Face detection, which can be defined as the task of determining the location and size of faces in digital images, is usually the first key step when building face processing applications (for example, facial expression recognition, drowsiness detection, gender classification, face recognition, head-pose estimation, or human-computer interaction). This is because the aforementioned applications requires as an input the location and size of the detected faces. Therefore, automatic face detection plays a critical role, and is one of the most studied topics in the artificial intelligence community.
Face detection seems like an effortless task for a human, but it is a very challenging task for computers because many problems/challenges are usually involved (for example, appearance variations, scale, rotation, facial expressions, occlusion, or lighting condition). Face...