Just been having major problems reliably imaging these MOSFETs
(http://electricstuff.co.uk/forumfiles/P1030235.jpg)
Problem was it was not consistently including or excluding one or both of the small tabs at the side of the large pad, causing random small rotations as the tabs are outside the outline of the 8 main pads.
I spent quite a while trying different combinations of threshold & blob size with no joy.
Then I figured the answer was to image the outline, with the aid of some tipp-ex on a spare nizzle to create a white background
(http://electricstuff.co.uk/forumfiles/P1030237.jpg)
Worked a treat - probably needs something a little more resilient than Tippex to stop it scraping off in the tool holder, but this was a 1-off build so it just about lasted.