1. A method comprising: a) computing the difference of the data value at each position in the plane and the data value at each position shifted by one in the x-direction in the plane, squaring the result and summing over said results for each plane to form the value of the xx entry of the metric at the (x, y) position, b) computing the difference of the data value at each position in the plane and the data value at each position shifted by one in the y-direction in the plane, squaring the result and summing over said results for each plane to form the value of the yy entry of the metric at the (x, y) position, c) multiplying the difference of the data value at each position in the plane and the data value at each position shifted by one in the x-direction in the plane with the difference of the data value at each position in the plane and the data value at each position shifted by one in the y-direction in the plane, and summing over said results for each plane to form the value of the xx entry of the metric at the (x, y) position, d) computing the result for the determinant of the metric at the (x, y) position as the result of the product of the xx entry of the metric and the yy entry of the metric minus the result of the square of the xy entry of the metric, e) computing the square root of the determinant of the metric at the (x, y) position, f) adding the result from the previous step to the result from the previous iteration of this inner most loop, if this is not the first iteration of said inner most loop, to form the loop result for the inner most loop, g) upon completion of the loop over the x-positions performing the next iteration over the loop of y-positions, h) upon completion of the loop over the y-positions assigning the final result for the loop result for the inner most loop as the value of the equivalence signature.