The principal goal of restoration methods (French: méthodes de restauration) is to improve an image by using methods based on objective considerations. On the contrary, enhancement methods that we have seen (e.g. in ) are mostly heuristic procedures designed to improve an image for a psychophysical purpose, that is, “to look visually fine”.
Restoration methods need knowledge of the phenomenon that has degraded the image. This knowledge gives a mathematical model of the degradation. The degradation phenomenon is “inversed” to recover the original image: hence restoration methods are said to be an inverse problem (French: problème inverse). This approach usually involves formulating a criterion of goodness (French: critère de qualité) to help the search for an optimal estimation: a criterion is a mathematical function whose minimum corresponds to the best possible restoration.