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Biometric Systems/notes/11. Fingerprints.md
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@ -82,11 +82,19 @@ Problemi nel matching:
### Segmentation
The term indicates the separation between the foreground fingerprint from the background which is isotropic (i.e. rotating the white background, the image stays the same)
Anisotropy: the property of being directionally dependent (as opposed to isotropy).
Anisotropy (*anisotropia in italiano*): the property of being directionally dependent (as opposed to isotropy).
Characteristic of the fingerprints are directionally dependent, we can use this to separate the fingerprint from the background.
Characteristic of the fingerprints are directionally dependent, we can use this to separate the fingerprint from the background:
- presence of a peak in a histogram of the local orientations
- the orientation of the ridge line is estimated at each pixel and a histogram is calculated for each block 16×16. The presence of a significant peak denotes an oriented pattern, while a "flat" histogram is characteristic of isotropic signal
- variance of the gray levels in the direction perpendicular to the gradient
- in noisy regions the pattern does not depend on the direction, while the fingerprint area is characterized by a very high variance in the direction orthogonal to the ridge orientation and very low along the ridge
- magnitude of the gradient
- because the area of the fingerprint is rich in edge due to the alternation of ridge and valley, the gradient is high in the foreground and lower elsewhere
- combination of several characteristics
- for each pixel we calculate some characteristics (consistency of the gradient, and average and variance of the intensity), and the allocation to foreground / background is operated by a classifier
Once a fingerprint has been segmentated we can start extracting macro-features such as:
Once a fingerprint has been segmentated we can start extracting **macro-features** such as:
- **ridge-line flow:** described by a structure called **directional map** (or directional image) which is a discrete matrix whose elements denote the orientation of the tangent to the ridge lines.
- analogously, the ridge line density can be synthesized by using a density map.
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