Function transform_spectral 

pub fn transform_spectral(
    image: &mut [i32],
    nx: usize,
    ny: usize,
    nz: usize,
    transform: SpectralTransform,
)

Apply the spectral transform along the z-axis for a 3D image.

Image layout: image[z * nx * ny + y * nx + x] (band-interleaved by pixel). nx × ny spatial, nz spectral bands.