CMYK grey vs. RGB grey

Discussion in 'Photoshop' started by Dr. J. Smith, Nov 8, 2003.

  1. Dr. J. Smith

    Dr. J. Smith Guest

    Why is it if I desaturate an image in RGB then convert it to CMYK and
    desaturate it again it's a different shade?
     
    Dr. J. Smith, Nov 8, 2003
    #1
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  2. Dr. J. Smith

    Flycaster Guest

    Rounding errors would be my guess, given that many mode changes. Maybe
    someone else has a better explanation.
     
    Flycaster, Nov 8, 2003
    #2
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  3. Dr. J. Smith

    Steve Guest

    methinks this happens with everything when converted....some just below our
    ability to detect - but shadows, my god!!! converting from RGB to CMYK, I
    have to reduce opacity to around 4% to be happy with the output.

    just my 2cents worth :)

    Steve
     
    Steve, Nov 8, 2003
    #3
  4. Dr. J. Smith

    Mike Russell Guest

    This turns out to be an interesting question, so the answer will be longer
    than the question by quite a bit :)

    Desaturate uses mathmatics based on the HSL color space. HSL is defined
    geometrically in terms of RGB ratios.

    For Hue/Sat/Lightness calculations, Photoshop treats a CMYK image as an RGB
    image, ignoring the K value. By this logic, when a CMYK image is
    desaturated, gray is generated by creating equal amounts of CMY inks. Since
    cyan is the weaker of the three inks, a desaturated CMYK image will take on
    a red cast.

    One servicable work-around is to convert to an RGB working space for the
    desaturate operation, then back to CMYK afterward.

    This quirk also affects the layer modes, the sponge tool,
    Image>Adjust>Variations, and Image>Adjust>Hue/Sat . For example a CMYK
    layer set to luminance mode will impart a red cast to the underlying layers.
    I recall reading that some of these operations have been changed in
    Photoshop CS, perhaps others will comment on this.

    --

    Mike Russell
    http://www.curvemeister.com
    http://www.zocalo.net/~mgr
    http://geigy.2y.net
     
    Mike Russell, Nov 8, 2003
    #4
  5. Dr. J. Smith

    belowme Guest

    i'm gonna have to agree with you on this answer. the three colors mixed will
    create a muddy brown so a slight amount of black ink is added where the
    shadows are. rgb can be made up into millions of colors, cmyk is made up
    with 4 colors when printing presses print 4 colors they only buy 4 colors of
    ink that can be mixed in only one way to create black thats why when you
    convert to cmyk it changes slightly to exactly match what a printing press
    can recreate.
     
    belowme, Nov 10, 2003
    #5
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