Facial characteristics can be organised into three levels of facial features:
Level 1 details consist of gross facial characteristics that are easily observable. These include general geometry of the face and global skin colour. Such features can be used to quickly discriminate between a short round face and a long thin face, between predominantly male or female characteristics, and between faces from different races.Level 2 details consist of localised face information such as the structure of face components (e.G. Eyes), the relationship between facial components and the precise shape of the face. These features are essential for accurate recognition and require a higher resolution face image. The characteristics of local regions of the face can be represented using geometric or texture descriptors.Level 3 details consist of unstructured, micro level features on the face, such as scars, freckles, skin discolouration, moles. One challenging face recognition problem where level 3 details may be critical is the discrimination of identical twins.