Patent GB-1452810-A (GB)
Title: Pattern recognition apparatus
IPC: G07C9/00
Filed: 1974-04-08
Abstract: 1452810 Fingerprint recognition systems CALSPAN CORP 8 April 1974 [9 April 1973] 15401/74 Heading G4R An apparatus for extracting minutiae from the electrical image of a fingerprint includes means to form a binary image in which each bit represents the black/white state of a corresponding point in the fingerprint, the means taking account of the grey level of the corresponding point and of points surrounding it. Each bit of the image is examined and portions of the image which are continuous with that bit and of the same sense are transferred to a unit which measures various parameters of the portions to determine the position, type, length, and orientation values of the minutia represented by portion. Values relating to the same minutia are combined, and the resultant used for recognition purposes. The fingerprint is scanned by, for example, a flying spot scanner whose output is digitized to provide, for each point in a notional matrix of image points covering the fingerprint, a 4-bit signal indicative of the grey level at that point. The signals are passed to a shift register store such that at any instant the store contains signals corresponding to a 9 x 9 point window. In effect the window scans over the image matrix. The grey level of the centre point is compared with the average grey level of all 9 Î 9 points in the window, to provide a first average value. The average grey levels of each of 12 "slits" radiating from the centre point are determined and each is compared with the average of all of them to provide second values. The second values indicate whether or not the corresponding slit lies along a ridge or a valley of the fingerprint, or along neither, and these values are used together with the first values to assign a single bit state to the centre point. This operation builds up in a store a binary image of the fingerprint which is free from voids, &c., resulting from scanning a poor fingerprint. The store feeds a shift register store which at any instant contains an 8 x 8 bit window. The bits in the window pass to logic circuitry in order to detect any string of black or string of white bits which includes one of the four centremost bits and extends from the centre bit to an edge of the window. Such a string is transferred to a unit (Fig. 3, not shown) which determines whether it passes through or terminates in the window, its width where it crosses a window edge, the length of its boundaries, its area, and the ratio of the square of the boundary length to the area. These values are fed to threshold units and if they all fall within a certain range, indicating the presence of a minutia at the centre of the window, and if the window does not relate to an area of the fingerprint that is too light or dark or of poor contrast, as determined by the grey level averaging circuits, a transfer circuit is enabled. In addition, the maximum length L and orientation # of the string are determined. The transfer circuit passes L and 0, together with the co-ordinates X, Y of the centre of the 8 Î 8 window, as determined by a time base, and a bit indicating the string type (black or white) to a store. The store feeds a circuit (Fig. 4, not shown) which merges the several values of L, 0, X and Y which relate to the same minutia into single representative values #, X, Y, which define the minutia. The several values arise because the same minutia will be detected more than once as the 8 x 8 window sweeps over the fingerprint. The representative values X, Y are formed by determining the average of the coordinates of minutiae having closely similar values of X, Y and #. The value # is that associated with the maximum value L of these minutiae. The representative values are stored. Those whose coordinates are closely similar and whose # differs by about 180 degrees are assumed to relate to false minutiae caused by a break in a fingerprint ridge and are discarded. The remainder define the fingerprint and are used for recognition.