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: dtd_covariance_pipe.m
    path of file: ./repos/md-dmri/methods/dtd_covariance
    
    the code of the file until where you have to start completion: function fn = dtd_covariance_pipe(s, paths, opt, ind)
% function fn = dtd_covariance_pipe(s, paths, opt)
%
% s     - input structure
% paths - either a pathname or a path structure (see mdm_paths)
% opt   - (optional) options that drive the pipeline
%         opt.mask.thresh = 0.1, you may want to adjust it 

    
function fn = dtd_covariance_pipe(s, paths, opt, ind)
% function fn = dtd_covariance_pipe(s, paths, opt)
%
% s     - input structure
% paths - either a pathname or a path structure (see mdm_paths)
% opt   - (optional) options that drive the pipeline
%         opt.mask.thresh = 0.1, you may want to adjust it 
%            
% fn    - a cell arary with filenames to generated nii files

if ( (nargin < 2) || isempty(paths) ), paths = fileparts(s.nii_fn); end