fn sti_dipole_kernel(
n: [usize; 3],
k: [&[f64]; 3],
bdir: (f64, f64, f64),
) -> Vec<f64>Expand description
STI’s dipole kernel D = 1/3 − (k·H)² / |k|² (0 at DC) on the unshifted padded grid, in
STI’s arithmetic: k from sti_freqs, k·H = k_x H_x + k_y H_y + k_z H_z and
|k|² = k_x² + k_y² + k_z² in that order, and H used as given when it is a unit vector
(normalised otherwise). See sti_freqs for why the rounding matters.