There has been proposed many methods for the minutia extraction, the traditional method consist of the following steps.
Binarization, thinning and minutiae detection.
- binarization- This process consist in converting the gray scale image in binary image, i.e, the intensity of the image has only two value: black, representing the ridges, and white, representing the valleys and the background. A simple method to binarize is to use a global threshold value, however, it is not well suited for noisy images, a more robust method consist of using some rectangular mask, rotate according the orientation of the ridges.
- thinning- The objective of thinning is to find the ridges of one pixel width. The process consist in performing successive erosions until a set of connected lines of unit-width is reached. This lines are also called skeletons. An important property of thinning is the preservation of the connectivity and topology which however can lead to generation of small bifurcation artifacts and consequently to detection of false minutiae. Therefore some procedure aiming the elimination of these artifacts must be performed after the thinning.
- minutiae detection- From the binary thinned image, the minutia are detected by using 3x3 pattern masks. Samples of masks used for identifying the ridge ending and bifurcations point are shown in the figure below. Although the process seems to be simple, it is necessary to consider the elimination of false detected minutiae.
 masks for bifurcation detection |
 masks of termination detection |
The results of each step for minutiae extraction are shown in the following figure:
 binarization of minutiae extraction |
 thinning |
 minutiae detection |
 result |
After a successful extraction of minutiae, they are stored in a template, which may contain the minutia position (x,y), minutia direction (angle), minutia type (bifurcation or termination), and in some case the minutia quality may be considered. During the enrollment the extracted template are stored in the database and will be used in the matching process as reference template or database template. During the verification or identification, the extracted minutia are also stored in a template and are used as query template during the matching.