It is for facial recognition software. I know nothing about the field, but my assumption would be that you need a plain base to work from. That way you start

It is for facial recognition software. 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.