Fingerprints are made up of lines known as ridges (peaks) and valleys that bifurcate and end to form several unique characteristics. These characteristics, such as Ridge Island and Ridge Endings, are commonly termed as Minutiae Points and the patterns they form are unique with every fingerprint.
To identify and extract these minutiae patterns from each fingerprint, the digital image undergoes a set of image processing techniques. This process can classified into the following phases:
- Image Enhancement
- Minutiae Detection
- As it may not be possible for all the people to keep all the fingers clean all the time, so may be the image of the fingerprint.
- Whatever be the image acquisition technology, the contrast and completeness of the digital image generated by the sensor, still requires enhancement, to get accurate results.
- Removing blurs, adding mixing pixels, improving the image contrast etc. are archived by adapting spatial methods and frequency methods for image enhancement and subjecting the image into transformations like Fourier and Gaussian.
- This enhanced image is a gray scale image, and all the pixels forming this image have an intensity value ranging between 0 and 255.
- Various parameters like average image pixel intensity, ridge pixel’s intensity distribution, valley pixel’s intensity distribution etc are analyzed to arrive at a threshold intensity value.
- Physically a fingerprint ridge (or a valley) will be around 300 – 500 microns. Since the image acquisition is carried out at 50 micron, the digital image of a ridge line (or a valley line) will be formed by around 6 – 10 pixels.
- The middle pixel of this ridge line maintained at intensity 255 and the rest of the pixel’s intensity are changed to 0, thereby thinning the ridge line width to a single pixel line.
- The thinned image is then analyzed to spot minutiae points. An eight pixel square is formed around the ridge pixel and this pixel square is moved along the ridge line.
- Any ridge pixel having more (or less) than two of the eight pixels in the square with intensity value other than 255 is noted as a minutiae point.
- The location (x, y coordinates) of all these minutiae points, (along with angle theta, in case it is a bifurcation minutiae) stored in an encrypted file format. This is called the minutiae template file.
- A fingerprint may have around 50 – 80 minutiae points and the minutiae template will be around 800 bytes as against a 80 kilobyte image. This makes it so easy to transmit it through even thin bandwidths for comparisons.