Tracing fibers requires computation of weighted averages of tensor
fields within the filter radius (1-3 voxel sizes) and finding tensor
eigenvalues and eigenvectors on every step. These are the most
expensive computations in the algorithm and are performed using SVD
decomposition and LU factorization using routines from
[Press et al. 1992]. We employ standard Euler and Runge-Kutta
forward integration scheme [Press et al. 1992] to evaluate the line integrals
in tracing procedure.