If it were that easy, the eyebrows and lips would be sunken inward because they are darker then the surroundings. More like (I'm guessing) it uses the change i

If it were that easy, the eyebrows and lips would be sunken inward because they are darker then the surroundings. More like (I'm guessing) it uses the change in reflected intensity with respect to the change in light position to estimate a surface normal (e.g., forehead is the same for all light positions, left side of your nose is bright when the left light is on, dark when the right light is on, right side of nose vice versa, etc). It then finds a surface that agrees with these normals by solving a partial differential equation. Clever, not simple. (Although I guess it's a compliment to say they make it look easy.)