1. A method for determining a gradient magnitude image from a range image, the range image including a plurality of intensity values at pixel locations, comprising: determining, for each pixel (i,j), a horizontal central difference dx, and a vertical central difference dy; and setting a 2D gradient magnitude at each pixel (i,j) in a gradient magnitude image I GM to 0.5*sqrt(dx 2 &plus;dy 2 &plus;4).