# Patent

## Title
Multiscale modulus filter bank and applications to pattern detection, clustering, classification and registration

## Abstract

A digital filter bank having a number J≧1 of stages is disclosed. For each integer j such that 1≦j≦J, the j-th stage includes a plurality of filtering units (20, 21) each receiving an input signal of the j-th stage. These filtering units include a low-pass filtering unit (20) using real filtering coefficients and at least one band-pass filtering unit (21) using complex filtering coefficients. Following each band-pass filtering unit of the j-th stage, a respective modulus processing unit (25) generates a processed real signal as a function of squared moduli of complex output values of the band-pass filtering unit. The input signal of the first stage is a digital signal supplied to the digital filter bank, while for 1<j≦J, the input signal of the j-th stage includes the processed real signal generated by at least one modulus processing unit of the (j−1)-th stage.

## Background
BACKGROUND OF THE INVENTION
The present invention relates to digital signal processing. It provides a representation of digital signals which can be used in a variety of applications including pattern detection, clustering, classification and registration.
For one dimensional or multidimensional signals, pattern detection, clustering, classification and registration problems require computing a reliable distance that measures the similarity of signals despite the existence of deformations.
For example, if two signals are translated, their Euclidean distance may become very large despite the fact that they are identical up to this translation. In images, such deformations include non-rigid translations, rotations, scaling (zoom in and out). For sounds, it includes frequency transpositions and scaling.
In pattern detection applications, the potential location of a family of predefined patterns in a given signal, where they may appear with deformations, is looked for. Detecting a human face in an image, despite its variability due to different morphologies, pose and/or scaling, is an example of pattern detection problem which requires using a distance that is not sensitive to such deformations.
A clustering problem in a signal involves segmentation to group signal patches that are similar relatively to some distance. Texture segmentation is an example of clustering problem which consists in dividing images into homogeneous texture regions. A clustering problem may also apply to several signals which need to be grouped in homogeneous classes.
A signal classification problem consists in finding the class to which a signal belongs. Classifying signals such as sounds, images, video signals or medical signals requires some measurement of their similarity. Signals that belong to the same class may differ by some important deformation. Such deformations should not alter their similarity distance to classify them appropriately.
Pattern registration is about recovering a deformation that maps a signal onto another signal. In stereo images, this deformation, also called disparity, carries depth information on the scene. In videos, such deformation provides the optical flow. In medical imaging, recovering this deformation allows registration of medical data to analyze potential anomalies.
Standard distances such as the Euclidean distance do not measure the similarity of signals when these signals have undergone some deformation, such as a translation. Indeed, the Euclidean distance between a signal and its translated version is often very large. Instead of measuring a Euclidean distance between signals, one may apply such a distance to some signal representation that is constructed to preserve important signal information while being insensitive to other signal properties. A major difficulty of signal detection, clustering, classification and registration then becomes to compute an appropriate signal representation over which state of the art detection, clustering, classification and registration algorithms may be applied using state of the art distances such as Euclidean distances.
Spectrograms are widely used in the field of speech recognition. They include removing the complex phase of the Fourier transform of an input audio signal restricted by a time window function. Indeed, the complex phase of a Fourier transform can be interpreted as a translation parameter which is removed by the complex modulus operation. However, spectrograms are local Fourier transforms and are thus sensitive to translations that are large relatively to the window size and to other deformations such as a signal scaling.
Multiscale transformations, such as wavelet transforms, have also been used for pattern recognition. In particular, a so-called scalogram, obtained with a complex wavelet transform followed by a modulus computation is a representation which is used for pattern recognition in U.S. Pat. No. 7,171,269. However, at fine scales wavelet coefficients are sensitive to translations and at coarse scales they do not carry enough information to discriminate signals.
In “A Steerable Complex Wavelet Construction and Its Applications to Image Denoising”, IEEE Transactions on Image Processing, Vol. 14, No. 7, July 2005, pp. 948-959, A. A. Bharath, et al. disclose a pyramid structure for a filter bank used for sub-band decomposition. The basic pyramidal unit has an isotropic lowpass filter followed by a downsampler, and complex bandpass filters. The structure extends over successive sub-band decomposition stages, the subsampled lowpass component being passed to the following stages. A similar kind of pyramid structure is presented in Portilla, et al., “A Parametric Texture Model Based on Joint Statistics of Complex Wavelet Coefficient”, International Journal of Computer Vision, Vol. 40, No. 1, October 2000, pp. 49-71.
Neural networks are also used in the context of pattern recognition. Multilayer neural networks define a cascade of linear and non-linear transformations that can construct efficient signal representations for pattern recognition. In particular, the convolution networks disclosed in “Large-Scale Learning with SVM and Convolutional Nets for Generic Object Categorization” (Fu-Jie Huang and Yann LeCun, Proceedings of the 2006 IEEE Computer Society Conference on Computer Vision and Pattern Recognition, Vol. 1, p. 284-291) perform a cascade of convolutions with real filters followed by some linear operations that typically include an absolute value and a sigmoid transformation. However, these neural networks require a heavy training phase to adjust the filters to the type of patterns encountered in the recognition task.
Accordingly, there is a need to find a generic signal representation that builds strong invariants to various deformations. It is desirable to implement such signal representation using standard band-pass filters which do not depend upon the specific pattern properties, and to avoid a learning stage.
SUMMARY OF THE INVENTION
A digital filter bank having J stages is proposed, where J is an integer greater than one. For each integer j such that 1≦j≦J, the j-th stage comprises:

    
    
        a plurality of filtering units each receiving an input signal of the j-th stage, including a low-pass filtering unit and at least one band-pass filtering unit using complex filtering coefficients; and
        following each band-pass filtering unit of the j-th stage, a respective modulus processing unit for generating a processed real signal as a function of squared moduli of complex output values of said band-pass filtering unit.
    
    


The input signal of the first stage is a digital signal supplied to the digital filter bank, while for 1<j≦J, the input signal of the j-th stage includes the processed real signal generated by at least one modulus processing unit of the (j−1)-th stage.
The filter bank converts the input digital signal into a multiscale invariant spectrum which can be made progressively more invariant to any specified group of deformations, for example translations, rotations, scaling, symmetries . . . . From this representation, a reliable similarity distance between signals can be computed by applying a standard distance such as a Euclidean distance. As opposed to state of the art multiscale algorithms, the multiscale invariant spectrum keeps the relevant signal information at all frequencies.
In addition to pattern detection, clustering and classification, building a representation that is progressively invariant to deformations also provides an efficient representation for coarse-to-fine recovery of deformations for signal registration.
The filter bank implements a filtering cascade with low-pass and complex band-pass filters. As opposed to state of the art filter bank algorithms (e.g. U.S. Pat. No. 5,526,299), convolutions are not computed on the output of a previous filter, but on the complex modulus of the previous filter output. As opposed to spectrograms or scalograms, the complex modulus is not applied once, at the output of the linear transform but in each stage of the filter bank, which builds stronger invariants. As opposed to convolution neural networks, the present filter bank uses low-pass and band-pass filters that do not need to be optimized according to a specific pattern recognition problem. It is implemented with a complex modulus operation which removes the complex phase of signals at each stage. Furthermore, the modulus filter bank can be designed to build multiscale invariants to rotation, scaling or any other type of group of deformation, by performing the convolution along rotation, scaling or other type of group parameters.
In an embodiment, the input signal of the j-th stage for 1<j≦J includes the processed real signal generated by each modulus processing unit of the (j−1)-th stage.
Advantageously, the low-pass filtering unit of the j-th stage (1≦j≦J) is arranged to compute a convolution of the input signal of the j-th stage with real filtering coefficients. Implementation is then simpler than with complex low-pass filtering coefficients, and the Fourier behavior of the low-pass component is improved.
For instance, the low-pass filtering unit in one or more stages comprises a Gaussian filter, and each band-pass filtering unit in the same stage comprises a respective Gabor filter.
In an embodiment, each of the filtering units in at least one of the stages includes a convolver for computing a convolution of the input signal of that stage with filtering coefficients of that filtering unit, output values of the filtering unit being produced as a sub-sampled signal from the computed convolution. Sub-sampling advantageously reduces the size of the multiscale invariant spectrum. Still, it can be dispensed with in certain embodiments or in some of the J stages of the filter bank. All the convolutions can be implemented with fast algorithms.
In at least one of the stages, each modulus processing unit following a respective band-pass filtering unit using complex filtering coefficients may include: a complex-to-real transformer receiving the complex output values of the respective band-pass filtering unit and producing respective real values depending on the squared moduli of said complex output values; and a post-filter for low-pass filtering said real values and outputting the processed real signal for said modulus processing unit. The optional post-filter filters out high frequency signal components introduced by the modulus operation.
In an embodiment, the input signal of the j-th stage for each integer j such that 1≦j≦J is defined over j variable indexes k0, . . . , kj−1 in which k0 is an index having d integer components, with d≧1 being the dimension of the digital signal supplied to the digital filter bank. For 1≦l<J, kl is an integer index associated with the plurality of filtering units of the l-th stage.
It is possible, in certain embodiments, that the filtering units of at least one of the stages include at least one further band-pass filtering unit using real high-pass filtering coefficients, followed by a processing unit for generating a processed real signal as a function of absolute values of real output values of said further band-pass filtering unit. In such an embodiment, this processing unit may include a post-filter for low-pass filtering the absolute values of the real output values.
Typically, the input signal of the j-th stage for each integer j such that 1≦j≦J is defined over j variable indexes k0, . . . , kj−1, where k0 is an index having d integer components, with d≧1 being the dimension of the digital signal supplied to the digital filter bank, and for 1≦l<j, kl is an integer index associated with the plurality of filtering units of the l-th stage.
In the j-th stage (for j>1), each value of the integer index kj−1 has a one-to-one correspondence with a respective filtering unit of the (j−1)-th stage. In a j-th stage (for j>2), the integer index kl with 1≦l≦j−2 may have a lower resolution with respect to the plurality of filtering units of the l-th stage if the filtering units of one or more stages between the l-th stage and the j-th stage perform filtering and sub-sampling along the variable index kl.
In a convenient implementation of the filter bank, each of the filtering units of a j-th stage (1≦j≦J) is arranged to compute a convolution of the input signal of the j-th stage with respective filtering coefficients along a single variable index kl, with 0≦l<j.
The low-pass filtering unit of a j-th stage may use real filtering coefficients h0j(m), and each band-pass filtering unit of the j-th stage may use filtering coefficients hkjj(m) such that hkjj(m)=hj(m)·exp(iξkjj·m) for a frequency offset value ξkjj, m being an index having a number d′ of integer components where d′ is a dimension of filtering of the input signal of the j-th stage, and hj(m) designating coefficients of a low-pass filter. For example, the low-pass filtering unit implements a Gaussian filter, and we can design the band-pass filtering units of the same stage as Gabor filters obtained from another Gaussian filter with respective frequency offsets.
An embodiment of the digital filter bank provides, for an integer l such that 1≦l<J, that the filtering units of the l-th stage compute convolutions of the input signal of the l-th stage with respective filtering coefficients along the variable index k0. Among those filtering units of the l-th stage, a first band-pass filtering unit uses complex filtering coefficients h1l(k0) while at least one second band-pass filtering unit designated by a respective integer index k, uses respective complex filtering coefficients hkl(k0)=Gk[h1l(k0)], where Gk is an operator acting on d-dimensional discrete signals and belonging to a set of operators having a group structure. The mapping of the group structure can be such that Gk·Gk′=Gk+k′ for any given pair k, k′ of integer indexes. In one or more following stages (j-th stage, with j>l), the band-pass filtering units can then compute respective convolutions of the input signal of such stage with respective filtering coefficients along the variable index kl. This is one way of designing filter banks to generate multiscale invariant spectra having strong invariance properties with respect to a family of deformations defined by the group G={Gk}k. Such group may cover translations, rotations, dilations, symmetries etc., and their combinations.
Efficient similarity distances for pattern recognition are obtained by applying standard distances (e.g. Euclidean distances) to the multiscale invariant spectrum obtained with the filter bank proposed here. Therefore, pattern detection, clustering, classification or registration of one-dimensional and multidimensional signals can be carried out by applying state of the art detection, clustering, classification or registration processes to the multiscale invariant spectrum proposed here. Other aspects of the invention relate to digital signal processing methods making use of the filter bank.
A pattern detection method according to the invention comprises:

    
    
        transforming a digital signal into a spectrum using a digital filter bank as defined above; and
        applying a pattern matching process to at least part of said spectrum and to at least one pre-stored spectrum associated with a respective pattern in order to detect said pattern in the digital signal.
    
    


A clustering method according to the invention comprises:

    
    
        transforming a digital signal into a spectrum using a digital filter bank as defined above; and
        applying a clustering process to at least part of said spectrum to identify a plurality of clusters and associate each of a plurality of patches of said spectrum with a respective one of the clusters, wherein the patches correspond to different regions of the spectrum or to at least part of different component spectra if the digital signal is made of a plurality of component signals transformed into respective component spectra using the digital filter bank.
    
    


A pattern classification method according to the invention comprises:

    
    
        transforming a digital signal into a spectrum using a digital filter bank as defined above; and
        applying a pattern classification process to at least part of said spectrum and to pre-stored training parameters corresponding to a plurality of pattern classes in order to associate the digital signal with one of said pattern classes.
    
    


A pattern registration method according to the invention comprises:

    
    
        transforming a first digital signal into a first spectrum and a second digital signal into a second spectrum using digital filter banks as defined above; and
        applying a pattern registration process to at least part of said first and second spectra in order to identify displacements mapping the first signal onto the second signal.
    
    


Other features and advantages of the devices and methods disclosed herein will become apparent from the following description of non-limiting embodiments, with reference to the appended drawings.



BRIEF DESCRIPTION THE DRAWINGS
FIG. 1 is a flow chart illustrating the operation of an embodiment of a digital filter bank in accordance with the invention.
FIG. 2 is a block diagram of a stage of such digital filter bank.
FIGS. 3-5 are schematic diagrams of three examples of filter banks configured in accordance with the invention.
FIGS. 6-9 are block diagrams of digital signal processing apparatuses making use of the present filter bank applied to pattern detection, clustering, classification and registration, respectively.



DESCRIPTION OF EMBODIMENTS
FIG. 1 shows steps of a process of filtering digital signals using a filtering structure referred to as a multiscale modulus filter bank (MMFB). The input of the filter bank is d-dimensional digital signal S(n)=S(n1, . . . , nd), where d is a positive integer (d≧1). This digital signal is available as a d-dimensional array of sample values.
In the following, for convenience and conciseness of the explanation, the terms “spatial variable” are used to designate the input signal variable n, although that variable n may correspond to time or any other signal parameter, as the reader will appreciate. Electrocardiograms (ECGs) or audio recordings are examples of one-dimensional signals (d=1) indexed by time. Still images are examples of two-dimensional signals (d=2), and video image sequences are examples of three-dimensional signals (d=3).
The process involves iterative filtering of signals, starting with the signal S(n) supplied to the filter bank. The process is initialized in step 10 by storing the signal samples in an array I0(k0)=S(k0) to form the input signal for the first stage of the MMFB, where k0=n=(n1, . . . , nd) is the d-dimensional spatial index defined on the sampling grid of the input signal S. In step 10, the iteration counter j (or stage counter) is also set to 1.
In each iteration j, the input signal of the j-th stage, noted Ij−1(k0, . . . , kj−1), is filtered in step 11 by a so-called modulus filter bank (MFB) having a number Kj of digital filtering units operating in parallel, where Kj is a number greater than one which may vary from one stage to another. The output of the MFB in the j-th iteration is noted Ij(k0, . . . , kj−1, kj) where kj is an integer variable index in the range of 0 to Kj−1. For each value of the index kj, the signal Ij(k0, . . . , kj−1, kj) as a function of the other variable indexes k0, . . . , kj−1 is the output signal of a respective one of the filtering units of the j-th stage. The set of those signals for j=0, 1, . . . , Kj−1 referred to as an invariant spectrum at scale j.
After the MFB filtering 11, the process checks in step 12 if the number of iterations j has reached the maximum number noted J set by the user or the designer of the MMFB, with J≧2. If j<J, the counter j is incremented by 1 in step 13 and the next iteration of the filtering step 11 is performed, with an input signal equal to the output of the previous stage of the MMFB. When j=J in step 12, the iterations stop and the multiscale invariant spectrum consisting of the signals I0(k0), I1(k0, k1), . . . , IJ(k0, k1, . . . , k J) can be output in step 14.
The multiscale invariant spectrum is thus made of the concatenation of the invariant spectra at the different scales j=0, 1, . . . , J. It will be appreciated, though, that depending on the application, it may be enough to use only part of the multiscale invariant spectrum, e.g. some of the scales j only, as the output of the MMFB.
FIG. 2 indicates a general layout of the j-th filtering stage of the MMFB (1≦j≦J). The Kj filtering units include a low-pass filtering unit 20 and Kj−1 band-pass filtering units 21. The low-pass filtering unit 20 preferably uses real filtering coefficients. In this case, it outputs a real signal (indicated by a thin arrow in FIG. 2) as a low-pass signal Ij(k0, . . . , kj−1, 0) included in the invariant spectrum for the j-th scale, i.e. for the variable index kj=0. Each band-pass filtering unit 21 uses complex filtering coefficients and outputs a complex signal (indicated by a thick arrow in FIG. 2).
Each filtering unit 20, 21 of the j-th stage includes a convolver 22 filtering digitally the input signal Ij−1(k0, . . . , kj−1) with an array of filtering coefficients which are real numbers H0j in the case of the low-pass filtering unit 20 and complex numbers Hkjj (kj≧1) in the case of the band-pass filtering unit(s) 21. Optionally, the output of each convolver 22 of the stage is sub-sampled, which is illustrated in FIG. 2 by a downsampling unit 23 downstream of the convolver 22. In practice, when sub-sampling is used, the convolver 22 may output directly the signal at the desired sampling rate. For example, when the sub-sampling factor is 2, the sampling rate at the output of the convolver 22 may be half that at the input.
The complex signals Îj(k0, . . . , kj−1, kj) output by the band-pass filtering units 21 (kj≧1) are transformed into real signals by respective modulus processing units 25. Each modulus processing unit 25 includes a complex-to-real transformer 26 which receives the output values of the respective filtering unit 21. For each received complex value Îj(k0, . . . , kj−1, kj)=z=z1+i·z2 (where i2=−1), the complex-to-real transformer 26 computes a real value r depending on the squared modulus |z|2=z12+z22 of the received complex value: r=ρ(|z2|).
A typical choice for the function ρ is the square root, in which case the complex-to-real transformer 26 computes the modulus of z. In a practical implementation, the square root function may be approximated for computation simplicity. It is also possible to combine the square root function with a gain saturation operator such as a sigmoid function, e.g. ρ(x)=tan−1(√{square root over (x)}).
Optionally, each modulus processing unit 25 of the j-th stage includes a post-filter 27 downstream of the complex-to-real transformer 26. The post-filter 27 is implemented as a low-pass filter, e.g. a Gaussian filter, to remove high frequency components introduced by the modulus operation in the transformer 26. If the output is further subdecomposed in a (j+1)-th stage, for computational efficiency, the post-filter 27 can also be integrated in the filters Hkj+1j+1 of the (j+1)-th stage by incorporating the low-pass filtering in these filters.
Each modulus processing unit 25 of the j-th stage outputs a real signal as a respective band-pass signal Ij(k0, . . . , kj−1, kj) of the j-th stage for the variable index kj=1, 2, . . . , Kj−1.
State of the art filtering algorithms can be used for fast implementation in the convolvers 22. In particular, separable multidimensional filters Hkjj are implemented with separable convolutions.
For kj=0, the convolution output on the sub-sampled grid is the next spectrum signal Ij(k0, . . . , kj−1, 0) which, for example, may be expressed as follows:




  
    
      
        
          
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For 0<kj<Kj, the convolution output Îj(k0, . . . , kj−1, kj) on the sub-sampled grid is, for example:




  
    
      
        
          
            
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In a preferred embodiment, the filtering units 20, 21 of a given stage j perform convolutions and sub-sampling along a single variable index kl with 0≦l<j. In other words, all the filters Hkjj in the convolvers 22 (0≦kj<Kj) filter Ij−1(k0, . . . , kj−1) along the index kl. If δ(k) designates the Kronecker delta function such that δ(k)=1 if k=0 and δ(k)=0 if k≠0, the separable filters are defined by:

Hkjj(k0, . . . , kl−1, kl, kl+1, . . . , kj−1)=δ(k0) . . . , δ(kl−1)·hkjj(kl)·δ(kl+1) . . . δ(kj−1)   (3)

where the coefficients hkjj(kl) are made of real numbers in the low-pass filtering unit 20 and of complex numbers in the band-pass filtering units 21. If l=0, namely if the signal is being filtered along the spatial index k0, the notation hkj(kl) is for a d-dimensional vector of real or complex coefficients. If l≠0, it stands for one real or complex coefficient.

Filtering in the convolvers 22 is thus implemented only along the variable kl:




  
    
      
        
          
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and it may be computed over values of kl that are sub-sampled relatively to the sampling grid of Ij−1.

The filter h0j is a real low-pass filter whereas the filters hkjj for 1≦kj<Kj are complex band-pass filters. A complex band-pass filter hkj((m) can be obtained by multiplying the coefficients of a low-pass filter hj(m) by a complex exponential exp(iξkj·m) of frequency ξkj=(ξk,1j, . . . , ξk,d′j) with d′ being the dimension of the filtering and ξkj·m designating the inner product ξkj·m=ξk,1j·m1+ . . . +ξk,d′j·md′. When separable filters are used, we have d′=d if the signal is filtered along the spatial index k0 (l=0). The complex exponential shifts the transfer function ĥj(ω) of the low-pass filter by a predetermined frequency offset value ξkj. The transfer function ĥkj(ω)=ĥj(ω−ξkj) of this complex band-pass filter is thus non-negligible only in the neighborhood of the frequency ξkj.
In the particular case where the non-zero frequency coordinates ξkj,pj of ξkjj (1≦p≦d′) are equal to π or −π, the complex exponential exp(iξkjj·m) is degenerated into a real number since the components of m are integers. In this case, the band-pass filtering is indeed a high-pass filtering in view of the sampling frequency of the signal. If hj(m) is a real low-pass filter, then the convolver 22 carrying out the high-pass filtering with the transfer function ĥkjj(ω)=ĥj(ω−ξkjj) belongs to a further band-pass filtering unit using real high-pass filtering coefficients, and the following processing unit 25 generates a processed real signal Ij(k0, . . . , kj−1, kj) as a function of the absolute values of the real output values from the further band-pass filtering unit. The absolute values can be computed in the transformer 26 by dropping the sign bit and they are optionally low-pass filtered in a post-filter 27.
A limit case of the MMFB occurs where any band-pass filtering unit in one or more stages j is a real high-pass filter generated as mentioned above. In other words, the j-th stage has:

    
    
        a plurality of filtering units each receiving the input signal of the j-th stage, including a low-pass filtering unit 20 using real filtering coefficients h0j(m) and at least one band-pass filtering unit 21 also using real filtering coefficients hkj(m) which can be chosen such that hkjj(m)=hj(m)·exp(iξkjj·m) for a frequency offset value ξkjj, m being an index having a number d′ of integer components where d′ is a dimension of filtering of the input signal of the j-th stage, where hj(m) designates coefficients of a real low-pass filter, wherein each component of the frequency offset value ξkjj is ±π; and
        following each band-pass filtering unit of the j-th stage, a respective modulus processing unit 25 for generating a processed real signal as a function of absolute values of complex output values of said band-pass filtering unit, whereby for 1<j≦J, the input signal of the j-th stage includes the processed real signal generated by each modulus processing unit of the (j−1)-th stage.
    
    


This limit case may not be optimal in terms of invariance properties, but it has an advantage of having a simpler implementation than with complex band-pass filters.
In an exemplary embodiment, h0j is a normalized Gaussian window of variance σ02:




  
    
      
        
          
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where λ is adjusted so that ∥h0j∥=1, and hj is also a normalized Gaussian window which may be the same as or different from h0j. The resulting filters hkjj are so-called Gabor filters. Typically, hj has a larger variance σ2 than h0j (σ2≧σ02).

The low-pass post-filter 27 of a modulus processing unit 25 computes a convolution along the same variable index kl as the preceding convolver 22, with an impulse response {tilde over (h)}(kl), i.e.:




  
    
      
        
          
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The low-pass filter {tilde over (h)}(kl) is normalized so that




  
    
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For example, the low-pass filter {tilde over (h)}(kl) can be a low-pass Gaussian filter.

In the exemplary embodiments illustrated in FIGS. 3 and 4, the MMFB computes multiscale invariant spectra relatively to spatial translation only. In this case, for all iterations j, the filtering takes place along k0 (l=0). Hence, the filters Hkjj filter the input signal Ij−1(k0, . . . , kj−1) only along the spatial variable k0 with a filter hkjj which is called a spatial filter.
FIG. 3 illustrates a simple embodiment of a MMFB having J=2 stages, in which the input signal S(n) is of dimension d=1 and Kj=2 filters are used in each stage j. The first stage 31 (j=1) transforms the input signal I0(k0)=S(n) of N samples into two signals I1(k0, 0) and I1(k0, 1) of size N with a low-pass filter 33 having real coefficients h01 and a band-pass filter 34 having complex coefficients h11 which convolve I0(k0) along the spatial parameter k0, with no sub-sampling. In the example, the modulus processing unit 35 computes the modulus of the complex output of the band-pass filter 34, i.e. using a gain function ρ(x)=√{square root over (x)}, and it convolves this modulus with a Gaussian post-filter {tilde over (h)}(k0) of variance 1⁄3. A possibility for the first stage 31 is to use a Gaussian low-pass filter 33 with σ=1/√{square root over (3)} and a Gabor band-pass filter 34 with σ=2/√{square root over (3)} and ξ11=3π/4.
In the next stage 32 (j=2), a low-pass filtering unit 36 and a band-pass filtering unit 37 both receive the input signal I1(k0, k1). In the representation of FIG. 3, each filtering unit 36, 37 is depicted as a parallel arrangement of two sub-units which receive the respective components I1(k0, 0) and I1(k0, 1) of the input signal. The two sub-units 36 use real coefficients h02 and output the real low-pass signals I2(k0, 0, 0) and I2(k0, 1, 0) of size N/2 if a sub-sampling factor of 2 is used. They may be implemented as Gaussian low-pass filters with σ=1. The two sub-units 37 use complex coefficients h12 and output complex signals Î2(k0, 0, 1) and Î2(k0, 1, 1) of size N/2 if a sub-sampling factor of 2 is used. They may be implemented as Gabor band-pass filters with σ=√{square root over (5)} and ξ12=2π/5. The second stage 32 further includes two identical modulus processing units 38 which perform the same operations as the modulus processing unit 35 of the first stage 31 downstream of the band-pass filtering sub-units 37 to output the respective real band-pass signals I2(k0, 0, 1) and I2(k0, 1, 1).
To compute an invariant spectrum at a larger scale if J>2, i.e. to compute Ij(k0, . . . , kj−1, kj) from Ij−1(k0, . . . , kj−1) with j>2, the above-described binary filter bank tree is continued, with the same filters h0j=h02 and h1j=h12 together with sub-sampling by a factor 2 and a modulus processing unit at the output of each complex band-pass filtering unit h12. For each integer l such that 0<l≦j, the spectrum Ij(k0, . . . , kj−1, kj) is then defined for kl=0 or kl=1 and includes N/2j−1 samples along k0.
Another exemplary embodiment suitable for analyzing images (d=2) is shown in FIG. 4. In this example, there are J=2 stages and Kj=5 filters per stage. The first stage 41 of the MMFB (j=1) transforms the input signal I0(k0)=S(n)=S(n1, n2) of N pixels into five signals I1(k0, k1) of size N for 0≦k1≦4, with a low-pass filter 43 having real coefficients h01 and four band-pass filters 44-47 having complex coefficients h11, h21, h31, h41 which convolve I0(k0) along the spatial parameter k0, without sub-sampling. Downstream of each band-pass filter hk11 with k1>0, a respective modulus processing unit 48 computes the modulus of the complex output of that band-pass filter, i.e. using a gain function ρ(x)=√{square root over (x)}, and it convolves this modulus with a post-filter {tilde over (h)}(k0) which can be chosen to be a Gaussian of variance 1⁄3. A possibility for the first stage 41 is to use a two-dimensional Gaussian low-pass filter 43 having coefficients h01(n) with σ=1/√{square root over (3)} and four Gabor band-pass filters 44-47 defined by hk1(n)=h1(n)·exp(iξk·n) where h1(n) is a Gaussian with σ=2/√{square root over (3)} and ξk=ξ·(cos θk, sin θk), θk=kπ/4 for 1≦k≦4, and ξ=3π4.
In the next stage 42 (j=2), a low-pass filtering unit 49 and K2−1=4 band-pass filtering units 50-53 all receive the input signal I1(k0, k1). In the representation of FIG. 4, each filtering unit 49-53 is depicted as a parallel arrangement of K1=5 sub-units which receive the respective components I1(k0, k1) of the input signal for k1=0, 1, 2, 3 and 4. The five sub-units 49 use real coefficients h02 and output the real low-pass signals I2(k0, k1) for 0≦k1≦4, of size N/4 if a sub-sampling factor of 2 is used along each spatial direction. They may be implemented as Gaussian low-pass filters with σ=1. For each integer k1 such that 1≦k1≦4, five filtering sub-units 50-53 convolve I1(k0, k1) with complex coefficients hk12 and output complex signals Î2(k0, k1, 1), Î2(k0, k1, 2), Î2(k0, k1, 3) and Î2 (k0, k1, 4) of size N/4 if a sub-sampling factor of 2 is used along each spatial direction. They may be implemented as Gabor band-pass filters defined by hk2(n)=h2(n)·exp(iξk·n) with ξ=√{square root over (5)}, ξk=ξ·(cos θk, sin θk), θk=kπ/4 for 1≦k≦4, and ξ=π/2. the second stage 42 further includes identical modulus processing units 54 which perform the same operations as the modulus processing units 48 of the first stage 41 downstream of the band-pass filtering sub-units 50-53 to output the respective real band-pass signals I2(k0, k1, k2) for 0≦k1≦4 and 1≦k2≦4.
To compute an invariant spectrum at a larger scale if J>2, i.e. to compute Ij(k0, . . . , kj−1, kj) from Ij−1(k0, kj−1) with j>2, the above-described filter bank tree is continued, with the same filters hkj=hk2 for 0≦k≦4 together with sub-sampling by a factor 2 along each spatial direction and a modulus processing unit at the output of each complex band-pass filtering unit hk2 for k>0. For each integer l such that 0<l≦j, the spectrum Ij(k0, . . . , kj−1, kj) is then defined for kl=0, 1, 2, 3 or 4 and includes N/22(j−1) pixels along k0.
Other embodiments of the MMFB compute spectra that are invariant relatively to translation but also relatively to other groups of deformations.
We denote by G={Gk}k a group of operators which act on discrete signals, with Gk·Gk′Gk+k′. To compute multiscale invariant spectra relatively to such a group in an exemplary embodiment, at an iteration j=l, the spectrum Il is computed with Kl−1 complex band-pass spatial filters hkl(k0) which are synthesized by transforming a single complex band-pass filter h1l(k0) with respective operators Gk of the group G:

hkl(k0)=Gk[h1l(k0)]  (7)

In dimension d=2, an operator Gk of the group may correspond to a rotation by an angle θk=π(k−1)/(Kl−1) for 1≦k<Kl. For n=(n1, n2), we have Gk[h1l(n)]=h1i(Rk[n]) where Rk[n1, n2]=(n1·cos θk−n2·sin θk, n1·sin θk+n2·cos θk) is a rotation by an angle θk.
For example, if h1l(k0) is a Gabor filter modulated at a frequency ξ1=(ξ, 0), then hkl(k0) is a Gabor filter whose center frequency ξk is rotated by −θk in the frequency plane: ξk=ξ·(cos θk, −sin θk).
An operator Gk of the group may also correspond to a dilation by a factor ak where a is a scaling parameter greater than one and 1≦k<K. In this case, hkl=Gk[h1l] is dilated by ak: hkl(n)=λkh1l(a−kn), where λk is adjusted so that ∥hkl∥=1.
Invariants may also be computed relatively to multidimensional groups such as the product of rotations and dilations. In the latter case the index of the group operators is k=(r, s) is multidimensional and specifies an operator Gk which is the composition of a rotation by an angle Or and a scaling by a factor as. Any other multidimensional group of deformation operators Gk may be used to select filters in the modulus filter bank, to compute a multiscale invariant spectrum relatively to such group.
To compute a multiscale invariant relatively to the group G over q scales, in the next q stages, i.e. for j going from l+1 to l+q, the filtering of Ij−1(k0, . . . , kj−1) is performed along the variable kl of the last spatial filtering index.
In an embodiment, the output h0l(k0) of the spatial low-pass filter of the l-th stage is not retransformed up to the (l+q)-th stage. For l<j≦l+q, we can then set:

Ij(k0, . . . , kl−1, 0, . . . , 0)=Ij(k0, . . . , kl−1, 0)   (8)

The output of the spatial band-pass filters is retransformed with a convolution along kl and, optionally, a sub-sampling along kl prior to the modulus processing:




  
    
      
        
          
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In an exemplary embodiment, for l<j≦l+q, the filter h0j(kl) is a Gaussian filter and the filters hkj(kl) are Gabor complex band-pass filters for k>0.
At the next iteration j=l+q+1, a spatial filtering may again be applied with filters derived according to (7) from a filter h1l+q+1(k0) operators Gk of a suitable group G′ (which may be the same as or different from G). The invariance relatively to the group G′ is obtained again with q′ filtering and modulus operations performed along the index l+q+1 of the last spatial filtering.
FIG. 5 illustrates an embodiment in which l=1 and q=2 (J=l+q=3). The input signal I0(k0)=S(n) of size N in dimension d is decomposed in the first stage 60 (j=1) with K1=5 spatial filtering units 63-67 which convolve the input signal I0(k0) with respective coefficients hk11(n) for 0≦k1<5. The low-pass filtering unit 63 may be implemented as a Gaussian low-pass filter. It uses real coefficients h01(n) and outputs the real low-pass signal I1(k0, 0) which will be kept all the way to the last stage l−q according to (8), namely I1(k0, 0)=I2(k0, 0, 0)=I3(k0, 0, 0, 0).
The coefficients hk11(n) of the complex band-pass filter units 65-67 (2≦k1<5) may be obtained by rotating or scaling those h11(n) of the complex band-pass filter unit 64 using operators Gk of a group: hk11(n)=Gk1[h11(n)]. In the illustrated embodiment, the outputs of the spatial filtering units 63-67 of the first stage are each of size N/2d, i.e. a sub-sampling factor of 2 is used along each spatial direction. The complex band-pass filter unit 64 from which the other filters are derived may comprise a Gabor filter having a bigger variance than the Gaussian filter of the low-pass filtering unit 63. Downstream of each band-pass filter hk11 with k1>0, a respective modulus processing unit 68 computes the squared modulus of the complex output of that band-pass filter, applies a gain function ρ(x), and optionally convolves the result with a post-filter, to obtain the real band-pass signals I1(k0, k1) for 1≦k1<5.
These spectrum signals I1(k0, k1) are retransformed by the filtering units of the second stage 61 (j=2), where K2=2 in the example shown in FIG. 5. They include a low-pass filtering unit 69 using real coefficients h02(n) and a band-pass filtering unit 70 using complex coefficients h12(n). Both units 69, 70 perform a convolution and a sub-sampling by 2 along the index k1 of the previous spatial filtering. The low-pass unit 69 yields the sub-sampled real low-pass signal I2(k0, k1, 0) and the band-pass unit 70 yields the sub-sampled complex band-pass signal Î2(k0, k1, 1), the range for k1 being {1, 2}, i.e. reduced from the original range {1, 2, 3, 4} by the sub-sampling factor of 2. The complex band-pass filtering unit 70 is followed by a modulus processing unit 71 which yields the two components of the real band-pass signal I2(k0, k1, 1).
The output signals I2(k0, k1, k2) of the second stage 61 still have N/2d samples along k0 because the convolution and sub-sampling are performed along the variable index k1. In this illustration, the convolution is over 4 samples corresponding to k1=1, 2, 3, 4, and the sub-sampling yields values only for k1=1, 2.
In the third stage 62 (j=2), a low-pass filtering unit 72 and a band-pass filtering unit 73 both receive the input signal I2(k0, k1, k2). In the representation of FIG. 5, each filtering unit 72-73 is depicted as a parallel arrangement of two sub-units which receive the respective components I2(k0, k1, k2) of the input signal for k2=1 and 2. The two sub-units 72 use real coefficients h03(n) and output the real low-pass signals I3(k0, k1, k2, 0) with a further sub-sampling factor of 2 along the variable index k1. The two sub-units 73 convolve I2(k0, k1, k2) with complex coefficients h13(n) and output complex signals I3(k0, k1, k2, 1) with the further sub-sampling factor of 2 along the variable index k1. The third stage 62 further includes two identical modulus processing units 74 which perform the same operations as the modulus processing unit 71 of the previous stage 61 downstream of the band-pass filtering sub-units 73 to output the real band-pass signal I3(k0, k1, k2, 1)
A multiscale spectrum may be computed for j>3 by first performing again a spatial transformation, for example with the same Kl−1 spatial band-pass filters as in the first stage, and then performing two more iterations of modulus filtering along the index of the last spatial filtering.
An MMFB as exemplified in FIGS. 1-5 can be implemented using dedicated hardware logic or programmable hardware logic such as field-programmable gate arrays (FPGA). In addition, it lends itself to software implementations using computer programs written in any suitable language for running on general-purpose computers, digital signal processors (DSP) or application-specific integrated circuits (ASIC).
The MMFB has a wide range of applications, of which some are discussed more particularly below.
FIG. 6 shows an exemplary configuration of a pattern detection apparatus which processes a d-dimensional input signal S(n). The pattern detection apparatus detects the location of a potential family of patterns in S(n) and outputs a pattern location array which specifies the position of each type of pattern in S(n). Face detection in images is an example of pattern detection.
The input signal S(n) is processed by the multiscale modulus filter bank unit 80, which determines a multiscale invariant spectrum {Ij}1≦j≦J as described above. The matching unit 81 takes in input at least one scale of this multiscale invariant spectrum and detects the location of a family of P potential patterns in the signal S(n). Any state of the art pattern detection algorithm may be used in unit 81.
In an exemplary embodiment, each of the P patterns is represented by a normalized signal Sp for 1≦p≦P. The pattern detection is performed at the coarsest scale, i.e. based on the invariant spectrum I J at the index j=J. For each p, the invariant spectrum Ip,J of Sp is pre-computed at the coarsest scale index J with the MMFB and stored in a memory 82.
The pattern detection is implemented by unit 81 with a match filtering procedure which finds the potential locations of different invariant spectra {Ip,J(k0, k1, . . . , kJ)}p in the signal invariant spectrum IJ(k0, k1, . . . , kJ) of the input signal S(n). We denote by TτIJ=IJ(k0−τ, k1, . . . , kJ) the translation of IJ along the spatial variable k0 by the vector τ. The Euclidean norm on the spectrum modulus is written:




  
    
      
        
          
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The sum is performed over all spatial indexes k0 on the support of Ip,J and over all other indexes kj for 2≦j≦J. If the spatial support of all the patterns Sp is smaller than a size Δ, then one may choose J as the integer part of log2(Δ) and look for different patterns on different spatial patches Pi of size Δ. On each such domain, the matching unit 81 computes the pattern index pi and the translation τi which minimize the distance defined by (10), i.e.:




  
    
      
        
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For each patch index i, the pattern index pi and its location τi in the input signal S(n) are provided at the output of the matching unit 81 as part of the pattern position array if a precision condition is fulfilled, e.g. ∥TτIJ−Ip,J∥2≦ε where ε is a precision parameter set by the user.
FIG. 7 shows an exemplary configuration of a clustering apparatus which processes a d-dimensional input signal S(n). A clustering system decomposes the signal into sub-parts that are homogeneous relatively to some criteria. It provides in output the signal locations that belong to the same cluster and a descriptor which characterizes each cluster. Image texture segmentation is an example of clustering problem.
The input signal S(n) is processed by the MMFB unit 85 which outputs an invariant spectrum {Ij}1≦j≦J as described above. The clustering unit 86 finds spatial locations having similar invariant spectra and outputs a cluster number for each spatial location as well as an invariant spectrum descriptor. In an exemplary embodiment, the set of spatial indexes k0 of the coarsest scale spectrum Ij(k0, k1, . . . , kJ) is segmented into several spatial patches {Pi}i. A patch Pi may be reduced to a single spatial location or could be a rectangular region. For each spatial patch Pi, we consider the array of coefficients {IJ(k0, k1, . . . , kJ)}k0∈Pi, k1, . . . , kJ for k0 in Pi and all k1, . . . , kJ. These arrays for all i are distributed in C clusters identified by the clustering apparatus. For example, C being a parameter set by the user, the clustering unit 86 identifies a family of C centroids {Īc}1≦c≦C which are arrays {Īc(k0, k1, . . . , kJ)}k0∈Pi, k1, . . . , kJ. In such example, the clustering also associates to each patch Pi the cluster index c(i) corresponding to the centroid Īc(i) which is the closest to the signal spectrum {IJ(k0, k1, . . . , kJ)}k0∈Pi, k1, . . . , kJ in the patch Pi.
The clustering may be performed with any state of the art process such as, for example, an iterative K-mean clustering algorithm, computed with a Euclidean distance. The clustering unit 86 outputs the centroid vectors {Īc}1≦c≦C and the cluster index c(i) for each patch Pi.
A clustering process may also be applied to several component signals {Si(n)}1≦i≦L of dimension d, which are grouped into clusters of similar signals. This type of clustering problem can be reduced to a clustering problem over a single signal that is constructed by aggregating all signals into a single signal. Each component signal Si(n), or part of it, is thus interpreted as a patch of a composite signal S(n)={S1(n), . . . , SL(n)}. The invariant spectra Ii,J of the component signals Si(n) at the largest scale J are considered as the previously mentioned spectrum patches. For example, the clustering unit finds a family of C centroids {Īc}1≦c≦C and it associates to each patch Si the cluster index c(i) corresponding to a centroid Īc(i) which is the closest to the signal spectrum Ii,J.
FIG. 8 shows an exemplary configuration of a pattern classification apparatus. The input signal S(n) is a d-dimensional signal with d≧1. The pattern classification apparatus outputs a label q which is an estimator of the class to which the input S(n) belongs. For example, if S(n) is an image of an object that belongs to an a priori defined set of Q objects, then there are Q possible classes and the label q may be chosen to be an integer between 1 and Q. Digit recognition in images is an example where there are Q=10 classes corresponding to the digits 0-9.
The input signal S(n) is processed by the MMFB unit 90, which outputs a multiscale invariant spectrum {Ij}1≦j≦J as described above. A classifier unit 91 processes the multiscale invariant spectrum to output a class label q associated with the input signal S(n).
Any state of the art classifier may be used. In an exemplary embodiment, the classifier unit 91 comprises a support vector machine (SVM) operating on the vector corresponding to the coarsest invariant spectrum I J(k0, k1, . . . , kJ). A conventional Gaussian kernel may be used in this SVM. Such a classifier unit is first trained on a sequence of {tilde over (L)}≧2 modulus spectra Il,J computed by means of the MMFB from a sequence of training signals {{tilde over (S)}l}1≦l≦{tilde over (L)} whose class labels are known. The training is implemented off-line to obtain training parameters which are stored in a memory 92 to which the classifier unit 91 accesses to process the invariant spectrum IJ(k0, k1, . . . , kJ) of the input signal S(n). The SVM associates a class label q (1≦q≦Q) to the modulus spectrum IJ of a signal S(n) computed by the MMFB unit 90.
FIG. 9 shows an exemplary configuration of a pattern registration apparatus making use of the MMFB. The input of the registration apparatus comprises two signals S1(n) and S2(n). It outputs registration parameters which specify a deformation operator which maps S1(n) onto an approximation of S2(n).
In an exemplary implementation, this deformation is specified by a displacement array τ[n] such that S1(n−τ[n]) is most similar S2(n). When S1(n) and S2(n) are two consecutive images in a video sequence, τ[n] specifies the optical flow. If S1(n) and S2(n) are two stereo images, τ[n] is referred to as the disparity, from which the depth of image points in the scene can be computed. The measurement of such displacements is also needed to register a medical signal S1(n) over a reference signal S2(n) to find potential anomalies. In this context, S1(n) and S2(n) may be one-dimensional, two-dimensional, three-dimensional or four-dimensional signals (d=1, 2, 3 or 4).
The signals S1(n) and S2(n) are input respectively to two MMFB units 95, 96 which output respective multiscale invariant spectrum arrays {I1,j}1≦j≦J and {I2,j}1≦j≦J. The registration of S1(n) relatively to S2(n) is performed by computing the registration of the multiscale invariant spectrum {I1,j} of S1(n) on the multiscale invariant spectrum {I2,j} of S2(n).
In an exemplary embodiment, the MMFB units 95, 96 compute a spatial spectrum with a MFB obtained with convolutions along the spatial variable k0. The registration unit 97 computes for each j, a displacement τj[k0] such that I1,j(k0−τj[k0], k1, . . . , kj) is most similar to I2,j(k0, k1, . . . , kj). For example, the displacement field τj is computed in order to minimize the Euclidean distance) D1,2j(τj):




  
    
      
        
          
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State of the art registration algorithms may be used by the unit 97 to compute the multiscale displacements τj(k0). For example, the displacements are computed using a coarse-to-fine process. A displacement τJ[k0] is first computed by minimizing the Euclidean distance D1,2J(τJ) at the coarsest scale j=J with any state of the art process. This displacement τJ[k0] is then used as an initial estimation to compute the displacement array τJ−1[k0] for the smaller index j=J−1 by minimizing D1,2J−1(τJ−1). Iteratively reducing j leads to progressively finer displacement estimations until τ0[k0] is obtained at the final scale index j=0. The registration unit 97 then outputs the displacement array τ(n)=τ0[k0].
It will be appreciated that the embodiments described above are merely illustrations of the invention disclosed herein and that various modifications can be made without departing from the scope as defined in the appended claims.
For example, while the input signal Ij−1(k0, . . . , kj−1) of a j-th stage for j>1 is shown to include the processed real signal generated by each modulus processing unit 35, 48, 71 of the preceding (j−1)-th stage in the embodiments lo depicted in FIGS. 3-5, it is also possible, in certain instances, not to fill completely the decomposition tree. In other words, it is possible to provide that only some of the modulus processing units of a (j−1)-th stage contribute to the input signal of the next stage.

## Claims

1. A digital filter bank having a number J of stages, where J is an integer greater than one, wherein for each integer j such that 1≦j≦J, the j-th stage comprises:
a plurality of filtering units each receiving an input signal of the j-th stage, including a low-pass filtering unit and at least one band-pass filtering unit using complex filtering coefficients; and
following each band-pass filtering unit of the j-th stage, a respective modulus processing unit for generating a processed real signal as a function of squared moduli of complex output values of said band-pass filtering unit,

wherein the input signal of the first stage is a digital signal supplied to the digital filter bank, and wherein for 1<j≦J, the input signal of the j-th stage includes the processed real signal generated by at least one modulus processing unit of the (j−1)-th stage.

2. The digital filter bank as claimed in claim 1, wherein for 1<j≦J, the input signal of the j-th stage includes the processed real signal generated by each modulus processing unit of the (j−1)-th stage.

3. The digital filter bank as claimed in claim 1, wherein the low-pass filtering unit is arranged to compute a convolution of the input signal of the j-th stage with real filtering coefficients.

4. The digital filter bank as claimed in claim 1, wherein each filtering unit in at least one of the stages includes a convolver for computing a convolution of the input signal of said one of the stages with filtering coefficients of said filtering unit, and wherein output values of said filtering unit are produced as a sub-sampled signal from the computed convolution.

5. The digital filter bank as claimed in claim 1, wherein each modulus processing unit following a respective band-pass filtering unit using complex filtering coefficients in at least one of the stages includes:
a complex-to-real transformer receiving the complex output values of said respective band-pass filtering unit and producing respective real values depending on the squared moduli of said complex output values; and
a post-filter for low-pass filtering said real values and outputting the processed real signal for said modulus processing unit.

6. The digital filter bank as claimed in claim 1, wherein the filtering units of at least one of the stages include at least one further band-pass filtering unit using real high-pass filtering coefficients, followed by a processing unit for generating a processed real signal as a function of absolute values of real output values of said further band-pass filtering unit.

7. The digital filter bank as claimed in claim 6, wherein the processing unit following said further band-pass filtering unit includes a post-filter for low-pass filtering said absolute values of the real output values.

8. The digital filter bank as claimed in claim 1, wherein the low-pass filtering unit of said j-th stage uses real filtering coefficients h0j(m), and each band-pass filtering unit of said j-th stage uses filtering coefficients hkjj(m) such that hkjj(m)=hj(m)·exp(iξkjj·m) for a frequency offset value ξkjj, m being an index having a number d′ of integer components where d′ is a dimension of filtering of the input signal of the j-th stage, and hj(m) designating coefficients of a low-pass filter.

9. The digital filter bank as claimed in claim 1, wherein the input signal of the j-th stage for each integer j such that 1≦j≦J is defined over j variable indexes k0, . . . , kj−1, where:
k0 is an index having d integer components, with d≧1 being the dimension of the digital signal supplied to the digital filter bank; and
for 1≦l<j, kl is an integer index associated with the plurality of filtering units of the l-th stage.

10. The digital filter bank as claimed in claim 9, wherein for 1≦j≦J, each of the filtering units of the j-th stage is arranged to compute a convolution of the input signal of the j-th stage with respective filtering coefficients along a single variable index kl, with 0≦l<j.

11. The digital filter bank as claimed in claim 9, wherein, for an integer l such that 1≦l<J, the filtering units of the l-th stage are arranged to compute convolutions of the input signal of the l-th stage with respective filtering coefficients along the variable index k0, wherein the filtering units of said l-th stage include:
a first band-pass filtering unit using filtering coefficients h1l(k0); and
at least one second band-pass filtering unit designated by a respective integer index k, using respective filtering coefficients hkl(k0)=Gk[h1l(k0)] where Gk is an operator acting on d-dimensional discrete signals and belonging to a group of operators.

12. The digital filter bank as claimed in claim 11, wherein the band-pass filtering units of at least one j-th stage, with j>l, are arranged to compute respective convolutions of the input signal of the j-th stage with respective filtering coefficients along the variable index kl.

13. A pattern detection method, comprising:
transforming a digital signal into a spectrum using a digital filter bank; and
applying a pattern matching process to at least part of said spectrum and to at least one pre-stored spectrum associated with a respective pattern in order to detect said pattern in the digital signal,

wherein the digital filter bank has a number J of stages, where J is an integer greater than one, wherein for each integer j such that 1≦j≦J, the j-th stage comprises:
a plurality of filtering units each receiving an input signal of the j-th stage, including a low-pass filtering unit and at least one band-pass filtering unit using complex filtering coefficients; and
following each band-pass filtering unit of the j-th stage, a respective modulus processing unit for generating a processed real signal as a function of squared moduli of complex output values of said band-pass filtering unit,

wherein said digital signal is the input signal of the first stage of the digital filter bank, and wherein for 1<j≦J, the input signal of the j-th stage includes the processed real signal generated by at least one modulus processing unit of the (j−1)-th stage.

14. A clustering method, comprising:
transforming a digital signal into a spectrum using a digital filter bank; and
applying a clustering process to at least part of said spectrum to identify a plurality of clusters and associate each of a plurality of patches of said spectrum with a respective one of the clusters, wherein the patches correspond to different regions of the spectrum or to at least part of different component spectra if the digital signal is made of a plurality of component signals transformed into respective component spectra using the digital filter bank,

wherein the digital filter bank has a number J of stages, where J is an integer greater than one, wherein for each integer j such that 1≦j≦J, the j-th stage comprises:
a plurality of filtering units each receiving an input signal of the j-th stage, including a low-pass filtering unit and at least one band-pass filtering unit using complex filtering coefficients; and
following each band-pass filtering unit of the j-th stage, a respective modulus processing unit for generating a processed real signal as a function of squared moduli of complex output values of said band-pass filtering unit,

wherein said digital signal is the input signal of the first stage of the digital filter bank, and wherein for 1<j≦J, the input signal of the j-th stage includes the processed real signal generated by at least one modulus processing unit of the (j−1)-th stage.

15. A pattern classification method, comprising:
transforming a digital signal into a spectrum using a digital filter bank; and
applying a pattern classification process to at least part of said spectrum and to pre-stored training parameters corresponding to a plurality of pattern classes in order to associate the digital signal with one of said pattern classes,

wherein the digital filter bank has a number J of stages, where J is an integer greater than one, wherein for each integer j such that 1≦j≦J, the j-th stage comprises:
a plurality of filtering units each receiving an input signal of the j-th stage, including a low-pass filtering unit and at least one band-pass filtering unit using complex filtering coefficients; and
following each band-pass filtering unit of the j-th stage, a respective modulus processing unit for generating a processed real signal as a function of squared moduli of complex output values of said band-pass filtering unit,

wherein said digital signal is the input signal of the first stage of the digital filter bank, and wherein for 1<j≦J, the input signal of the j-th stage includes the processed real signal generated by at least one modulus processing unit of the (j−1)-th stage.

16. A pattern registration method, comprising:
transforming a first digital signal into a first spectrum and a second digital signal into a second spectrum using first and second digital filter banks, respectively; and
applying a pattern registration process to at least part of said first and second spectra in order to identify displacements mapping the first signal onto the second signal,

wherein each of the first and second digital filter banks has a number J of stages, where J is an integer greater than one, wherein for each integer j such that 1≦j≦J, the j-th stage of each of the first and second digital filter banks comprises:
a plurality of filtering units each receiving an input signal of the j-th stage of said filter bank, including a low-pass filtering unit and at least one band-pass filtering unit using complex filtering coefficients; and
following each band-pass filtering unit of the j-th stage of said filter bank, a respective modulus processing unit for generating a processed real signal as a function of squared moduli of complex output values of said band-pass filtering unit,

wherein said first digital signal is the input signal of the first stage of the first digital filter bank, wherein said second digital signal is the input signal of the first stage of the second digital filter bank, and wherein for 1<j≦J, the input signal of the j-th stage of each of the first and second digital filter banks includes the processed real signal generated by at least one modulus processing unit of the (j−1)-th stage of said filter bank.