dors write
    TIFF images with tags that are unregistered; in this case interpreting
    their contents is usually a waste of time.
  </p>
  <p>
    <tt>libtiff</tt> reads the contents of a directory all at once
    and converts the on-disk information to an appropriate in-memory
    form.  While the TIFF specification permits an arbitrary set of
    tags to be defined and used in a file, the library only understands
    a limited set of tags.
    Any unknown tags that are encountered in a file are ignored