Mean (left) and standard deviation (right) of the error of estimation of the eigenvalues for 3, 5, and 15 gradient directions. (a) Mean of the error, 3 directions. (b) Standard deviation of the error, 3 directions. (c) Mean of the error, 5 directions. (d) Standard deviation of the error, 5 directions. (e) Mean of the error, 15 directions. (f) Standard deviation of the error, 15 directions.