better way to get a light(white)-shadow(black) gradation of object?

    Jul 17 2010 | 6:56 pm
    I am trying to get as smooth as possible gradation between lightened and shadowed of the object. I am currently using rgb2luma which shows white/black with thresholds. which is not showing detailed steps between light-shadow. do I have to use grayscale to get better gradation steps to be used? ultimately, the lightened part include fades (exclude. black) that controlled by threshold setting will be used as visible part of original color video. It is like a gradation mask in the photoshop.
    how can I make gradated mask based on light-shadow of the object?