Rotation-invariant operators applied to enhancement of fingerprints

Rotation-invariant operators of order 0 and 2 are used first to detect and then to enhance lines. The enhancement procedure, suitable for fingerprints, is based on suppression of line responses with an orientation that creates a mismatch with the locally dominating direction of the fingerprint. The rotation-invariant operators also serve as basis functions. By construction they are mutually orthogonal and by proper normalization they form an orthonormal set. At the origin any pattern a(r, φ) can be expanded or at least approximated by such a set of basis functions