Faces are often the most important element in a photograph. However, it is often the case that a photograph does not have the right exposure which, in turn, can lead to the faces in the photograph being under-exposed and too dark. This happens, for example, when the lighting is behind the subject thus causing the background to be exposed properly and for the face to be too dark.
The color and tone of faces in photographs can also be degraded in post-process image editing. For example, people often apply different stylization filters to a photograph to enhance the photograph. However, these filters are typically applied globally, without any special consideration for semantically important regions likes faces. This, in turn, can create poor skin tones and under-exposed faces.
In the past, some attempts at face exposure correction relied on isolating skin pixels and then using luminance histograms to correct for shadows and exposure issues. In these instances, however, detecting skin regions can be difficult and can lead to unsatisfactory results. Moreover, luminance histograms do not always provide a visually smooth result. This, in turn, can lead to artifacts and other undesirable visual effects, such as unnatural, abrupt color transitions in the image, which are readily detectable by the human eye.