x0-3:and :y0-3:define the four control points of the contrast-s curve.  the default is a straight line. the curve will be a cubic hermite spline.
black:this controls the logarithmic shaper. intuitively, it is the ev number  where you want to locate black. the more you go negative, the further away  you are pushing the black point. this means, relative to that, all pixel values  will become brighter and contrast is compressed more. use this to flatten high  dynamic range images. a large value of black (say 1.0) will result in a more  contrasty image.
