// Assumes that f is no smaller than x.
func copyRealToChannel(f *rimg64.Multi, p int, x *fftw.Array2) {
for u := 0; u < f.Width; u++ {
for v := 0; v < f.Height; v++ {
f.Set(u, v, p, real(x.At(u, v)))
}
}
}
// Avoids f.Set(x, y, d, f.Get(x, y, d, ...)).
func addToMulti(f *rimg64.Multi, x, y, d int, v float64) {
f.Set(x, y, d, f.At(x, y, d)+v)
}