Extract frame from video - aviread replacement

[Evangelos Rozos]

Hi,

Currently, the video package is being rewritten. The new version will be 
able to handle a variety of codecs. Until then, if you need to load a 
frame from a video, you can use the following function. It calls ffmpeg 
to do the job and is as fast as a turtle, but it does the job. The last 
argument is a Boolean flag  whether to delete or keep the temporary file 
of the extracted image.


% Copyright (C) 2019 Evangelos Rozos
%
% This program is free software: you can redistribute it and/or modify it
% under the terms of the GNU General Public License as published by
% the Free Software Foundation, either version 3 of the License, or
% (at your option) any later version.
%
% This program is distributed in the hope that it will be useful, but
% WITHOUT ANY WARRANTY; without even the implied warranty of
% MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
% GNU General Public License for more details.
%
% You should have received a copy of the GNU General Public License
% along with this program.  If not, see
% <https://www.gnu.org/licenses/>.

% -*- texinfo -*-
% @deftypefn {} {@var{img} =} extractframe (@var{projectpath}, 
@var{videofilename}, @var{framenumber}, @var{deleteimg} )
%
% @seealso{}
% @end deftypefn

% Author: Evangelos Rozos
% Created: 2019-05-22

function img = extractframe (projectpath, videofilename, framenumber, 
deleteimg)

  if (nargin ~= 4)
    error ('img = extractframe (projectpath, videofilename, 
framenumber, deleteimg)');
  end

  video= [projectpath filesep videofilename];
  if not(exist (video, 'file' ) )
    error([video ' does not exist!'])
  end

  extractedframe= [projectpath, filesep(), 'extractedframe_', ...
num2str(framenumber), '.jpg'];


  cmd= ['ffmpeg -i "' video '" -vf "select=gte(n\,' ...
         num2str(framenumber-1), ')"' ' -vframes 1' ' -f image2 ' ...
         '"' extractedframe '"  2> "' projectpath filesep() 'log.txt"' ];
  system(cmd);
  if exist(extractedframe, 'file')
    img= imread(extractedframe);
  else
    img=[];
    return
  end
  if deleteimg
    delete(extractedframe);
  end

end