eli5: How do tech companies do it so that face ID doesn’t recognize a picture of that person?

Question

(edit: for the purposes of this question I’ll only consider Apple’s Face ID system but thank you to everyone who addressed Android and others) 🙂

In: 242

Answer 1

It projects a grid of dots, and the sees how they lie in your face. It then gets a good impression of the depth of the features as well as the flat image