ideo files.  We use a plugin 
   architecture in the backend to do the actual writing.  For example, 
   on Windows, DirectShow will typically be used and on Linux, the 
   ffmpeg library is often used.

   vw = videoWriter(url)
     Opens the given video file for writing using the default plugin.
     On Windows, 'DirectShow' is used by default and on Linux,
     'ffmpegPopen2' is used by default.  For most plugins, the url will 
     really be a filename.

   vw = videoWriter(url,..., 'plugin',