Find the flow line of angle θ in the field h. In other words, solve the DE η'(t) = exp(ih(η(t))/χ + iθ)".

torus_fgf(m,n,s,rng=GLOBAL_RNG)

torus_fgf(n,s,rng=GLOBAL_RNG)

Return a sample of the FGF on an m × n square grid with periodic boundary conditions