I don't know the precise rules, but my understanding is - and apologies for repeating what I already mentioned in the link - a partial view is acceptable, at the agent's discretion. That's similar to the manual check, since an officer may or may not ask someone to remove their hat, scarf, spectacles, abaya, turn their head etc (but generally won't). The machine has a photo and a separate set of video feeds, incidentally, if I'm not much mistaken. Unfortunately I've not had the pleasure yet, since the machines were switched off on both evenings I went through last week.

Something else for you to ponder: what would happen if you grew a beard (or shaved one off)?