Non-neutral colors in gray-scale gradient in Photoshop. Why?

Discussion in 'Photoshop' started by Malte, Sep 10, 2004.

  1. Malte

    Malte Guest

    Hi all,

    I recently did a little test in Photoshop and came up with two strange
    observations. Here is the experiment:

    - Create a new image being 256 (or an integer multiple of this) wide
    and, say, 100 pixels high.
    - Specify white to be the foreground and black to be the background
    color (press D).
    - Draw an exact horizontal linear gradient from the very left to the
    very right of the image, or vice versa (hold SHIFT to ensure it is
    horizontal).

    Here are the observations:
    - Use the eyedropper tool to browse over the image and look at the
    info palette. There are some pixels that have non-neutral color values
    (i.e. the values for red, green and blue differ by 1). Why is that? A
    gray-scale gradient should only contain neutral colors, shouldn't it?

    - Have a look at the histogram (either use the Levels dialog or select
    Image/Histogram from the menu). The shadows and highlight are higher
    than the midtones. Why is that? A gray-scale gradient should have an
    exact flat histogram, shouldn't it?

    I tried to do this experiment in different color spaces, such as Adobe
    RGB (1998) and sRGB. The results are always the same. BTW, I used
    Photoshop 7 but I suppose the result is not much different in
    Photoshop CS, is it? Does anyone have a nice explanation for this?

    Best regards,
    Malte.
     
    Malte, Sep 10, 2004
    #1
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  2. Malte

    Mike Russell Guest

    Sure should. I haven't found a way around this. It appears to be a one bit
    +- dither, but with vertical stripes that run perpendicular to the gradient.
    You and I think so, but try to realize - heh - that a spline is a much more
    natural way to start and stop a gradient. In the bad old days of PS 5.0,
    this was not optional. Luckily as of 6.0 you may disable this "feature" by
    editing the gradient, and cranking the smoothness down to zero.
    The explanation is that vitamin and hormonal additives found their way into
    the gradient tool after version 4.0. Thank God, or another power with a
    three letter name, that we at least have the smoothness adjustment.
     
    Mike Russell, Sep 10, 2004
    #2
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  3. Malte

    Mike Russell Guest

    [re dithering in a gradient]

    Yay! There is a dither checkbox in the gradient options palette that
    determines whether dither is added to a gradient. So perfectly clean
    gradients are achievable after all.
     
    Mike Russell, Sep 10, 2004
    #3
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