Fingerprint Image Enhancement Using STFT Analysis

The performance of a fingerprint feature extraction and matching algorithm depend heavily upon the quality of the input fingerprint image. While the ‘quality’ of a fingerprint image cannot be objectively measured, it roughly corresponds to the the clarity of the ridge structure in the fingerprint image. A ‘good’ quality fingerprint image has high contrast and well defined ridges and valleys. A ‘poor’ quality fingerprint is marked by low contrast and ill-defined boundaries between the ridges and valleys. There are several reasons that may degrade the quality of the fingerprint image. 1. The ridges are broken by presence of creases, bruises or wounds on the fingerprint surface 2. Excessively dry fingers lead to fragmented ridges 3. Sweaty fingerprints lead to bridging between successive ridges The quality of fingerprint encountered during verification varies over a wide range .It is estimated that roughly 10% of the fingerprint encountered during verification can be classified as ‘poor’ . Poor quality fingerprints lead to generation of spurious minutiae. In smudgy regions, genuine minutiae may also be lost, the net effect of both leading to loss in accuracy of the matcher. The robustness of the recognition system can be improved by incorporating an enhancement stage prior to feature extraction. Due to the non-stationary nature of the fingerprint image, general purpose image processing algorithms are not very useful in this regard but serve as a pre processing step in the overall enhancement scheme. Furthermore pixel oriented enhancement schemes like histogram equalization , mean and variance normalization , weiner filtering improve the legibility of the fingerprint but do not alter the ridge structure. Also, the definition of noise in a generic image and a finger print are widely different. The noise in a fingerprint image consists of breaks in the directional flow of ridges. In the next section, we will discuss some filtering approaches that were specifically designed to enhance the ridge