The face is schematically divided into aesthetic units and subunits to account for different features of the skin and contours. The aesthetic units of the face are the eyes, forehead, temples, cheeks, lips, chin, ears and nose.
For example, skin color, skin thickness, and the number and size of skin pores differ from one aesthetic unit to another (for example, eyelid and nostril). Subunits are simplified as two-dimensional surfaces in the three-dimensional face. The units and subunits are bounded by natural dividing lines.
Separation lines are the intersection curves of two surfaces that run at a certain angle to each other (for example, transition from the bridge of the nose to the slope of the nose). A similar principle is applied, for example, in computer graphics when representing a face by polygons (see “Non-uniform rational B-Splines” or NURBS for short).
Other dividing lines are formed by crease lines (for example, nasolabial fold at the transition from the cheek to the mouth). Separation lines are suitable for planning the sectioning, since scars in the border area of units or subunits appear particularly inconspicuous.