What is the luminosity blend mode, exactly?

Discussion in 'Photoshop Tutorials' started by fersman4, Jan 15, 2005.

  1. fersman4

    fersman4 Guest

    I have PSE3, and this is what I did. I have a color image and duplicate
    it. Between the two layers I create a new layer and fill it with 50%
    gray. Then I set the top layer (the copy) blend mode to "Luminosity".
    This produces a gray-scale version of the image.

    Okay, now with the image copy layer (top layer) selected, I take my
    eyedropper tool and record a color in a fairly uniform
    hue/saturation/brightness area as the foreground color. I then open up
    the foreground color picker and change the hue to something else,
    leaving the saturation and brightness the same. Next I paint the new
    color in a small area inside the uniform area.

    Since I didn't change the brightness or saturation of this area, I
    would expect that the luminosity (tone) would not have changed. But
    when I have all the layers visible, there is a distinct spot on my
    image where I painted where the gray-scale value is different. My very
    small understanding of color theory says that if you seperate the color
    from an image but keep the saturation and brightness, you will end up
    with a gray-scale image where pixels with the same saturation and
    brightness will have the same gray-scale value.

    What gives?
    fersman4, Jan 15, 2005
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  2. fersman4

    Mike Russell Guest

    Luminosity is a weighted sum of the RGB components, and is not the same as
    Brightness in HSB, which gives equal weight to each RGB component.

    So, for example, pure blue and pure green will have an HSB Brightness value
    of 100, but their luminances will be different, so the two colors will have
    a different effect in a layer in luminance mode.
    Mike Russell, Jan 15, 2005
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