Dealing with the minutiae of fingerprint analysis

Wen Li and Srik Gurrapu, Texas Instruments

April 4, 2011

Wen Li and Srik Gurrapu, Texas Instruments

Binarization. So far, the image we discussed is in gray-level and has many levels of intensity. The goal of binarization is to convert the gray-level image to binary level “1” or “0.” In other words, this space-changing operation converts the image to black or white with no levels in between. The key in this operation is to set the threshold value between black and white. Depending on the implementation, there are various ways to set the threshold value. Also, some filtering, such as band-stop, can be applied to prevent information loss.

Thinning. After the binarization, the ridges and valleys are in black and white, respectively, but the width of lines may be wider than one pixel. To further reduce the complexity of minutiae extraction, a thinning algorithm will be applied to the image. Usually a 3x3-pixel window is applied to each pixel across the image. In total, nine neighboring pixels will be used at one time. Depending on the calculation result, a decision will be made whether or not to select the current pixel in the skeleton line.

Feature extraction. Figure 5 illustrates the fingerprint image for each processing step during the entire fingerprint analysis flow. After these steps of signal processing, we will obtain the final fingerprint image. The minutiae-based features, such as ridge ending and bifurcation, will be found and extracted from the final image.

To illustrate how a minutia is found, let us take a closer look at the last image of Figure 5 and zoom in on a ridge-ending area as an example. In the checker diagram in Figure 6, each grid represents one pixel. By comparing the center pixel values with the neighboring pixels in this window, a minutia can be detected. From the algorithm’s aspect, the result of this comparison is defined by the “crossing number” cn(p) numerically. The value of the crossing number will determine if a minutia is there and what kind of minutia it is.


Click on image to enlarge.


Click on image to enlarge.