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Thread: Grayscale image turned into color

  1. #1
    TomG is offline Fiery Forum Expert Contributor TomG is on a distinguished path
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    Unhappy Grayscale image turned into color

    Refers to a Xerox Color 550/560 with EX-560 external Fiery controller and also applies to Xerox 1000 (both Fiery System 9).

    I have a document containing just black text and on some pages grayscale images. I double-checked if they are really gray (proven by PitStop or Acrobat Pro > edit image in Photoshop).

    I import the PDF file into Command WorkStation (not using the PostScript driver). Fiery turns the images into CMYK, the text is kept real black. No difference if you enable or disable the Adobe PDF Print Engine (APPE).

    Fiery only keeps the image grayscale either if you disable conversion to the output profile or if you enable the gray-settings for CMYK text/graphics/images. Why disabling the conversion works is clear but it's not the desired way to do this because all other colors in the document are not converted correctly.

    The question is why does enabling the gray-setting for CMYK work? In my opinion this should have an effect only for CMYK images containing gray pixels. But here the complete image is grayscale, not CMYK color space. And why does Fiery turn the grayscale image into color although it has a kind of gray-detection or gray-preservation?

    Thanks for any advices.

    See the PDF with screenshots and (German) annotations attached to this thread to have a look at the Fiery color settings in detail.
    Attached Images          
    Last edited by TomG; 12-13-2011 at 04:35 AM. Reason: wrong attachment

  2. #2
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    Default RE: Grayscale image turned into color

    The problem of greyscale images being converted to color is not limited to Fiery. It is a general challenge of color management. Once source spaces (RGB, CMYK, Greyscale) are converted into the profile connection space (LAB or XYZ) in the middle of the color management conversion, the color system no longer knows what the sources were. So for example if you have an input color of C/M/Y/K = 0/0/0/95, this gets converted to something like 15/0/0 in LAB connection space. When this goes through the output profile it will come out as color, some combination of CMYK that gives those LAB values in your output profile.

    The reason for the Grey options on Fiery is to solve this problem. So leave that set to Text/Graphics/Images and it will fix this problem in all the cases it can (will fail if there is any color on the page). The text stays 100% since you set Pure Black on (Gray and Black Processing tab in expert color settings).

    You probably also want to turn on Grey processing for RGB Text/Graphics at least. This will solve the same problem for RGB sources if all the pixels are R=G=B.

    ~lou

  3. #3
    TomG is offline Fiery Forum Expert Contributor TomG is on a distinguished path
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    This is clear to me. But why does the CMYK gray-setting solve the problem? This setting should fix an image in CMYK color space containing gray pixels having C=M=Y=0. It should not affect real grayscale images.

    Grayscale is neither CMYK nor RGB color space... but Fiery seems to handle it like CMYK. That's what's unclear to me.

  4. #4
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    Default RE: Grayscale image turned into color

    You are right, it is treated like color. Since it is color managed into the profile connection space, it is not known at that point whether it was originally RGB, CMYK, or Greyscale. The Fiery CMS does preserve black in these special cases by turning on the gray handling as you have done. The reason this is an option is because sometimes this behavior is not desired, such as if a designer puts a 1-C black and a rich or 4C "black" next to each other on the page.

  5. #5
    TomG is offline Fiery Forum Expert Contributor TomG is on a distinguished path
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    I'd swear that with Fiery System 8 (e.g. DC260) a grayscale image was kept gray even if not using this CMYK-gray-setting... Maybe I just remember wrong.

    Thanks, now I understood.

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