contrast-stretch action only on one hand of the histogram ?

Discussion in 'Photoshop Tutorials' started by Fabrice, Oct 26, 2006.

  1. Fabrice

    Fabrice Guest

    Hello,

    I would like to create an action that adjusts only the left boundary of the
    histogram (in the shadows), without moving the right boundary corresponding
    to the highlights. I tried to use the "auto-levels" feature in PS CS2, by
    specifing a cut-off threshold of 0.01% for the shadows, and 0.00% for the
    highlights. The problem is that CS2 still moves the cut-off boundary of the
    highlights to the first non-populated value at the right of the histogram.
    For example, with an histogram beginning at 10 and finishing at 230,
    auto-levels sets the boundaries at 12 or so (which is fine) and at 230
    (which is not what I want). I would like auto-levels to set the boundaries
    at 12 and 255.
    How can this be done in an action in PS CS2 ?

    Thank you,
    Fabrice.
     
    Fabrice, Oct 26, 2006
    #1
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  2. Fabrice

    KatWoman Guest

    don't use AUTO
    just use LEVELS
    make the setting of levels manually
    move sliders you want or change numbers in the boxes
     
    KatWoman, Oct 27, 2006
    #2
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  3. Fabrice

    Fabrice Guest

    For example, with an histogram beginning at 10 and finishing at 230,
    The problem is that I want to specify a *percentage* of the range to be
    cut-off, and not a constant value, so that the threshold will be adapted to
    the images content. I want a "generic" action for different images. That's
    why I cannot change numbers in the boxes, since they are absolute values,
    aren't they ?

    Fabrice.
     
    Fabrice, Oct 28, 2006
    #3
  4. Yes, absolute values won't work in an action. The only thing I could
    think of is a bit complicated, but should work. Let your action do the
    following:

    1. Copy the image to a new layer
    2. Set advanced blending mode of this layer so that the darker half is
    opaque and the lighter half is transparent
    3. Use Auto Levels on this layer
    4. Flatten
     
    Johan W. Elzenga, Oct 28, 2006
    #4
  5. Fabrice

    Fabrice Guest

    Yes, absolute values won't work in an action. The only thing I could
    Hi,
    Thank you for the suggestion, it is indeed a bit complicated but works.
    Anyway, I find it quite incredible that PS should not be able to do directly
    such an
    easy thing ! Finally, I have turned to Imagemagick: one single option does
    the job !

    Thank you,
    Fabrice.
     
    Fabrice, Nov 1, 2006
    #5
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