I would think that they focus more on the location of facial features that are set in stone^Wbone. On that note though, could you construct lenses that change where your eyes appear to be located?
I know nothing about the field, but my assumption would be that you need a plain base to work from. That way you start with a known value and you compare from the base to the image you're trying to match.
If you had a smiling photo as the base, and you're trying to match a photo where the person is frowning, that must be that much harder to work out.
Remember that smiling changes not on the mouth but the eyes as well. I would guess that eye shape is a key factor in making facial recognition work.
For the record, I am favor of facial recognition for identity checks with passports. It means I can use the automated immigration processing queues, which are much, much faster than standing in a line waiting for a guy to ask you a couple of benign questions and stamp your passport.
If current facial recognition software can't handle smiles, then you just have to remember to plaster on a creepy politician-rictus-smile while walking through airports. I also wonder how some pols get passports...
Watch BBC's The Human Face presented by John Cleese and Elizabeth Hurley, it is from 2001 but it has a part on facial recognition by machines.
What part of facial recognition relies on the lack of a smile? I was hoping that would get explained here, and am disappointed that it wasn't.
I think everyone assumes it's for facial recognition...