Adjusts the levels of an image by scaling the colors falling
   between specified white and black points to the full
   available quantum range. The parameters provided represent
   the black, mid, and white points. The black point specifies
   the darkest color in the image. Colors darker than the black
   point are set to zero. Mid point specifies a gamma
   correction to apply to the image.  White point specifies the
   lightest color in the image. Colors brighter than the white
   point are set to the maximum quantum value.