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Learn about fingerprint classification methods, challenges in search, and techniques like Fingerprint Sub-Classification, Minutae Points, and Flash Method for accurate matching and indexing.
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Fingerprint Classificationsections 5.3 - 5.5 Fingerprint matching using transformation parameter clustering R. Germain et al,IEEE And Fingerprint Identification Using Delaunay Triangulation G. Bebis et al ,IEEE
Performance of Classification Techniques (cont..) • Confusion Matrix
Two Databases NIST DB4 - contains 2000 fingerprint pairs NIST DB14 – contains 27000 fingerprint pairs Consist of 8-bit grey level images Two different fingerprint instances Classified into 5 classes
Fingerprint Indexing and Retrieval • Problems with classification schemes • Number of classes is small • Fingerprints are unevenly distributed • More than 90% of fingerprints belong to only 3 classes • Difficult to search a single fingerprint form the large database
These problems can be handled with 2 different approaches Fingerprint sub classification Continuous Classification
Continuous Classification and Other Indexing Techniques • Uses vectors summarizing their main features • Feature vectors are created through a similarity preserving transformation • Avoids ambiguous fingerprints • System efficiency and accuracy will be balanced by adjusting the size of the neighborhood.
Indexing Techniques • Using Minutae points • Identifies all the minutae triplets in the fingerprints • Uses geometric hashing to retrieve a similar fingerprints from the database • This is built by quantizing all the possible triplets • If the same fingerprint is hit by more triplets, then a voting technique is applied to get the final rank
Other Indexing techniques • Based on matching scores between the fingerprints • In some papers, different Indexing techniques are combined to improve the performance • Continuous classification with MKL –based approaches • Finger code feature vectors are combined with a simplified version of the minutae triplet approach
Retrieval Strategies • If exclusive classification is used for indexing then, • Hypothesized class only • Fixed search order • Variable search order
If continuous classification is used for indexing then, Fixed radius Incremental search order
Fingerprint matching using transformation parameter clustering Fingerprint Identification Using Delaunay Triangulation
Flash Method • Flash algorithm uses a higher a dimensional indexing scheme than geometric hashing by adding invariant properties of the feature subset to the index • Second stage uses, transformation parameter clustering to accumulate evidence
Flash Method • When adding a model to the database, invariant information computed from each subset of feature points forms a key or index • Key labels an entry that is added to a multimap, • This entry contains the identifier of the model that generated the key and information concerning the feature subset
When servicing a query, each key generated by the query object is used to retrieve any items in the multimap that are stored under the same index. Each item retrieved represents hypothesized match between subsets of features in the query object and the reference model This hypothesized match is labeled by the reference model by parameters characterizing the geometric transformation bringing the two subsets of features into closest correspondence Votes for these hypothesized matches accumulate in another associative memory structure
How it applies to fingerprint matching • In the fingerprint application, class of transformations that connects different object instances is assumed to be of two-dimensional distance preserving transformations • A least squares estimation methodology is used to solve the over constrained pose estimation problem for each hypothesized local correspondence generated by the index lookup process
Data abstraction and index generation • Minutae provides a natural choice for feature points • A triplet of numbers (X, Y, Ө ) represent each feature point
Flash matcher uses skeletonized version of the ridge pattern on the finger • If a line is drawn between each pair of minutae, the number of ridges crossed by this line can be computed • Ridge counting procedure repeats for each pair of minutae in the fingerprint, and the results become part of the flash index
The flash algorithm uses redundant combination of three feature points when forming indices This gives some immunity against noise To keep the number of indices generated within bounds, the algorithm restricts the acceptable combinations of feature points used to form an index
The search engine requires the generation of indices used for table lookup These indices are descriptive of the objects stored in the database. Each component of the index is invariant under rotations and translations The full index consists of nine components: Length of each side Ridge count between each pair Angles measures with respect to the sides
Accumulating evidence • During the query phase, each index generated by the query fingerprint • This is used to retrieve all the objects in the database that are labeled with same index • Each retrieved model objects represents a hypothesized correspondence between 3 points in the query print and three in the model
Accumulating evidence • If a large number of feature points can be brought into correspondence by rigid transformation of the coordinate system, all of the indices generated by the combinations of three feature points belonging to this set generate the same coordinate transformation parameters
Accuracy Issues • Four scenarios are possible H0 is true, and test says H0 is true H0 is false, and test says H0 is true H1 is true, and test says H1 is true H1 is true, and test says H1 is true Two distinct types of errors can be made False Negative: incorrectly assigned mated to non mated False Positive: incorrectly assigned non mated to mated The number of matching triangles that generate a consistent rigid transformation serves as the basis for assigning pairs to the mated or non-mated pair population
With the decision criteria, it is straightforward to determine the two error rates from the conditional probability densities computed from the test populations The error rate for incorrectly assigning a mated pair to the nonmated population is given by The error rate for incorrectly assigning a nonmated pair to the mated population is given by
Consider one to many identification query The candidate list of hypothesized matches is formed by taking all prints from the reference database Assuming the presence of one mate to the query, the FPR and FNR for and identification search against a database N is shown below The FPR increases drastically with database size because each additional entry in the database provides another opportunity to randomly achieve a high score
Results • Data set • 97492 inked dab images • 657 queries, against this database • Query set of prints was a subset of the models • They made 657 X 97492 comparisons of pairs These pairs divided into 3 groups identical fingerprints(657 pairs) diff. impressions of the same finger( 768 pairs) impressions of different fingers( 64,050,819 pairs)
Advantages of using this technique • Preserves index selectivity • Reduces memory requirements • Improves recognition time • Considers only O(N) minutae triangles
Important issues to be consider when using Indexing • memory requirements : • In the case of fingerprints, memory requirements can become much higher since fingerprints contain more features on the average than typical objects • Index selectivity: • relates to the discrimination power of the groups considered for indexing • groups with low discrimination power give rise to very similar indices • large number of hypothetical matches are generated during recognition
To deal with this problems Increasing index dimensionality using large size groups Additional information can be computed from each group and added to index
Indexing based methods have two phases of operation Preprocessing features which remain unchanged under geometric transformations are extracted from groups of model points and used to form indices Indexed locations are filled with entries containing references to the models Recognition Features from groups of image points are extracted and used to form indices again The models listed in the indexed entries are collected into a list of candidate models and the most often indexed models are selected for further verification
Delaunay triangulation has certain properties Non degenerate set of points is unique A circle through the three points of a Delaunay triangle contains no other points The minimum angle across all the angles in all the triangles in a delaunay triangulation is greater than the minimum angle in any other triangulation of the same points
Indexing using Delaunay Triangulation • Minutae triangulation
Building the Index Table • The index table is built by considering the minutae triangles formed by the Delaunay triangulation • From each minutae triangle, information invariant to similarity transformations is computed. • Then, an index is formed using the invariants and appropriate information is stored in the indexed table location • the Delaunay triangulation, yields O(N).
Given a minutae triangle, Compute 3 invariants These based on sides and angles of the triangle First sort the sides of the triangle to avoid considering all possible orders of three points
