Numerical differential functions.

• diffx - numerical aproximation to first derivative of TOS in x using (-1,0,1) filter.
• diffy - numerical approximation to first derivative of TOS in y.
• grdsq - sum squared of diffx and diffy of TOS image.
• lap - sum of the second derivatives in x and y of TOS image.
• ddn - second order image derivative invariant of TOS image.
• ddt - second order image derivative invariant of TOS image.
• crv - curvature of TOS image with IP(Precision).