Fingerprint ridge distance computation methodologies

The average ridge distance of "ngerprint images is used in many problems and applications. It is used in "ngerprint "lter design or in identi"cation and classi"cation procedures. This paper addresses the problem of local average ridge distance computation. This computation is based on a two-step procedure: "rst, the average distance is de"ned in each signi"cant portion of the image and then this information is propagated onto the remaining regions to complete the computation. Two methods are considered in the "rst step: geometric and spectral. In the geometric approach the central points of ridges are estimated on a regular grid and straight lines passing through these points and parallel to the ridge directions are used. The second method is based on the computation of harmonic coe$cients leading to e!ective estimates of the average ridge period. In order to complete the average distance map a di!usion equation is used so that maps with minimum variations are favored. Finally, some experimental results on NIST SDB4 are reported. ( 1999 Patern Recognition Society. Published by Elsevier Science Ltd. All rights reserved.