Keywords
Fingerprint recognition, fingerprint image segmentation, evaluation benchmark, manually marked ground truth, directional Hilbert transform, Riesz transform, Butterworth bandpass filter, texture extraction
1 Introduction
Nowadays, fingerprint recognition is used by millions of people in their daily life for verifying a claimed identity in commercial applications ranging from checkin at work places or libraries, access control at amusement parks or zoos to unlocking notebooks, tablets or mobile phones. Most fingerprint recognition systems are based on minutiae as features for comparing fingerprints [1]
. Typical processing steps prior to minutiae extraction are fingerprint segmentation, orientation field estimation and image enhancement. The segmentation step divides an image into foreground, the region of interest (ROI), and background. Two types of error can occur in this step and both have a negative impact on the recognition rate: ’true’ foreground can be labelled as background and features like minutiae can be lost, or ’true’ background can be misclassified as foreground and spurious features may be introduced. It is desirable to have a method that controls both errors.
1.1 The Factorized Directional Bandpass Method, Benchmark and Evaluation
In order to balance both errors we take the viewpoint that – loosely speaking – fingerprint images are highly determined by patterns that have frequencies only in a specific band of the Fourier spectrum (prior knowledge). Focusing on these frequencies occuring in true fingerprint images (FOTIs), we aim at the following goals:

Equally preserving all FOTIs while attenuating all nonFOTIs.

Removing all image artifacts in the FOTI spectrum, not due to the true fingerprint pattern.

Returning a (smooth) texture image containing only FOTI features from the true fingerprint pattern.

Morphological methods returning the ROI.
In order to meet these goals we have developed a factorized directional bandpass (FDB) segmentation method.
The FDB method
At the core of the FDB method is a classical Butterworth bandpass filter which guarantees Goal 1. Notably Goal 1 cannot fully be met by Gaussian based filtering methods such as the Gabor filter. Obviously, due to the Gaussian bell shaped curve, FOTIs would not be filtered alike. Because straightforward Fourier methods cannot cope with curvature (as could e.g. curved Gabor filters [2]) we perform separate filtering into a few isolated orientations only, via directional Hilbert transformations. The composite directional Hilbert Butterworth bandpass filter (DHBB) incorporates our prior knowledge about the range of possible values of ridge frequencies (between and pixels) or interridge distances (between 3 and 25 pixels) [2], assuming a sensor resolution of 500 DPI and that adult fingerprints are processed. In the case of adolescent fingerprints [3] or sensors with a different resolution, the images can be resized to achieve an age and sensor independent size – not only for the first segmentation step, but also for all later processing stages. Our parameters can be tuned to reach an optimal tradeoff between treating all realistic frequencies alike and avoiding Gibbs effects. Moreover we use a data friendly rectangular spectral shape of the bandpass filter employed which preserves the rectangular shape of the spatial image.
A second key ingredient is the factorization of the filter into two factors in the spectral domain, between which a thresholding operation is inserted. After preserving all FOTIs and removing all nonFOTIs in application of the first factor, all FOTI features not due to the true fingerprint pattern (which are usually less pronounced) are removed via a shrinkage operator: softthresholding. Note that albeit removing less pronounced FOTI features, thresholding introduces new unwanted high frequencies. These are removed, however, by application of the second factor, which also compensates for a possible phase shift due to the first factor, thus producing a smoothed image with pronounced FOTI features only.
At this stage, nonprominent FOTI features have been removed, not only outside the ROI, but also some due to true fingerprint features inside the ROI. In the final step, these “lost” regions are restored via morphological operations (convex hull after binarization and twoscale opening and closing).
The careful combination of the above ingredients in our proposed FDB method yields segmentation results far superior to existing segmentation methods.
Benchmark
In order to verify this claim, because of the lack of a suitable benchmark in the literature, we provide a manually marked ground truth segmentation for all 12 databases of FVC2000 [4], FVC2002 [5] and FVC2004 [6]. Each databases consists of 80 images for training and 800 images for testing. Overall this benchmark consists of 10560 marked segmentation images. This ground truth benchmark is made publicly available, so that other researchers can evaluate segmentation algorithms on it.
Evaluation against existing methods
We conduct a systematic performance comparison of widely used segmentation algorithms on this benchmark. In total, more than 100 methods for fingerprint segmentation can be found in literature. However, it remains unclear how these methods compare with each other in terms of segmentation performance and which methods can be considered as stateoftheart. In order to remedy the current situation we chose four of the most often cited fingerprint segmentation methods and compared their performance: a method based on mean and variance of gray level intensities and the coherence of gradients as features and a neural network as a classifier
[7], a method using Gabor filter bank responses [8], a Harris corner response based method [9] and an approach using local Fourier analysis [10].1.2 Related Work
Early methods for fingerprint segmentation include Mehtre et al. [11] who segment an image based on histograms of local ridge orientation and in [12] additionally the graylevel variance is considered. A method proposed by Bazen and Gerez [7] uses the local mean and variance of graylevel intensities and the coherence of gradients as features and a neural network as a classifier. Similarly Chen et al. [13] use block based features including the mean and variance in combination with a linear classifier. Both methods perform morphology operations for postprocessing. A method by Shen et al. is based on Gabor filter bank responses of blocks [8]. In [2], all pixels are regarded as foreground for which a valid ridge frequency based on curved regions can be estimated. Wu et al. [9] proposed a Harris corner response based method and they apply Gabor responses for postprocessing. Wang et al. [14]
proposed to use GaussianHermite moments for fingerprint segmentation. The method of Zhu
et al. [15] uses a gradient based orientation estimation as the main feature, and a neural network detects wrongly estimated orientation and classifies the corresponding blocks as background. Chikkerur et al. [10] applied local Fourier analysis for fingerprint image enhancement. The method performs implicitly fingerprint segmentation, orientation field and ridge frequency estimation. Further approaches for fingerprint enhancement in the Fourier domain include Sherlock et al. [16], Sutthiwichaiporn and Areekul [17] and Bartůněk et al. [18, 19, 20].1.3 Setup of Paper
The paper is organized as follows: in the next section, we describe the proposed method beginning with the design of the DHBB filter for texture extraction in Section 2.1. Subsequently, the extracted and denoised texture is utilized for estimating the segmentation as described in 2.2 which summarizes the FDB segmentation procedure. In Section 3, the manually marked ground truth benchmark is introduced and applied for evaluating the segmentation performance of four widely used algorithms and for comparing them to the proposed FDB segmentation method. The results are discussed in Section 4.
2 Fingerprint Segmentation by FDB Methods
Our segmentation method uses a filter transforming an input 2D image into a feature image
(1) 
Due to our filter design, the product above as well as all convolutions, integrals and sums are understood in the principal value sense
Having clarified this, the symbol “pv” will be dropped in the following. At the core of (1) is the DHBB filter conveyed by ( counts directions, and are tuning parameters providing sharpness). In fact, we suitably factorize the filter conveyed by in the Fourier domain where with the argument reversion operator “” and apply a thresholding procedure “in the middle”. Underlying this factorization is a factorization of the bandpass filter involved. The precise filter design will be detailed in the following. Note that the directional Hilbert transform is also conveyed by a nonsymmetric kernel. Reversing this transform (as well as the factor of the Butterworth) restores symmetry. It is inspired by the steerable wavelet [21, 22, 23] and to some extend similar in spirit to the curvelet transform [24], [25] and the curved Gabor filters [2]. We deal with curvature by analyzing single directions separately before the final synthesis.
Via factorization, possible phase shifts are compensated and unwanted frequencies introduced by the thresholding operator are eliminated, yielding a sparse smoothed feature image. This allows for easy binarization and segmentation via subsequent morphological methods, leading to the ROI.
Note that (1) can be viewed as an analog to the projection operator in sampling theory with the analysis and synthesis steps (e.g. [26]). In this vein we have the following three steps:

Forward analysis (prediction): A first application of the argument reversed DHBB filter to a fingerprint image corresponds to a number of directional selections in certain frequency bands of the fingerprint image giving above.

Proximity operator (thresholding): In order to remove intermediate coefficients due to spurious patterns (cf. [27]) we perform soft tresholding on the filtered grey values yielding above.

Backward synthesis: Subsequently we apply the filter (nonreversed) again giving assembled from all subbands. A numerical comparison to other synthesis methods, summation (corresponding to a naive reconstruction) and maximal response in the appendix 5, shows the superiority of this smoothing step.
Due to the discrete nature of the image , we work with the discrete version of at in Eq. (1).
2.1 Filter Design for Fingerprint Segmentation
The features of interest in a fingerprint image are repeated (curved) patterns which are concentrated in a particular range of frequencies after a Fourier transformation. In principle, the frequencies lower than these range’s limits correspond to homogeneous regions and those higher to small scale objects, i.e. noise, respectively. Taking this prior knowledge into account, we design an algorithm that captures these fingerprint patterns in different directional subbands in the frequency domain for extracting the texture.
In this section, we design angularpass and bandpass filters. The angularpass filter builds on iterates of the directional Hilbert transformation, a multidimensional generalization of the Hilbert transform called the Riesz transform. It can be represented via principal value convolution kernels. The bandpass filter builds on the Butterworth transform which can be represented directly via a convolution kernel. We follow here a standard technique designing a bandpass filter from a lowpass filter which has an equivalent representation in analog circuit design.
2.1.1 The Order Directional Hilbert Transform of a Butterworth Bandpass
Although a fingerprint image
is a discrete signal observed over a discrete grid, we start our considerations with a signal
assuming values in a continuum . The frequency coordinates in the spectral domain will be denoted by .
As usual, the following operators are defined first for functions in the Schwartz Space of rapidlydecaying and infinitely differentiable test functions:
and continuously extended onto
In our context we only need . Further, we denote the Fourier and its inverse transformations by
where denotes the imaginary unit with .
Butterworth bandpass
For and frequency bounds , setting , , the onedimensional () Butterworth bandpass transform is defined via
cf. [28]. It is easy to verify that tends to zero for and
and has unique maximum at the geometric mean
with value . In consequence, for high values of , this filter approximates the ideal filterThe ideal filter, however, suffers from the Gibbs effect. Letting , we factorize the bandpass Butterworth as
with (), the negative squares of which representing the different complex roots of . Then, with the below complex valued factor of called the transfer function,
we use the approximation: to obtain
This approximation is often called the bilinear transform, which turns out to reduce the frequency bandwidth of interest, cf. Figure 7.
The 1D filter is then generalized to a 2D domain. The McClellan transform [29], [30], [31], [32], would be one favorable method. Also, recently, bandpass filtering with a radial filter in the Fourier domain has been proposed by [33], [34] and [35] et al. for enhancing fingerprint images. However, for a simpler reconstruction of 2D filter and a datafriendly alternative to the polar tiling of the frequency plane, a Cartesian array is used instead (see [24], [25], [36], [37]).
Thus, on a rectangular domain with common cuttoff frequencies
and the two characteristic functions
(see Figure 9), define
(2) 
as the spectrum of our twodimensional Butterworth filter . Note that since , there is a well defined
th order directional Hilbert transformations
For more detail on the Hilbert transform and the Riesz transform , we refer the reader to the literature for an indepth discussion [21], [23], [38], [39], [40], [41, 42, 43], [44], and [45].
Consider a vector
and set and compute, respectively, for(3)  
(4) 
The first line (3) called the Riesz transform has a representation as a principal value integral
where
Setting and we have for the third line (4) called the order directional Hilbert transform that
Since and high powers preserve the values near while forcing all other values in towards , this filter gives roughly the same result as an inverse Fourier transform of a convolution of the signal’s Fourier transform with
for small . The directional Hilbert transform, however, suffers less from a Gibbs effect than this sharp cutoff filter.
In 2D, the direction vector is with the discretized and , where is the total number of orientation. Rewrite the impulse response of the order directional Hilbert transform in (4) as
(5) 
2.1.2 Thresholding
For given , softthresholding is defined as follows
(7) 
Thus, the thresholded coefficients are Note that is a solution of the shrinkage minimization problem
yielding softthresholding (cf. [46]). Figure 4 visualizes the effect of the softthresholding and the comparison with the others (such as: hard [46], semisoft [47] and nonlinear [48] thresholding operators).
2.2 Fingerprint Segmentation
After having designed the FDB filter, let us now ponder on parameter selection, image binarization and morphological processing.
2.2.1 Parameter Choice for Texture Extraction
A fingerprint image will be rescaled such that its oscillation pattern stays in a specific range in the Fourier domain, the coordinates of which are . For choosing the cutoff frequencies and , we incorporate our prior knowledge about adult fingerprint images at resolution of 500 DPI: Valid interridge distances remain in a known range approximately from 3 to 25 pixels [2]. This corresponds exactly to as a limit for high frequencies. A limit of for low frequencies of the Butterworth bandpass filter corresponds to an interridge distance of about 12 pixels. The range contains the small scale objects which are considered as noise. The range contains the low frequency objects, corresponding to homogeneous regions.
The number of directions in and the order of the directional Hilbert transform involves a tradeoff between the following effects. We observe that with increased order the filter’s shape becomes thinner in the Fourier domain. Although this sparsity smooths the texture image in the spatial domain, in order to fully cover all FOTIs, needs to grow with . However, a disadvantage of choosing large and is that errors occur on the boundary due to the oversmoothing effect as illustrated in Figure 5 (o).
The next parameter to select is the order of the Butterworth filter . An illustration of the filter for different orders and with cutoff frequencies and is shown in Figure 6, its bilinear approximation in Figure 7. As increases the filter becomes sharper. For very large values of , it approaches the ideal filter which is known to cause the unfavorable Gibbs effect.
The thresholding value separates large coefficients corresponding to the fingerprint pattern (FOTIs) (which are slightly attenuated due to softthresholding) from small coefficients corresponding to nonFOTIS and FOTIs which are not features due to the fingerprint pattern (these are eliminated). On the one hand, if is chosen too large, more prominent parts of true fingerprint tend to be removed. On the other hand, if is chosen too small, not all all unwanted features (as above) are removed which may cause segmentation errors.
In order to find good tradeoffs, as described above, , , and are trained as described in Section 3.1. In fact, since different fingerprint sensors have different properties, is adaptively adjusted to the intensity of coefficients in all subbands as
(8) 
Thus, instead of , is trained for each sensor.
2.2.2 Texture Binarization
In the first step, the texture is decomposed by the operator (1) to obtain the reconstructed image . Then, is binarized using an adaptive threshold adjusted to the intensity of . Thus, the threshold is chosen as , with from (8). If is larger than this threshold, it will be set to 1 (foreground), otherwise, it is set to 0 (background) as illustrated in Figure 1.
2.2.3 Morphological Processing
In this final phase, we apply mathematical morphology (see Chapter 13 in [49]),
to decide for each pixel whether it belongs to the foreground or background.
Firstly, at each pixel
, we build an block centered at and 8 neighboring blocks
(cf. Figure 3). Then, for each block, we count
the white pixels and check whether their number exceeds the threshold with another parameter .
If at least blocks are above threshold, the pixel
is considered as foreground.
Then, the largest connected white pixel component is selected by a region filling method. Its convex hull is then the ROI. For better visualization we have inverted white and black, i.e. display the background by white pixels and the ROI by black pixels, cf. Figure 1.
3 Evaluation Benchmark and Results
The databases of FVC2000, 2002 and 2004 [4, 5, 6] are publicly available and established benchmarks for measuring the verification performance of algorithms for image enhancement and fingerprint matching. Each competition comprises four databases: three of which contain real fingerprints acquired by different sensors and a database of synthetically generated images (DB 4 in each competition).
It has recently been shown that real and synthetic fingerprints can be discriminated with very high accuracy using minutiae histograms (MHs) [50]. More specifically, by computing the MH for a minutiae template and then computing the earth mover’s distance (EMD) [51] between the template MH and the mean MHs for a set of real and synthetic fingerprints. Classification is simply performed by choosing the class with the smaller EMD.
The nine databases containing real fingerprints have been obtained by nine different sensors and have different properties. The fingerprint image quality ranges from good quality images (especially FVC2002 DB1 and DB2) to low quality images which are more challenging to process (e.g. the databases of FVC2004). Some aspects of image quality concern both the segmentation step and the overall verification process, other aspects pose problems only for later stages of the fingerprint verification procedure, but have no influence on the segmentation accuracy.
Aspects of fingerprint image quality which complicate the segmentation:

dryness or wetness of the finger

a ghost fingerprint on the sensor surface

small scale noise

large scale structure noise

image artifacts e.g. caused by reconstructing a swipe sensor image

scars or creases interrupting the fingerprint pattern
Aspects of fingerprint image quality which make an accurate verification more difficult, but do not have any influence on the fingerprint segmentation step:

distortion, nonlinear deformation of the finger

small overlap area between two imprints
Each of the 12 databases contains 110 fingers with 8 impressions per finger. The training set consists of 10 fingers (80 images) and the test set contains 100 fingers (800 images). In total there are 10560 fingerprint images giving 10560 marked ground truth segmentations for training and testing.
3.1 Experimental Results
Segmentation Performance Evaluation
Let and be the width and height of image in pixels. Let be number of pixels which are marked as foreground by human expert and estimated as background by an algorithm (missed/misclassified foreground). Let be number of pixels which are marked as background by human expert and estimated as foreground by an algorithm (missed/misclassified background). The average total error per image is defined as
(10) 
The average error over 80 training images is basis for the parameter selection. In Table 3, we report the average error over all other 800 test images for each database and for each algorithms.
Parameter Selection
Parameters  Description 

a constant for selecting the threshold in Eq. (8)  
which removes small coefficients corresponding to noise.  
the order of the directional Hilbert transform which  
corresponds to the angularpass filter in Eq. (4).  
the number of orientations in the angularpass filter in Eq. (4).  
the order of the Butterworth bandpass filter in Eq. (2).  
the window size of the block in the postprocessing step in Eq. (9).  
a constant for selecting the morphology threshold in Eq. (9).  
the number of the neighboring blocks in Eq. (9). 
FVC  DB  

2000  1  0.06  4  5 
2  0.07  2  5  
3  0.06  4  4  
4  0.03  1  5  
2002  1  0.04  1  4 
2  0.05  1  7  
3  0.09  1  5  
4  0.03  1  6  
2004  1  0.04  1  7 
2  0.08  2  5  
3  0.07  1  6  
4  0.05  1  5 
Experiments were carried out on all 12 databases and are reported in Table 3. For each method listed in Table 3, the required parameters were trained on each of the 12 training sets: the choice of the threshold values for the Gabor filter bank based approach by Shen et al. [8], and the threshold values for the Harris corner response based method by Wu et al. [9]. The parameters of the method by Bazen and Gerez are chosen as described in [7]
: the window size of the morphology operator and the weights of the perceptron which are trained in
iterations due to the large number of pixels in the training database. For the method of Chikkerur et al., we used the energy image computed by the implementation of Chikkerur, performed Otsu thresholding and mathematical morphology as explained in [49].For the proposed FDB method, the involved parameters are summarized in Table 1 and the values of the learned parameters are reported in Table 2. Also, the mirror boundary condition with size 15 pixels is used in order to avoid boundary effects. In a reasonable amount of time, a number of conceivable parameter combinations were evaluated on the training set. The choice of these parameters balances the smoothing properties of the proposed filter attempting to avoid both undersmoothing and oversmoothing.
FVC  DB  GFB [8]  HCR [9]  MVC [7]  STFT [10]  FDB  
2000  1  13.  26  11.  15  10.  01  16.  70  5.  51 
2  10.  27  6.  25  12.  31  8.  88  3.  55  
3  10.  63  7.  80  7.  45  6.  44  2.  86  
4  5.  17  3.  23  9.  74  7.  19  2.  31  
2002  1  5.  07  3.  71  4.  59  5.  49  2.  39 
2  7.  76  5.  72  4.  32  6.  27  2.  91  
3  9.  60  4.  71  5.  29  5.  13  3.  35  
4  7.  67  6.  85  6.  12  7.  70  4.  49  
2004  1  5.  00  2.  26  2.  22  2.  65  1.  40 
2  11.  18  7.  54  8.  06  9.  89  4.  90  
3  8.  37  4.  96  3.  42  9.  35  3.  14  
4  5.  96  5.  15  4.  58  5.  18  2.  79  
Avg.  8.  33  5.  78  6.  51  7.  57  3.  30 
This systematic comparison of fingerprint segmentation methods clearly shows that the factorized directional bandpass method (FDB) outperforms the other four widely used segmentation methods on all 12 databases. An overview of visualized segmentation results by the FDB method is given in Figure 12. A few challenging examples for which the FDB method produces a flawed segmentation are depicted in Figure 13. Moreover, a comparison of all five segmentation methods and their main features for five example images are shown in Figure 14 to 18.
4 Conclusions
In this paper, we designed a filter specifically for fingerprints which is based on the directional Hilbert transform of Butterworth bandpass filters. A systematic comparison with four widely used fingerprint segmentation showed that the proposed FDB method outperforms these methods on all 12 FVC databases using manually marked ground truth segmentation for the performance evaluation. The proposed FDB method for fingerprint segmentation can be combined with all methods for orientation field estimation like e.g. the line sensor method [52] or by a global model based on quadratic differentials [53] followed by liveness detection [54] or fingerprint image enhancement [2, 55]. It can also be used in combination with alternative approaches, e.g. as a preprocessing step for locally adaptive fingerprint enhancement in the Fourier domain as proposed by Bartůněk et al. [20]
or before applying structure tensor derived symmetry features for enhancement and minutiae extraction proposed by Fronthaler
et al. [56].Notably, the filter is similar to the Gabor filter which could have been used instead of the DHBB filter. Similarly, Bessel or Chebbychev transforms as well as Bsplines as generalizations ([57]) could replace the Butterworth. We expect, however, for reasons elaborated, relying on the DHBB filter gives superior segmentation results.
The manually marked ground truth benchmark and the implementation of the FDB method
are available for download at
www.stochastik.math.unigoettingen.de/biometrics/.
In doing so, we would like to facilitate the reproducibility of the presented results and promote the comparability of fingerprint segmentation methods.
Acknowledgements
S. Huckemann and C. Gottschlich gratefully acknowledge the support of the FelixBernsteinInstitute for Mathematical Statistics in the Biosciences and the Niedersachsen Vorab of the Volkswagen Foundation. The authors would like to thank Benjamin Eltzner, Axel Munk, Gerlind PlonkaHoch and Yvo Pokern for their valuable comments.
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