RU2051415C1 - Method for output of papillary pattern image - Google Patents

Abstract

FIELD: computer engineering. SUBSTANCE: picture is divided into elements, the latter are analyzed against presence of image. This results in generation of frame filling matrix and in determination of line of optimal pressure of finger against roller. When rolling is over, direction and speed of rolling are determined and rolling speed is averaged. EFFECT: increased stability to screen pollution and to blur in image, increased sharpness and contrast of image. 7 cl

Description

 Over the years, a widely used method of obtaining an image of a papillary pattern is to roll a pre-painted surface of the skin onto a sheet of paper. Most often this method produces fingerprints ten-finger cards used by law enforcement agencies. A standard dactocard contains fingerprints rolled on paper from nail to nail, palm prints, as well as control prints formed by the simultaneous application of four fingers of one hand — index, middle, ring and little fingers.

 The laborious and unpleasant procedure for the manufacture of dactocard using dye has obvious disadvantages. Firstly, prints of satisfactory quality are not always obtained, and therefore successively rolling all ten fingers is very difficult. Secondly, very often the imprints in the upper part contain the lubrication zone caused by deformation of the spherical part of the finger when it is deployed to a plane. Lubrication is observed in other parts of the fingerprint, due to the complex curved shape of the finger.

 Several methods have been proposed for obtaining fingerprints without the use of dye.

 Known methods [1,2] in which the image of the papillary pattern of the finger is formed on the surface of a conical prism. But even using a set of prisms of different curvature, it is not possible to achieve a complete fit of the surface of the finger from the nail to the surface of the prism, since the complex curved surface of the finger has little in common with the conical surface.

 A method [3] of imaging a fingerprint is proposed, in which, during the advancement of the finger, the position of the light source is adjusted to provide an optimal illumination angle. This method has not found application, because there is no control over the quality of the resulting image.

 In the known method [4], the position of the line along which the fingerprint image is currently observed is controlled. This method also does not solve all problems, since it is impossible to roll a finger with one constant predetermined speed, and during the rolling process there will always be either a forward run or a lag of the actual position of the finger from the indicated indicator position. Such an inconsistent movement leads to loss of information at the edges of the image with a slight mismatch and to rupture of the image with a large.

Two methods of imaging a fingerprint are also known [5]
In the first method, dynamically changing images are summed over a rolling time of 1-3 s. At the same time, one can rely on obtaining a clear, non-lubricated image only if absolute adhesion is ensured between the rolling surface and the finger and slipping is completely absent, which is not feasible for the reasons described above.

 In the second method, the final image is formed from several intermediate images as follows. Before rolling, the final image is initialized with brightness values corresponding to the brightness of the empty rolling surface, then the first intermediate image is compared with the final one and in those places where the intermediate image is darker than the final one, the corresponding brightness values in the final image are replaced by the brightnesses from the intermediate image. The process is repeated for the second and subsequent images and as a result, a detailed image of the fingerprint is formed. There are two drawbacks to this method. The first is that each time the extreme part of the intermediate image is added to the final image, i.e. the part that is characterized by a weak fit of the finger to the rolling surface. As a result, the clarity and contrast of the image are lost. The second drawback is due to the fact that two adjacent image fragments can be formed by non-adjacent intermediate images and due to the elasticity of the skin between these fragments there will be no continuous transition.

 Closest to the proposed system is the formation of rolling images of fingerprints [6] in which movement along the rolling surface is monitored by the dynamic characteristics of the image in several adjacent lines of the current image. On each frame, the position of the left and right edges of the current image is fixed. The total image is formed from the intermediate ones in much the same way as in [5] with the only difference being that the image is not replaced everywhere in the final image, but in the range from the left to the right edge. Another difference is that, in addition to the substitution procedure, there is a procedure for averaging the final and intermediate images.

 The disadvantages of this system include both described disadvantages of the second method [5] The low accuracy of tracking the edges of the current image is due to the fact that not all the image is analyzed, but only a small number of its lines. The system has a serious limitation caused by the fact that the analysis of the image movement and the formation of the final image are performed in real time, which leads to a limitation on the frequency of polling the current image, since the processor needs some time to complete these calculations. This limits the system's ability to reduce fingerprint blur.

 The proposed method has increased resistance to screen contamination and the appearance of smear in the image and provides a sharper and more contrast image compared to existing ones.

 The current image, consisting of N rows and M columns, is divided into square areas of n * n image elements. The elements of the partition are sequentially analyzed in order to find out which of the elements of the current frame contain the image. For this, the values of the elements are summed up and the sum is compared with the Q parameter. If the sum exceeds Q, then it is considered that the element contains an image.

i,j),
F(k,l)=1, еcли S(k,l)>Q k=1.i/n,
F(k,l)=0, еcли S(k,l)<=Q l=1.j/n,
F(k,l)- матрица заполненноcти кадра.S (k, l)

i, j),
F (k, l) = 1 if S (k, l)> Q k = 1.i / n,
F (k, l) = 0 if S (k, l) <= Q l = 1.j / n,
F (k, l) is the matrix of frame occupancy.

 The parameter Q is set only greater than zero so that the noise of the digitizer does not introduce errors into the correct determination of the matrix.

 On the frame occupancy matrix, you can find a line along which the optimal fit of the finger and the rolling surface is ensured. Intuitively, this line can be defined as passing through the center of a given current image. This line is optimal for several reasons. Firstly, this line is far enough away from the leading edge, and the image along this line is formed, i.e. It has the highest possible clarity and contrast. Secondly, the blur of the image across this line is minimal due to the fact that the finger pressure on the rolling surface is approximately symmetrical with respect to this line. Thirdly, the image along this line has a maximum height, and as a result, the image does not lose the image along the upper and lower edges.

 Here it is necessary to explain what is meant by the formed image. Since the papillary line has irregularities on the surface, immediately after the line touches the rolling surface, the image is weak. For a short period of time, the irregularities are filled with grease artificially applied to the surface or formed by the sweat substance of the finger, and the image becomes as sharp and sharp as possible.

k 1.i/n
l 1.j/n
Обозначим $\genfrac{}{}{0ex}{}{\text{ΣΣ}}{\text{kl}}$ F(k,l) через U.The center line is the vertical line passing through the center of gravity of the current image (you can use the center line in the form of a curve):
X

k 1.i / n
l 1.j / n
We denote $\genfrac{}{}{0ex}{}{\text{ΣΣ}}{\text{kl}}$ F (k, l) via U.

If the above calculations are performed for all intermediate frames, then as a result we obtain the following set of parameters characterizing the rolling process:
x (i), M (i), i 0,1.N _k , N _{k the} number of frames.

A-priory:
X (O) 0, U (O) 0,
X (N _k ) M, U (N _k ) 0
The rolling process may end before the end of the last frame N. In order to determine the end time of rolling, the current image area of the frame is compared with the maximum frame area. The rolling process is considered complete, then the following condition is met:
U (i) <U _max / pp> 2
U _max max (U (i) i 1,2.N _k
The rolling direction (Fig. 2) is determined by the position of the finger in the first frame, i.e. by X (I):
If X (I)> M / 2 roll from right to left.

 If X (1) M / 2 is rolling from left to right.

The rolling speed calculated by the difference X (n) X (n-1) changes in steps from frame to frame. This is caused by the error in determining the midline. To eliminate this undesirable phenomenon, a simple procedure is used several times:
X ^t (2) (X (I) + X (3)) / 2
X ^t (3) (X (2) + X (4)) / 2

X ^t (N _k -3) (X (Nk-4) + X (N _k -2)) / 2
X ^t (N _k -2) (X (Nk-3) + X (N _k -1)) / 2
After applying this procedure, the change in speed becomes smooth, as shown in FIG. 3, which allows you to maximize the use of information from intermediate frames and strengthen the system's capabilities in the fight against lubrication.

The final image is formed from intermediate as follows. From the first frame, a strip from X (1) to X (2) is taken, from the nth strip from X (n-1) to X (n), from the N _kth strip from X (N _l-1 ) to X (N _k ).

 As another variant of the gluing procedure, one can apply averaging over several intermediate frames, i.e. the image strip is formed as follows.

In the band from X (i-1) to X (i), the average of the images of frames with the numbers ip, i-p + 1, i + p-1, i + p (p> 0) is taken
B ^x (i, j) [B ^ip (i, j) + B ^{i-p + 1} (i, j) ++ B ^{i + p} (i, j)] / 2-p + 1), where B ^x (i, j) an element of the generated image;
B ⁿ (i, j) picture element from frame n.

Claims (7)

 1. METHOD FOR PAPILLARY PATTERN IMAGE FORMATION, which consists in rolling fingerprint images, splitting a fingerprint image into elements in frames, determining the end of rolling, forming the final image when averaging images of intermediate elements, characterized in that after splitting the current image, the presence of elements in the elements is determined frames of image areas, according to the results, a matrix of frame occupancy is built and the line of optimal fit of the finger to the pr rink, after the end of the rolling direction and determined by the rolling speed, the rolling average value of the recording speed.  2. The method according to claim 1, characterized in that when determining the presence of image areas, each element is assigned a conditional value by the intensity of its color, and the conditional values of neighboring elements are summarized and compared with a threshold value.  3. The method according to claim 1, characterized in that the line of optimal fit of the finger to the rolling is defined as the geometric center of the current image.  4. The method according to claim 1, characterized in that the line having the maximum image height is selected as the line of optimal fit of the finger to the rolling.  5. The method according to claim 1, characterized in that the line of maximum fit of the finger to the rolling is determined by the line of maximum clarity, maximum contrast and minimum smear across this line.  6. The method according to claim 1, characterized in that the rolling speed is defined as the ratio of the difference in the position of the centers of the images of two adjacent frames to the time interval between these frames.  7. The method according to claim 1, characterized in that the rolling speed is eliminated by adjacent frames, and the number of frames 3 is selected.