How do they match fingerprints?

Question

I'm aware that the techniques have changed since fingerprints were introduced in the 1800's (I think), and that they used to search the archives manually before the computers came.

But do they still do a manual search of the archives, or have they managed to create computer programs that can do the job when analyzing fingerprints from crime scenes?

Answer 1

The short answer is yes, they do computer matching of fingerprints to a database of prints..

Fingerprint matching is process that involves scanning an image, then testing that image against a database of known fingerprints to find out if a match has occurred. This process of finding a "correlation" between two pictures can be executed in one of two ways using signal processing: In the Time/Spatial Domain or in the Frequency Domain. In addition to written analysis, be sure to check out the Matlab .m files on the sidebar.

Time/Spatial Domain

The Time/spatial domain process of matching involves using 2D Convolution to find a correlation between two image matrices. In order to find a correlation, first we can use the Matlab command "conv2" as our workhorse. Once the two images are convolved, the maximum value in the matrix is obtain using the command "max."

Properties of the Spatial Convolution Method

Advantages: Known Method that works, fairly robust

Disadvantages: Calculations take up many clock cycles i.e. fairly expensive, timewise

Frequency Domain

Just like in the spatial domain, in the frequency domain we use a Matched Filter to determine the correlation between two images. The frequency domain counterpart to 2D convolution is multiplication using Fourier manipulation. That is, instead of spending a great deal of clock cycles convolving two large images (256x256), we simply can perform the FFT on the two image matrices and then multiply the result. In order to get good results, it is also necessary to use the "normalized" matrices with Matlab's "norm" command. More specifically, we used the Frobenius Norm, also called the Euclidean Norm.

Answer 2

They have computer programs but I think they still have to go in and manually make sure they match.

Answer 3

Your fingerprints are in a large data base on a computer server, they use 12 points I believe it is to match prints.