Face detection illustrate the presence and location of a face in an image, by determining the face from all other patterns present in the scene. This needs appropriate face modeling and segmentation. The approach should also take into account the sources of variation of facial appearance like viewing geometry (pose), illumination (color, shadowing, and self-shadowing), the imaging process (resolution, focus, imaging noise, perspective effects), and other factors like Occlusion .
There are some other method also in which face detection are carried out by using the entire face , making occlusion difficult to handle. Face detection techniques classified on the basis of the image data used to aid in detection—color, geometric shape, or motion information .The following figure illustrate the process of face detection in a still image or image sequence.