psf shift question

[Starsmith2]

psf = circshift(flip(psf,3),ceil(Nz/2)+2,3)/norm(psf(:)); %Shift impulse stack
I cant understand this code.
h(�, �;-z) = h(�, �; Nz - z) is a z-stack that is flipped in the zdirection.
Why circshift is needed?

[antipa]

It's just an fft convention thing. The way I implemented the laplacian is with the center of the kernel being in the top, rear, left of the volume, to match the convention of fft algorithms. This is what one would get if the laplacian kernel were placed in the center of the volume then ifftshift were applied. By circshift-ing the PSF in z, the DC term of the z-direction fft comes out in the first layer, which is what we crop out. It would also have been fine to leave out the circ shift and define the 3D cropping to take the center section of the volume after 3D convolution.