x% gray in digital

Discussion in 'Digital Photography' started by Roy G, Jan 16, 2007.

  1. Roy G

    Roy G Guest

    Eh ????

    Roy G
     
    Roy G, Jan 16, 2007
    #1
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  2. Roy G

    Steven Woody Guest

    what's the pixel value ( from 0 to 255 ) representing a x% gray ? where
    x ranged from 0 to 100 and gamma is supposed to be 2.2 ( or 1.8 ).

    thanks.

    -
    woody
     
    Steven Woody, Jan 16, 2007
    #2
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  3. Roy G

    acl Guest

    Hi. If the blackpoint is set to 0, then I suppose it will fall at
    255*(x/100)^gamma
     
    acl, Jan 16, 2007
    #3
  4. Roy G

    acl Guest

    I should have added that I assumed x% gray to mean "would fall at
    2.55*x or gamma=1 (linear mapping)". I have no clue if this is what it
    is supposed to mean photographically (eg maybe it already is defined
    using some sort of logarithmic scale etc; I have no clue, as I said).

    I suspect that was your question too.
     
    acl, Jan 16, 2007
    #4
  5. Roy G

    Mike Russell Guest

    There are a number of valid ways to answer your question. Here's a simple
    one: taking RGB(255,255,255) as 100 percent, for gamma 2.2, the percentages
    map to RGB values like this:

    00 RGB(0.0,0.0,0.0)
    10 RGB(88.0,88.0,88.0)
    20 RGB(123.0,123.0,123.0)
    30 RGB(148.0,148.0,148.0)
    40 RGB(169.0,169.0,169.0)
    50 RGB(187.0,187.0,187.0)
    60 RGB(203.0,203.0,203.0)
    70 RGB(217.0,217.0,217.0)
    80 RGB(231.0,231.0,231.0)
    90 RGB(243.0,243.0,243.0)
    100 RGB(255.0,255.0,255.0)

    These numbers were calculated in Photoshop by converting a gamma 1.0
    gradient to sRGB, posterizing to 11 steps, and outputting the numbers as
    text from the Curvemeister plugin.
     
    Mike Russell, Jan 16, 2007
    #5
  6. Roy G

    acl Guest

    oops, typo: should have been
    255*(x/100)^(1/gamma)
    as it is meant to compensate for the nonlinearity. Maybe someone can
    tell me if this is correct, or if x% gray means something else than
    what I think? Thanks.
     
    acl, Jan 16, 2007
    #6
  7. Roy G

    Steven Woody Guest

    the data seem close to the formula acl provided.
     
    Steven Woody, Jan 16, 2007
    #7
  8. Looks good to me. 18% grey is about mid-scale.

    David
     
    David J Taylor, Jan 16, 2007
    #8
  9. For 18 % grey (i.e Zone V), and gamma 2.2, the answer is (126,126,126).

    For pixel values for the other zones - and some background, please
    see: http://hannemyr.com/photo/zonesystem.html
     
    Gisle Hannemyr, Jan 22, 2007
    #9
  10. Roy G

    Steven Woody Guest

    Steven Woody, Jan 26, 2007
    #10
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