GRADIENT Approximate gradient.
    [FX,FY] = GRADIENT(F) returns the numerical gradient of the
    matrix F. FX corresponds to dF/dx, the differences in the
    x (column) direction. FY corresponds to dF/dy, the differences
    in the y (row) direction. The spacing between points in each
    direction is assumed to be one. When F is a vector, DF = GRADIENT(F)
    is the 1-D gradient.

    [FX,FY] = GRADIENT(F,H), where H is a scalar, uses H as the
    spacing between points in each direction.