Detect Modifications

One important thing is to assure that the thumbnail image displays the same information than the original, only in a downscaled version. To make this possible we use the modification time stored in the required 'Thumb::MTime' key and check if it's equal the current modification time of the original. If not we must recreate the thumbnail.

if (file.mtime != thumb.MTime) {
      recreate_thumbnail ();
}
         

It is not sufficient to do a file.mtime > thumb.MTime check. If the user moves another file over the original, where the mtime changes but is in fact lower than the thumbnail stored mtime, we won't recognize this modification.

If for some reason the thumbnail doesn't have the 'Thumb::MTime' key (although it's required) it should be recreated in any case.