Method for analyzing fingerprint images
View Patent ↗A method for analyzing fingerprint images includes picking up a sequence of images at very short time intervals, while a finger is being placed onto an electronic fingerprint sensor. A criterion for recognizing a live finger is extracted from variations in the images. The images are defined as a pixel raster having different gray values, with the aid of which the pixels are assigned binary values of zero or one. Significant changes in the values are detected and compared, with the aid of distinctive numbers or the like, to known typical changes that occur regularly in a particular image class.
1. A method for analyzing electronically generated fingerprint images, which comprises the steps of:
a) generating a plurality of fingerprint images subdivided into pixels in chronological succession, while a fingerprint is registered by a fingerprint sensor;
b) determining, for each fingerprint image, a number of pixels containing a gray value exceeding a prescribed gray value;
c) determining, for each subsequent fingerprint image, a difference between the number of pixels and the corresponding number of pixels of the previous fingerprint image;
d) determining, for each of the differences, a ratio of the difference and one of the numbers of pixels of the previous fingerprint image and the subsequent fingerprint image;
e) selecting the fingerprint images for which the ratio exceeds a prescribed limit value;
f1) binarizing the selected fingerprint images by assigning a binary value one to the pixels containing the gray value exceeding the prescribed gray value and a binary value zero to remaining pixels;
g1) superimposing the binarized fingerprint images by adding the binary values of the corresponding pixels to generate an image having gray value gradations; and
h1) deciding by a distribution of the gray values whether the fingerprint is regarded as pertaining to a real live finger.
2. The method according to claim 1 , which further comprises the steps of:
i1) subdividing the image of step g1) into blocks;
j) calculating, in each block, an average value of the gray values;
k) calculating, in each block, a standard deviation of the gray values from the average value;
l) calculating an average value of the standard deviations across all blocks;
m) calculating a standard deviation of the average values of the gray values;
n) calculating a quotient of the average value of the standard deviations and the standard deviation of the average values of the gray values; and
o) deciding by the quotient whether the fingerprint is regarded as pertaining to a real live finger.
3. A method for analyzing electronically generated fingerprint images, which comprises the steps of:
a) generating a plurality of fingerprint images subdivided into pixels in chronological succession, while a fingerprint is registered by a fingerprint sensor:
b) determining, for each fingerprint image, a number of pixels containing a gray value exceeding a prescribed gray value:
c) determining, for each subsequent fingerprint image, a difference between the number of pixels and the corresponding number of pixels of the previous fingerprint image;
d) determining, for each of the differences, a ratio of the difference and one of the numbers of pixels of the previous fingerprint image and the subsequent fingerprint image;
e) selecting the fingerprint images for which the ratio exceeds a prescribed limit value;
f2) determining, for each selected fingerprint image, a number of pixels exceeding the prescribed gray value that did not exceed the prescribed gray value in the preceding fingerprint image and are adjacent to a pixel exceeding the prescribed gray value in the preceding fingerprint image; and
g2) deciding by the numbers determined in step f2) whether the fingerprint is regarded as pertaining to a real live finger.
4. The method according to claim 3 , which further comprises the steps of:
h2) determining, for each selected fingerprint image, a ratio of the difference determined in step c and one of the numbers of pixels of the appertaining fingerprint images determined in step f2); and
i2) deciding by the ratio determined in step h2) whether the fingerprint is regarded as pertaining to a real live finger.
5. The method according to claim 4 , which further comprises the steps of:
determining, for a last generated fingerprint image, a special number of pixels exceeding the prescribed gray value and located successively along one of finger lines rendered in the fingerprint image, imaginary center lines of the finger lines and corresponding center lines generated with a computer by thinning the finger lines into curves; and
dividing the numbers determined in step f2) by the special number.
6. The method according to claim 3 , which further comprises the steps of:
determining, for a last generated fingerprint image, a special number of pixels exceeding the prescribed gray value and located successively along one of finger lines rendered in the fingerprint image, imaginary center lines of the finger lines and corresponding center lines generated with a computer by thinning the finger lines into curves; and
dividing the numbers determined in step f2) by the special number.