As described in the previous section, the quality of the fingerprint image is determined by many factors which sometime may be difficult to control, therefore a fingerprint system must be able to handle also the image of medium and low quality (recoverable). In some cases it is possible to improve significantly the image quality by applying some image enhancement technique. The main purpose of such procedure is to enhance the image by improving the clarity of ridge structure or increasing the consistence of the ridge orientation. In noisy regions, it is difficult to define a common orientation of the ridges. The process of enhancing the image before the feature extraction is also called pre-processing. Here we show two technique of pre-processing fingerprint images. The first consist in a simple normalization and the second uses Fourier transformation (FFT). There a lot of other more complex techniques and there still place for future researches on this topic. The enhancement may be useful for the following cases
- connect broken ridges (generally produced by dry fingerprint or cuts, creases, bruises)
- eliminate noises between the ridges
- improving the ridge contrast