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
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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
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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, Nov 8, 2003
  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
    Mike Russell, Nov 8, 2003
  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
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