Fingerprint Analysis (part 1) Pavel Mrázek

What is fingerprint Ridges, valleys Singular points –Core –Delta Orientation field Ridge frequency

Fingerprint classes

Small scale: Minutia 150 types in theory 7 used by human experts 2 types for the machine: –Ending –Bifurcation

Minutia examples

Sensing Traditional (off line): rolled ink impression + paper scan Plus: big area Minuses: –Inconvenient –Distortion –Too much/little ink

Sensing Optical sensors

Sensing Optical sensors Good: large area possible, good image quality, contactless scanning available Bad: size

Sensing Silicon sensors Capacitive Electric field Thermal

Sensing Silicon sensors Good image quality, small form factor Price proportional to size

Sensing Silicon sensors Area Swipe

Fingerprint types

Minutia detection overview

Orientation field Orientation field (or ridge flow) estimation: Crucial step before image enhancement Various methods: –Gradient-based –Gabor filters –FFT

Orientation estimation Gradient direction –local characteristics –same ridge orientation, opposite gradients –more global view needed Classical solution: Structure tensor (second moment matrix, interest operator) –start from a 2x2 matrix (positive semidefinite) –safe to average information

Orientation estimation Structure tensor Local: Larger scale: average componentwise (Gaussian window, linear/nonlinear smoothing) 2 nonnegative eigenvalues –both small: backgroung / low contrast –one big, one small: regular ridge area –both big: multiple orientations (core, delta, scar)

Orientation estimation Structure tensor system of 2 orthogonal eigenvectors shows dominant direction

Orientation estimation

Problematic images Solution –Enforce smoothness –Use prior knowledge

Orientation model

References Maltoni et al.: Handbook of Fingerprint Recognition. Springer Maltoni. A tutorial on fingerprint recognition. In LNCS 3161, Springer Hong, Wan, Jain. Fingerprint image enhancement: algorithm and performance evaluation. IEEE PAMI Zhou, Gu. A model-based method for the computation of fingerprints’ orientation field. IEEE TIP Weickert. Coherence enhancing shock filters. DAGM Contact: mrazekp -at- cmp.felk.cvut.cz