You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: spm_smooth.m
    path of file: ./repos/bspm/thirdparty/spm8
    
    the code of the file until where you have to start completion: function spm_smooth(P,Q,s,dtype)
% 3 dimensional convolution of an image
% FORMAT spm_smooth(P,Q,S,dtype)
% P     - image to be smoothed (or 3D array)
% Q     - filename for smoothed image (or 3D array)
% S     - [sx sy sz] Gaussian filter width {FWHM} in mm (or edges)
% dtype - datatype [default: 0 == same datatype as P]
%____________________________________________________________________________

    
function spm_smooth(P,Q,s,dtype)
% 3 dimensional convolution of an image
% FORMAT spm_smooth(P,Q,S,dtype)
% P     - image to be smoothed (or 3D array)
% Q     - filename for smoothed image (or 3D array)
% S     - [sx sy sz] Gaussian filter width {FWHM} in mm (or edges)
% dtype - datatype [default: 0 == same datatype as P]
%____________________________________________________________________________
%
% spm_smooth is used to smooth or convolve images in a file (maybe).
%
% The sum of kernel coeficients are set to unity.  Boundary
% conditions assume data does not exist outside the image in z (i.e.
% the kernel is truncated in z at the boundaries of the image space). S
% can be a vector of 3 FWHM values that specifiy an anisotropic
% smoothing.  If S is a scalar isotropic smoothing is implemented.
%
% If Q is not a string, it is used as the destination of the smoothed
% image.  It must already be defined with the same number of elements
% as the image.
%
%_______________________________________________________________________
% Copyright (C) 2008 Wellcome Trust Centre for Neuroimaging

% John Ashburner & Tom Nichols
% $Id: spm_smooth.m 4172 2011-01-26 12:13:29Z guillaume $


%-----------------------------------------------------------------------
if numel(s) == 1, s = [s s s]; end
if nargin < 4, dtype = 0; end;

if ischar(P), P = spm_vol(P); end