A fingerprint is the pattern of ridges that appear on the surface of the fingertip. It is perhaps the most popular and reliable biometric characteristic used for human authentication. These observations rely basically on two of its properties, namely, individuality and persistence. The former refers to the fact that the fingerprint is unique across individuals and across fingers of the same individual, whereas the second property means that the basic fingerprint characteristics do not change over time. Because of these reasons, fingerprint-based authentication systems have been widely adopted in many applications where high security levels are a need. Moreover, fingerprints have been adopted as a proof of criminalistic evidence in many law courts over the world, becoming therefore a standard mechanism in forensics.
The fingerprint structure comprises two levels of features. At the most global level, the fingerprint features correspond to the patterns of ridges and valleys that appear on the fingerprint. These patterns are typically used for fingerprint classification purposes and they do not own any kind of properties for establishing the identity of an individual. Due to these facts, those patterns are denominated macro features. In a typical fingerprint image, the dark lines are related to the ridge patterns, whereas the brighter ones are referred to the valleys. The figure below shows some examples of valleys and ridges.
At the local level, the fingerprint features are known as minutiae. They correspond to the points where the ridge patterns either bifurcate (ridge bifurcations) or terminate abruptly (ridge endings). These features posse the discriminating information for establishing the individuality of fingerprints and therefore the identity of an individual. Some examples of minutiae are depicted in the figure below.
