The MPEG2 format, which is used in DVD .VOB files, has a number of peculiarities worth mentioning.
The Microsoft Expression Encoder 4 SP1 SDK — which CLAutoThumbnailer uses to actually generate thumbnails — needs to do something it calls “Indexing” before it can fully open MPEG2 encoded video files. This adds an extra step that can be pretty long. This is reported in CLAutoThumbnailer.log (or --debug option output) as the time to run CalculateDuration line.
MPEG2 doesn’t seem to be as good at exact time positioning as other video formats. In particular you will see small errors in the timestamps of multi-part MPEG2 encoded video files (like DVD title sets, VTS_NN_*.VOB).
MPEG2 videos can have a “Display aspect ratio” specified for them that is different than their typical 720x480 (1.50:1) video frame size. This is how DVDs support the display of widescreen movies. The --aar option controls whether the “Display aspect ratio” is used to automatically crop thumbnails.
Thumbnailing directly from an unencypted DVDROM is much slower than thumbnailing from a copy on your hard drive (because of the vastly slower speed of DVDROM drives). If you plan on processing files that are on a DVDROM more than once or twice, it’s probably worthwhile to make a copy if you have the room. Also some DVDROM drives will automatically run slower to make them quieter while watching movies. You might want to search the Internet for free utility programs that can force your DVDROM drive to run at its maximum speed.