are presented the better, I guess.

     
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