System and Method for Determining the Quality of Fingerprint Images

This invention relates to the field of image processing. More specifically, the invention relates to a system and method for processing fingerprint images and rejecting poor quality and/or partial fingerprint images.

There exist systems that accomplish automatic verification or identification of a person using his/her fingerprint. A fingerprint of a person comprises a distinctive and unique ridge pattern structure. For identification/verification purposes, ridge pattern structure could be characterized by endings and bifurcations of the individual ridges. These features are popularly known as minutiae. An example fingerprint is shown in FIG. 1A. The minutiae for the fingerprint shown in FIG. 1A are shown in FIG. 1B as being enclosed by "boxes." For example, box 101B shows a bifurcation minutia of a bifurcated ridge 101A in FIG. 1A and box 103B shows a ridge ending minutia of ridge 103A in FIG. 1A. Note that minutiae on the ridges in fingerprints have directions (also called orientations) 105 associated with them. The direction of the minutia at a ridge end 103B is the direction in which the end of the ridge points. The direction of a bifurcation minutia 101B is the direction in which the bifurcated ridge points. Minutiae also have locations which are the positions of the minutiae on the fingerprint with respect to some coordinate system.

The prevalent methods of fingerprint identification and verification methods are based on minutiae features. These systems need to process the fingerprint images to obtain accurate and reliable minutiae features to effectively determine or verify the identity of a person.

FIG. 2 is a flow chart showing the steps generally performed by a typical prior art system 200.

In step 210, the image is acquired. This acquisition of the image could either be through a CCD camera and framegrabber interface or through a document scanner communicating with the primary computing equipment.

Once the image is acquired into the computer memory or onto disk, relevant features minutia features are extracted (220). Not all of the features thus extracted are reliable. Some of the unreliable features are optionally pruned (step 230), e.g. manually edited. The resultant reliable features are used for matching two fingerprint images (step 240). That is, matching the acquired fingerprint image with stored minutiae representations of previously acquired fingerprint images.

In semi-automatic systems, the unreliable features could be manually pruned visually by a human expert before the minutiae are used for matching (step 240). The fingerprint feature extraction, pruning, and matching system constitute the primary backbone of a typical minutiae-based automatic fingerprint identification systems (AFIS). The matching results are typically verified by a human expert (step 260 in FIG. 2). The verification may also be performed automatically. The following reference describes examples of the state of the prior art:

Nalini K. Ratha and Shaoyun Chen and Anil K. Jain, Adaptive flow orientation based texture extraction in fingerprint images, Pattern Recognition, vol. 28, no. 11, pp. 1657-1672, November, 1995.

This reference is incorporated herein by reference in its entirety.

FIG. 3 is a flow chart showing the prior art steps performed by a feature extraction process that are similar to some of the feature extraction proposed by Ratha, Jain, and Chen in the article mentioned above.

It is often not desirable to directly use the input fingerprint image for feature extraction. It might need an enhancement or preprocessing before one could further extract minutiae. Typically, a smoothing process is employed to reduce the pixel-wise noise (step 305).

After the preprocessing stages, prior art systems find the directions of the ridge flow (step 310). The next important step in the processing is finding the exact location of the finger in the image, a process referred to as the foreground/background segmentation (step 315). Once the finger is localized, the next step is to extract the ridges from the finger image (step 320). The ridges thus extracted are thick and might contain some noisy artifacts which do not correspond to any meaningful structures on the finger. The small structures could be safely removed (step 325). The longer structures are thinned to one-pixel width and then processed to remove any other artifacts using morphological operators (step 330). The locations and orientations of ridge endings and bifurcations are then extracted from the thinned structures (step 335) to obtain the minutiae. In some systems, post-processing 340 is performed on the extracted minutiae.

Some prior art systems fail to correctly image fingerprints that have scars or other imperfections. Also, some prior art fails to overcome problems with optics, lighting and image acquisition (e.g., finger contact). Accordingly, there are a large number of poor quality images produced by these systems. Therefore, these systems are not effective because the matching based on these poor quality features is poor. It is therefore critical to design an automatic scheme which examines the quality of an acquired fingerprint image before it is processed so that fingerprints with poor quality caused by conditions like dryness or smudging can thereafter be identified and rejected.

For instance, FIG. 4A is a prior art drawing of a typical fingerprint 400 of poor quality. Specifically, region 410A is smudged. This smudging is common in fingerprints and can be caused by excessive pressure, sweat, excess ink, skin diseases, etc. FIG. 5A is a prior art drawing of another typical fingerprint of poor quality. Region 510A is caused by dryness of the finger in which case very little impression is produced in the image. Other reasons for poor quality regions include: poor optics, poor illumination, motion blur, etc. FIGS. 4B and 5B are prior art drawings of the unreliable minutia that result from the poor quality fingerprint in FIGS. 4A and 5A, respectively. Both images have excess minutiae (410B, 510B) from the smudging 410A and from the dryness 510A. These "noisy" fingers result in a number of unreliable minutiae, i.e., the minutiae do not uniquely identify their respective fingerprint and therefore can not be reliably matched to minutiae in a database that were extracted from an image of the same fingerprint.

Furthermore, it is desirable to determine if an image of a fingerprint is produced by capturing only a portion of the fingerprint. In these cases, the image quality of the portion can be high but the portion will not contain all of the minutia of the fingerprint. For example, FIGS. 6A and 6B are two partial impressions of separate portions of the same finger. Each impression has good quality but lacks minutiae (610A, 620B) that exist in the other impression. Specifically, FIG. 6A shows the left part of a finger, while FIG. 6B shows of the right part of the same finger. Such fingerprint images are hereafter referred to as "partial prints." Irrespective of how reliable the extracted minutiae are, there are not many common minutiae to be matched and therefore, use of these partial impressions can lead to non-identification or non-verification. Prior art does not recognize or address this problem.

An object of this invention is an accurate and reliable fingerprint image pre-processing system that quantifies fingerprint image quality.

An object of this invention is an accurate and reliable fingerprint image pre-processing system that quantifies fingerprint image quality and reject images of poor quality.

An object of this invention is an accurate and reliable fingerprint image pre-processing system that detects images containing partial prints.

Another object of this invention is an accurate and reliable fingerprint image pre-processing system that detects smudged fingerprint images.

Another object of this invention is an accurate and reliable fingerprint image pre-processing system that detects fingerprint images that arise from dry fingers.

The invention is a computer system and method that determines a quality measure of a fingerprint image by dividing the image into blocks of pixels. (The invention permits the blocks to be created by down sampling pixels from the original image.) The blocks are marked to be directional (having a direction/orientation) or non-directional by determining whether there is a "prominent" number of pixels with the same direction within the block. The blocks are also determined to be within the foreground or background of the image. One embodiment of the invention makes these determinations by summing the intensity differences between the pixels in the block and their neighbors along one or more directions to classify the pixels as either foreground or background pixels. Blocks with over a threshold number of background pixels are background blocks, the other blocks are foreground blocks. Contiguous regions of blocks that are in the foreground and are directional are selected. In some embodiments, regions that have too few blocks are ignored. The quality measure is the ratio of the area of all these selected contiguous regions to the total area of the fingerprint image (i.e., the foreground.)

In one preferred embodiment, the contribution of each of the blocks of the regions to the quality measure is weighted (i.e., given by) by a distance from a block containing a reference point (a block of reference) within the foreground. In a more preferred embodiment, this block of reference is located at the centroid of the foreground.

Images are judged to be of poor quality if the quality measure is below a quality threshold and are judged to be of good quality if the quality measure is above the quality threshold. Optionally, poor quality images are examined further to determine if they are smudged or come from a dry finger. Good quality images are further processed to determine if they are partial or entire images of the fingerprint.

The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of preferred embodiments of the invention with reference to the drawings that include the following:

FIG. 1A is a prior art drawing of a typical fingerprint.

FIG. 1B is a prior art drawing showing minutia of the fingerprint in FIG. 1A.

FIG. 2 is a flow chart showing the method steps performed by a typical prior art system.

FIG. 3 is a flow chart showing the prior art steps performed by an feature extraction process.

FIG. 4A is a prior art drawing of a typical smudged fingerprint of poor quality.

FIG. 4B is a prior art drawing of the unreliable minutia that result from the poor quality fingerprint in FIG. 4A.

FIG. 5A is a prior art drawing of a typical dry fingerprint with poor quality.

FIG. 5B is a prior art drawing of the unreliable minutia that result from the poor quality fingerprint in FIG. 5A.

FIG. 6A is a prior art drawing of a (portion) left part of a fingerprint, i.e., "partial print."

FIG. 6B is a prior art drawing of a (portion) right part of a fingerprint, i.e., "partial print."

FIG. 7 is a block diagram of one preferred embodiment of the present system.

FIG. 8 is a flow chart showing the steps performed by the fingerprint quality checker.

FIG. 8A is a flow chart showing a preferred embodiment of image segmentation and block direction determination.

FIG. 8B is a flow chart showing the steps for characterizing whether a poor quality fingerprint image is dry or smudged.

FIG. 8C is a flow chart showing the steps for determining whether a fingerprint image contains a complete or partial impression.

FIG. 9 is a figure showing the sets of neighboring pixels for four directions.

FIG. 10 is an illustration of a fingerprint with directional blocks used in a quality measurement.

FIG. 11 is a figure illustrating how to identify an image containing only partial impressions.

FIG. 12 is a drawing that shows how to resolve the ambiguities in directions of fixed size regions.

FIG. 13 is a drawing that shows how abrupt changes in block directions are handled.

Referring now to the drawings, and more particularly to FIG. 7, there is shown the block diagram representation of a general computer hardware environment that is used as the image processing system 700. This computer 710 may be one of International Business Machines Corporation (IBM) Personal System/2 (PS/2) family of Personal Computers, a RISC System/6000, or Power Parallel System (SP/x), or equivalent. The system 700 includes one or more central processing units (CPU) 715, which may conform to any general computer architecture (e.g. Intel or a reduced instruction set microprocessor.) The CPU 715 is attached to a system bus (not shown) to which are attached a read/write and/or random access memory (RAM) 720 that can include one or more cache memories, a read only memory (ROM) 740, and an input/output adapter 725. The RAM 720 provides temporary storage for one or more application program processes 800 (FIG. 8) containing code and/or data while the ROM typically includes the basic input/output system (BIOS) code. Direct Access Storage Devices (DASDs), here represented by hard disk drive 730, are also connected to the CPU by appropriate adapter (not shown.) The hard disk drive 730 typically stores the computer's operating system (OS), such as IBM's OS/2 operating system, and various application programs, data, and/or databases. These databases include intermediate results and fingerprint image data 735. Typically, the input/output adapter 725 has attached to it a keyboard 727, a mouse 728, and/or other user interface devices (not shown).

The system 700 also can include a display 738, here represented as a cathode ray tube (CRT) display but which may be a liquid crystal display (LCD) or other suitable display and/or graphic user interface (GUI) 738. The display 738 is connected to the system bus via a display adapter.

The computer 710 is also interfaced with a framegrabber 750 and an image acquisition device, e.g. a camera 760, arrangement to capture the fingerprint image into the computer memory/disk. Alternatively, the computer might be communicating with a document scanning device 765 that scans the fingerprint image 766 from a document 770 like an inked fingerprint card 770. Any other known means can be used to enter a fingerprint image to the memory 735, e.g., transmitting an image over a network 786 to the system 700 from another computer 710A, e.g., a server 710A, that is connected to the network 786 and has access to one or more databases 787.

The hardware for system 700 and equivalents of these systems are well known to those skilled in the art.

Personal System/2, PS/2, OS/2, RISC System/6000, Power Parallel System, SP/x, and IBM are trademarks of the International Business Machines Corporation.

FIG. 8 is a flow chart showing the steps of the novel fingerprint image quality checker 800.

Given a fingerprint image, e.g. 766, a reduced size image is first obtained at step 810. In a preferred embodiment, the accessed image is reduced to one forth the actual image size to reduce the number of pixels in the image. The invention can determine quality using reduced size images and therefore allows for reduced storage usage and faster processing times.

In this embodiment, one pixel is selected out of 4, e.g. the upper left pixel is selected to represent the four pixels in every 2.times.2 matrix of pixels on the image. Therefore, the input fingerprint image is reduced by a factor of two in both directions. For the determination of the quality of a fingerprint image, the information lost at this step is negligible.

In one embodiment, step 820 extracts (segments) the fingerprint image from the background using known techniques.

Then step 830 selects a block of (representative) pixels from the extracted foreground fingerprint image 820 (e.g., of the reduced image 810). For instance, a block is 8.times.8 (representative) pixels. Then a block direction is extracted (i.e., orientation) from the block. In a preferred embodiment, the block direction is the direction of the maximum number of pixels in the block where each pixel is oriented in one of four directions. For example, in a block with 64 pixels, each pixel has one of the following orientations (directions): 0, 45, 90, and 135 degrees. If more pixels have an orientation of 45 degrees than any other pixel orientation, the block direction (orientation) is chosen to be 45 degrees. To be chosen in a preferred embodiment as the prominent block direction, the direction of more than half of the pixels in the block has to be the same. If the block does not contain more than half of its pixels in a given direction, the block is marked as having no prominent direction. See the following reference:

B. M. Mehtre, Fingerprint Image Analysis for Automatic Identification, Machine Vision and Applications, Springer-Verlag, vol. 6, pp. 124-139, 1993.

which is herein incorporated by reference in its entirety.

In a preferred embodiment, steps 820 and 830 are combined as shown in process 820A in FIG. 8A. In this process 820A the direction extraction and segmentation are novelly combined.

Process 820A (FIG. 8A) performs both the foreground and background segmentation of step 820 and the orientation computation of step 830.

In step 821, the entire image is divided into blocks, e.g., 8.times.8 pixels. Then the orientation of each pixel in the block is determined 822 by selecting a direction (i.e., a line of pixels containing the given pixel) that has the smallest variation in intensity. In a preferred embodiment, four directions are used: 0, 45, 90, and 135 degrees. Similar methods are well known, and are explained by Mehtre in the article mentioned above.

In step 823, the intensity along each direction from the pixel is novelly used to determine whether the pixel is part of the foreground or part of the background. In other words, the intensity variations along selected directions containing each of the given pixels is used to segment the fingerprint image from the background.

Regions of background and areas of leftover impressions (e.g. those impressions having faint residual portions of different figure prints--from dirty input devices) usually exhibit small intensity variation around their neighborhoods. To determine whether a pixel belongs to the background, step 823 first computes the intensity differences, D.sub.d, between the pixel of interest and its neighboring pixels on the (four) directional lines, e.g. in a preferred embodiment d=0, 45, 90 and 135 degrees respectively, and the intensity difference is determined as follows:

where f(i,j) is the intensity of pixel (i,j) and f.sub.d (i,j) are the intensities of the neighbors of pixel (i,j) along the d direction. This indicates the summation of differences between the given pixel of interest, pixel (i,j), and a number n, e.g. 6, neighboring pixels along each of the directions. FIG. 9 illustrates the neighbors of pixel (i,j), where pixels labeled as 1, 2, 3, 4 correspond to neighbors on the directions 0(910), 45(920), 90(930) and 135(940) degrees, respectively. Note that in this embodiment, three neighbor pixels are selected in each of the positive (950) and negative (960) directions along each of the 0, 45, 90, and 135 degree directions/paths.

The intensity variation D at the pixel of interest is then obtained by summing up the differences at four directions, e.g., all 24 differences, as follows: ##EQU1##

A comparison 824 is made to determine whether the pixel's summed intensity difference 823 is greater or less than a background threshold. The pixel is classified as a background pixel 826 if the summed intensity variation D 823 is less than the background threshold. The pixel is classified as a foreground pixel 825 if the summed intensity variation D 823 is more than the background threshold. If the intensity variation is equal to the background threshold, one of the branches (825, 826) is consistently taken.

Subsequently, for a block of fixed size, say N by N (in a preferred embodiment, N is set to eight), step 827 determines whether the block of interest belongs to the foreground of a fingerprint image. The criterion is that if less than certain block threshold amount of pixels within the block are in the background, the block is marked as foreground 829. If the block contains more than the block threshold of pixels in the background, the block is marked as background 828. In a preferred embodiment, if more than five percent of the block's pixels are background, the block is marked as background. Otherwise, the block is marked as a foreground block. Background blocks are not considered in the rest of the processing, e.g., the remaining orientation computations 830/830A.

Optionally, the segmentation can have additional features. In one preferred embodiment, step 827 examines the blocks in the neighborhood surrounding the given block before marking the block as foreground/background rather than performing the above foreground extraction scheme on each block independently. If all the neighbors surrounding the given block are marked as foreground (background) than the given block is also marked as foreground (background). In a preferred embodiment, a 3 by 3 neighborhood of blocks around the given block is considered.

Optionally, step 827 can perform an area-based test to determine if the fingerprint image (foreground) is too small. If the area of the segmented foreground is less than a desired size (in a preferred embodiment, the desired size is twenty percent of the total image size), the fingerprint image is rejected right away because it is too small. The fingerprint image is then said to be of poor quality because of the small area it occupies.

Referring back to FIG. 8, step 840 determines whether there is a block direction (as described above) or not. If there is a block direction, i.e., a prominent direction, the direction is noted, and the block is marked as having a prominent direction, e.g., with a binary value of 1. If the block has no prominent direction, as determined above, the non-directional block is also marked, e.g., with a 0.

After the foreground blocks are marked, step 840 (FIG. 8) determines whether the resulting direction for each block is prominent. The idea is that a block with a prominent direction should exhibit a clear ridge/valley direction which is consistent with most of the pixel directions. On the contrary, a block with no direction does not convey any consistent directional information. Specifically, step 840 first computes the directional histogram, for each block, based on the directions of pixels within it. Then if the maximum value of the histogram is greater than a prominent threshold T.sub.1, the block is said to have a prominent direction, and is labeled as such. In addition, bifurcations of ridges may result in two prominent directions in a block. Therefore, if two or more directions of the direction histogram are greater than a bifurcation threshold, T.sub.2, the corresponding block is also labeled as having a direction. Typically, the bifurcation threshold, T.sub.2, is less than the prominent threshold, T.sub.1. Accordingly, step 840 generates a binary image with blocks having prominent directions being labeled as 1, while the others having no prominent direction, being labeled as 0.

Up to step 840, the decision as to whether a block conveys a consistent directional information is made independently, i.e, on a block by block basis. However, it is desirable to remove "noise" from the obtained binary image by taking into account the properties of the neighboring blocks. Steps 850 and 860 play such roles in our fingerprint image quality checker. Specifically, step 850 performs post-processing to remove blocks which are inconsistent with their neighbors. If a "directional" block is surrounded by "non-directional" blocks, it is relabeled as a non-directional block. Similarly, a non-directional block being surrounded by neighboring directional blocks is changed to a directional block.

For example, an optional post-processing step 850 examines all the blocks in a neighborhood of a given block to determine if all of the blocks in the neighborhood are marked as having a direction (no direction). If the given block is marked as having no direction (a direction) and all of the blocks in the neighborhood are marked as having a direction (no direction), the marking of the given block is changed to that of having a direction (no direction.) In a preferred embodiment, the blocks in the neighborhood are those that are adjacent to the given block and form a 3.times.3 matrix of blocks around the given block. For example, if the given block is marked as having no direction and all the blocks in its neighborhood are marked as having a direction, the given block is marked as having a direction.

Step 860 does a connected component analysis based on

Dana H. Ballard and Christopher M. Brown,

Computer Vision, 1982,

Prentice-Hall, Inc., Englewood Cliffs, N.J. 07632

of which chapter 5 is herein incorporated by reference in its entirety. Step 860 determines which blocks have a direction and are contiguous with other block that have a direction. (Note that the directions do not have to be the same.) Once these contiguous regions of blocks with directions are found, the number of blocks in each contiguous region is determined. The blocks in contiguous regions have fewer than a region threshold number of blocks, i.e., a region threshold, are discarded for the purposes of the remaining steps in this process 800. This is because regions with small areas, i.e., with fewer than the region threshold number of blocks, i.e. a region threshold, are assumed to be noise. In a preferred embodiment, the region threshold number of blocks is 10. The end result is that the fingerprint foreground image is partitioned into regions of contiguous direction blocks and blocks without direction or non contiguous blocks with direction.

The quality value of the acquired fingerprint image is determined in step 870. If the computed quality value is small, below a quality threshold, the image is rejected 871. Step 880 performs a dryness/smudginess test on rejected 871 images that provides feedback to the system operator for reference. On the other hand, if the quality value, is above the quality threshold 872, the image is further processed to determine 890 if the good image is an entire or partial fingerprint image.

After the regions of prominent block directions and large support (greater than the region threshold 860) are identified, step 870 determines the quality of these selected regions of the fingerprint image.

Refer to FIG. 10. The quality value computation step 870 selects contiguous regions 1010 of blocks 1020 that have greater than a threshold area 1015 (greater than the region threshold 860), that are in the foreground 1030 of the fingerprint image 1050. Note that in a preferred embodiment, each member block of the region 1010 has its own prominent direction 1040 and the region 1010 is larger than the threshold area 1015, i.e., the region threshold 860 determines the minimum number of blocks in the region 1010. The quality measure is the ratio of the area of all these selected contiguous regions (all the regions 1010) to the total area 1030 of the fingerprint image (i.e., the foreground 1030.)

In a preferred embodiment, the contribution of each of the blocks to the quality measure is weighted by a distance 1045 from a block containing a reference point 1060 (a block of reference) within the foreground. In a more preferred embodiment, the block of reference 1060 is located at the centroid of the foreground.

A preferred embodiment, employs a weighed scheme to determine the quality of the fingerprint image 1050. More emphasis is put on regions 1010 close to the centroid 1060. To do this, a weight is assigned to a block based on the geometric distance 1045 from the block 1020 to the centroid 1060. This is appropriate since regions (or accordingly minutiae) near the centroid are likely to provide more information for the Finger Identification System (FIS). Specifically, for block x.sub.i (which is a foreground block), the associated weight w.sub.i is

where x.sub.c is the centroid of foreground, and q reflects the contribution (relative weight) for blocks with respect to the distance from the centroid. In a preferred embodiment, q is set to five.

In a preferred embodiment, since all of the block have an equal area, the quality (i.e., the ratio of the areas above) of a fingerprint image Q is therefore obtained by computing the ratio of total weights of directional blocks to the total weights for each of the blocks in the foreground 1030, i.e.

The computed quality Q is used as a measure of how much reliable directional information is available for an acquired fingerprint image. If the computed Q is less than the quality threshold, TH, the image is rejected 871.

Many reasons cause the loss of directional information of a fingerprint image. For example, excessive ink results in a smudged impression, insufficient ink leads to a dry impression, etc. It is desirable to be able to identify what type of low quality fingerprint images are captured. Step 880 is therefore designed to identify whether a to-be rejected fingerprint image is dry or smudged. The idea is that for a dry impression, there are lots of "dry" blocks where the contrasts of corresponding neighbors vary a lot. And for a smudged impression, there are lots of "smudged" blocks whose contrasts are very small.

FIG. 8B shows the procedures involved in optional step 880. For a poor quality fingerprint image, a dryness test is first conducted. If it is found to contain the dry impression of a finger, the smudginess test is ignored. If not, a smudged test is performed afterward.

In step 8801, the mean intensity of pixels within each foreground block is computed. For each block, step 8802 next computes the mean intensity (.mu.) of pixels whose intensities are smaller than the mean intensity of all pixels within the block. (The pixels whose intensities are smaller than the mean intensity of all the pixels in the block are considered to be pixels on a ridge, i.e. ridge pixels. Also .mu. is assumed to be an approximation of the mean intensity of ridge pixels.) Step 8802 also computes the standard deviation (.sigma.) of intensities of all pixels within the same block.

For a block with good contrast, .mu. is small and .sigma. is large. But for a block with low contrast due to dryness, .mu. is large and .sigma. is small. Subsequently, to measure the contrast within a block, step 8803 computes the ratio of corresponding .mu. to corresponding .sigma., i.e.

where c.sub.d is the contrast measure. A block is then considered to be dry 8803A if c.sub.d is lower than a first dryness threshold, DTH1. Alternatively the fingerprint is also considered to be dry if the contrasts of its neighboring blocks vary a lot. Specifically, step 8804 compares the difference between the maximum and minimum values of the corresponding set of contrasts 8803 to a second dryness threshold DTH2. If the difference is greater than DTH2, then the block is a dry block 8805. Otherwise, the block is a non-dry block 8806. (In a preferred embodiment, 3 by 3 neighboring blocks are chosen, and DTH2 is between 2 and 4. In a more preferred embodiment, DTH2 is set to 3.5. In a preferred embodiment, DTH1 is greater then 4.0, more preferably 4.5.) The dryness measure of the image is next computed at step 8807 as the ratio of the total number of dry blocks 8805 to the total number of foreground blocks 829. If 8807 the resulting measure is greater than a third threshold, DTH3, step 880 (FIG. 8) reports that a dry impression caused the quality measure to be poor and stop. If not, the following smudginess test (8808 et seq.) is performed.

To check whether a non-dry fingerprint image is smudged, step 8808 computes another measure to determine the smudginess of each block. The measure is computed as the product (c.sub.s) where

of the corresponding .mu. and corresponding .sigma. obtained at step 8802. A comparison 8809 is made to determine whether a block is a smudged block. If the smudginess measure 8808 is smaller than a smudginess threshold STH, then the block is classified as a smudged block 8810. If not, the block is classified as a non-smudged block. (In a preferred embodiment, c.sub.s is normalized by a typical maximum value of c.sub.s, in a preferred embodiment, the number is 128.times.128. Further, STH is between 0 and 0.2. In a more preferred embodiment, STH is set to 0.12. Lastly, step 8812 computes the ratio of the number of smudged blocks to total number of foreground blocks. If the resulting ratio is larger than a second threshold, STH2, a smudged impression is reported 880.

Step 880 (FIG. 8) considers the association of dry or smudged description to a low quality fingerprint image.

Step 890 (FIG. 8) performs a partial impression analysis to ensure that the complete impression (not a partial print) of a finger is acquired in those images with an acceptable quality measure. If not, the operator is notified that the image only contains a partial impression.

Without such a procedure, an image consisting of a partial impression of a fingerprint may be captured and used. In these images, reliable directional information exists for extracting minutiae. However, it is likely that there is less common area between mated (fingerprint images from the same finger) impressions. Additional effort is thus required to match them. Moreover, the worst case is if two non-overlapped fingerprint images corresponding to the same finger are considered. (For example, a query fingerprint image consists of left part of a finger, while the mated fingerprint image in the database represents the right part of the same finger.)

Step 890 (FIG. 8) applies an automatic scheme for identifying images containing such partial impressions. FIG. 8C is a flow chart showing the procedures involved in step 890. In this procedure, FIG. 8C, a fingerprint is considered complete if at least one ridge is found to form a parabolic-like curve (i.e., a curve that exists in both a left and right half of a coordinate system centered at the centroid of the foreground image of the fingerprint) and that has path ends that both terminate sufficiently close to the horizontal axis on which the centroid lies.

First, step 891 constructs a coordinate system (1110) whose origin x.sub.c (1120) locates at the centroid of the foreground and x and y axes are parallel to columns and rows, respectively, as shown in FIG. 11. The idea is that, if an image consists of the complete impression of a fingerprint, there should exist at least one ridge which lies on both left half plane and right half plane with respect to the constructed coordinate system and terminates at both ends of the path sufficiently close to the x axis.

However, instead of detecting ridges which requires additional work, this process relies on the directions of foreground blocks. Specifically, a path is defined to be a sequence of blocks whose member blocks are connected (see 897, 898, and 899 below) by the associated block directions. Then the path is determined to exist or not exist in both the right and left half planes about the centroid. Note that if there is no prominent direction within a non directional block, but here the block direction 830 still exists and is used.

Step 892 selects a starting block of a path by choosing a block, (x.sub.1, 0) 1130, on the left plane and on the x-axis (i.e. x.sub.1 <0). If the direction of the selected block happens to correspond to 0 degree 893, the scheme goes back to step 892 and selects another block. Otherwise, based on the block direction, the next block 894 of the current path is found. Step 895 subsequently determines whether the path has indicated a complete impression of a fingerprint image by checking whether block 894 is located at the right half plane, and whether it is also close to the horizontal axis. If such case occurs, then the image is said to consist of a complete impression. (In a preferred embodiment, denote the coordinate of block 894 by (x.sub.2,y.sub.2) 1140. If x.sub.2 >0 and y.sub.2 <c, then a complete impression is acquired. (In a more preferred embodiment, c is set to 3, i.e., within 3 blocks of the x axis.) If not, step 896 is performed to determine whether block 894 is a background block. When the path leads to a background block, the corresponding path is terminated, and the scheme goes back to step 892 to start another new path. This is because the terminated path does not come close enough to the horizontal axis. If block 894 is not a background block according to step 897, the current path is to be extended by the following steps.

Step 897 concerns about resolving the ambiguity in choosing which direction to proceed from block 894. For example, if the direction of block 894 is 0 degree, either moving to the right or to the left of the current block is possible. In a preferred embodiment, the assumption is employed that ridges do not change direction abruptly. Specifically, given the heading direction of a path at the block immediately before block 894, step 897 chooses the direction (from two possible ones) which results in less heading change of a path to proceed. For example, in FIG. 12, if the current block is b.sub.2 1210, while the previous block and associated path direction are b.sub.1 1220 and the direction shown in the FIG. 12, then b.sub.3 1230 is chosen to extend the path instead of b.sub.4 1240.

Once the direction ambiguity is resolved, 898 checks whether there is an abrupt change in the heading direction of the path. If the change is 90 degrees, step 899 replaces the direction of block 894 with the direction of the member block which is immediately before block 894. For clarity, FIG. 13 shows such an example, where b.sub.2 1320 is the current block and b.sub.1 1310 is the previous block, while the direction of the current path is indicated by the direction of b.sub.1. The computed direction of b.sub.2 1320 shows a 90 degree heading change irrespective of choosing b.sub.3 1330 or b.sub.4 1340. Block b.sub.5 1350 is chosen accordingly.

The scheme then extends the path according to the modified direction to find another next block 894. For example, in FIG. 11, the path from 1130 to 1140 consisting of blocks {(-4,0), (-3,1), (-2,2), (-1,3), (0,3), (1,3), (2,2), (3,1), (3,0)} is shown along with their block directions. Since this path meets the aforementioned criterion of a complete impression, the corresponding image is retained for further processing. In a preferred embodiment, if all the paths whose starting blocks are on the left half plane indicate a partial impression is captured, the scheme will start tracing paths whose starting blocks are on the right half plane, and similar procedures are applied. This is an alternative way to find at least one path that meets the criteria. Lastly, if the fingerprint is determined to be partial according to the criteria above, the partial print is reported and is rejected 890.

Given this disclosure alternative equivalent embodiments will become apparent to those skilled in the art. These embodiments are also within the contemplation of the inventors.