Wavelet transform

In mathematics, a wavelet series is a representation of a square-integrable (real- or complex-valued) function by a certain orthonormal series generated by a wavelet. This article provides a formal, mathematical definition of an orthonormal wavelet and of the integral wavelet transform.^{[1][2][3][4][5]}

Definition[]

A function ${\displaystyle \psi \,\in \,L^{2}(\mathbb {R} )}$ is called an orthonormal wavelet if it can be used to define a Hilbert basis, that is a complete orthonormal system, for the Hilbert space ${\displaystyle L^{2}\left(\mathbb {R} \right)}$ of square integrable functions.

The Hilbert basis is constructed as the family of functions ${\displaystyle \{\psi _{jk}:\,j,\,k\,\in \,\mathbb {Z} \}}$ by means of dyadic translations and dilations of ${\displaystyle \psi \,}$,

${\displaystyle \psi _{jk}(x)=2^{\frac {j}{2}}\psi \left(2^{j}x-k\right)\,}$

for integers ${\displaystyle j,\,k\,\in \,\mathbb {Z} }$.

If under the standard inner product on ${\displaystyle L^{2}\left(\mathbb {R} \right)}$,

${\displaystyle \langle f,g\rangle =\int _{-\infty }^{\infty }f(x){\overline {g(x)}}dx}$

this family is orthonormal, it is an orthonormal system:

${\displaystyle {\begin{aligned}\langle \psi _{jk},\psi _{lm}\rangle &=\int _{-\infty }^{\infty }\psi _{jk}(x){\overline {\psi _{lm}(x)}}dx\\&=\delta _{jl}\delta _{km}\end{aligned}}}$

where ${\displaystyle \delta _{jl}\,}$ is the Kronecker delta.

Completeness is satisfied if every function ${\displaystyle f\,\in \,L^{2}\left(\mathbb {R} \right)}$ may be expanded in the basis as

${\displaystyle f(x)=\sum _{j,k=-\infty }^{\infty }c_{jk}\psi _{jk}(x)}$

with convergence of the series understood to be convergence in norm. Such a representation of f is known as a wavelet series. This implies that an orthonormal wavelet is self-dual.

The integral wavelet transform is the integral transform defined as

${\displaystyle \left[W_{\psi }f\right](a,b)={\frac {1}{\sqrt {|a|}}}\int _{-\infty }^{\infty }{\overline {\psi \left({\frac {x-b}{a}}\right)}}f(x)dx\,}$

The wavelet coefficients ${\displaystyle c_{jk}}$ are then given by

${\displaystyle c_{jk}=\left[W_{\psi }f\right]\left(2^{-j},k2^{-j}\right)}$

Here, ${\displaystyle a=2^{-j}}$ is called the binary dilation or dyadic dilation, and ${\displaystyle b=k2^{-j}}$ is the binary or dyadic position.

Principle[]

The fundamental idea of wavelet transforms is that the transformation should allow only changes in time extension, but not shape. This is affected by choosing suitable basis functions that allow for this.^[how?] Changes in the time extension are expected to conform to the corresponding analysis frequency of the basis function. Based on the uncertainty principle of signal processing,

${\displaystyle \Delta t\Delta \omega \geq {\frac {1}{2}}}$

where ${\displaystyle t}$ represents time and ${\displaystyle \omega }$ angular frequency (${\displaystyle \omega =2\pi f}$, where ${\displaystyle f}$ is temporal frequency).

The higher the required resolution in time, the lower the resolution in frequency has to be. The larger the extension of the analysis windows is chosen, the larger is the value of ${\displaystyle \Delta t}$^[how?].

When ${\displaystyle \Delta t}$ is large,

1. Bad time resolution
2. Good frequency resolution
3. Low frequency, large scaling factor

When ${\displaystyle \Delta t}$ is small

1. Good time resolution
2. Bad frequency resolution
3. High frequency, small scaling factor

In other words, the basis function ${\displaystyle \psi }$ can be regarded as an impulse response of a system with which the function ${\displaystyle x(t)}$ has been filtered. The transformed signal provides information about the time and the frequency. Therefore, wavelet-transformation contains information similar to the short-time-Fourier-transformation, but with additional special properties of the wavelets, which show up at the resolution in time at higher analysis frequencies of the basis function. The difference in time resolution at ascending frequencies for the Fourier transform and the wavelet transform is shown below. Note however, that the frequency resolution is decreasing for increasing frequencies while the temporal resolution increases. This consequence of the Fourier uncertainty principle is not correctly displayed in the Figure.

This shows that wavelet transformation is good in time resolution of high frequencies, while for slowly varying functions, the frequency resolution is remarkable.

Another example: The analysis of three superposed sinusoidal signals ${\displaystyle y(t)\;=\;\sin(2\pi f_{0}t)\;+\;\sin(4\pi f_{0}t)\;+\;\sin(8\pi f_{0}t)}$ with STFT and wavelet-transformation.

Wavelet compression[]

Wavelet compression is a form of data compression well suited for image compression (sometimes also video compression and audio compression). Notable implementations are JPEG 2000, DjVu and ECW for still images, JPEG XS, CineForm, and the BBC's Dirac. The goal is to store image data in as little space as possible in a file. Wavelet compression can be either lossless or lossy.^[6] Wavelet coding is a variant of discrete cosine transform (DCT) coding that uses wavelets instead of DCT's block-based algorithm.^[7]

Using a wavelet transform, the wavelet compression methods are adequate for representing transients, such as percussion sounds in audio, or high-frequency components in two-dimensional images, for example an image of stars on a night sky. This means that the transient elements of a data signal can be represented by a smaller amount of information than would be the case if some other transform, such as the more widespread discrete cosine transform, had been used.

Discrete wavelet transform has been successfully applied for the compression of electrocardiograph (ECG) signals^[8] In this work, the high correlation between the corresponding wavelet coefficients of signals of successive cardiac cycles is utilized employing linear prediction.

Wavelet compression is not good for all kinds of data: transient signal characteristics mean good wavelet compression, while smooth, periodic signals are better compressed by other methods, particularly traditional harmonic compression (frequency domain, as by Fourier transforms and related).

See Diary Of An x264 Developer: The problems with wavelets (2010) for discussion of practical issues of current methods using wavelets for video compression.

Method[]

First a wavelet transform is applied. This produces as many coefficients as there are pixels in the image (i.e., there is no compression yet since it is only a transform). These coefficients can then be compressed more easily because the information is statistically concentrated in just a few coefficients. This principle is called transform coding. After that, the coefficients are quantized and the quantized values are entropy encoded and/or run length encoded.

A few 1D and 2D applications of wavelet compression use a technique called "wavelet footprints".^[9][10]

Evaluation[]

Requirement for image compression[]

For most natural images, the spectrum density of lower frequency is higher.^[11] As a result, information of the low frequency signal (reference signal) is generally preserved, while the information in the detail signal is discarded. From the perspective of image compression and reconstruction, a wavelet should meet the following criteria while performing image compression:

• Being able to transform more original image into the reference signal.
• Highest fidelity reconstruction based on the reference signal.
• Should not lead to artifacts in the image reconstructed from the reference signal alone.

Requirement for shift variance and ringing behavior[]

Wavelet image compression system involves filters and decimation, so it can be described as a linear shift-variant system. A typical wavelet transformation diagram is displayed below:

The transformation system contains two analysis filters (a low pass filter ${\displaystyle h_{0}(n)}$ and a high pass filter ${\displaystyle h_{1}(n)}$), a decimation process, an interpolation process, and two synthesis filters (${\displaystyle g_{0}(n)}$ and ${\displaystyle g_{1}(n)}$). The compression and reconstruction system generally involves low frequency components, which is the analysis filters ${\displaystyle h_{0}(n)}$ for image compression and the synthesis filters ${\displaystyle g_{0}(n)}$ for reconstruction. To evaluate such system, we can input an impulse ${\displaystyle \delta (n-n_{i})}$ and observe its reconstruction ${\displaystyle h(n-n_{i})}$; The optimal wavelet are those who bring minimum shift variance and sidelobe to ${\displaystyle h(n-n_{i})}$. Even though wavelet with strict shift variance is not realistic, it is possible to select wavelet with only slight shift variance. For example, we can compare the shift variance of two filters:^[12]

Biorthogonal filters for wavelet image compression

		Length	Filter coefficients	Regularity
Wavelet filter 1	H0	9	.852699, .377402, -.110624, -.023849, .037828	1.068
	G0	7	.788486, .418092, -.040689, -.064539	1.701
Wavelet filter 2	H0	6	.788486, .047699, -.129078	0.701
	G0	10	.615051, .133389, -.067237, .006989, .018914	2.068

By observing the impulse responses of the two filters, we can conclude that the second filter is less sensitive to the input location (i.e. it is less shift variant).

Another important issue for image compression and reconstruction is the system's oscillatory behavior, which might lead to severe undesired artifacts in the reconstructed image. To achieve this, the wavelet filters should have a large peak to sidelobe ratio.

So far we have discussed about one-dimension transformation of the image compression system. This issue can be extended to two dimension, while a more general term - shiftable multiscale transforms - is proposed.^[13]

Derivation of impulse response[]

As mentioned earlier, impulse response can be used to evaluate the image compression/reconstruction system.

For the input sequence ${\displaystyle x(n)=\delta (n-n_{i})}$, the reference signal ${\displaystyle r_{1}(n)}$ after one level of decomposition is ${\displaystyle x(n)*h_{0}(n)}$ goes through decimation by a factor of two, while ${\displaystyle h_{0}(n)}$ is a low pass filter. Similarly, the next reference signal ${\displaystyle r_{2}(n)}$ is obtained by ${\displaystyle r_{1}(n)*h_{0}(n)}$ goes through decimation by a factor of two. After L levels of decomposition (and decimation), the analysis response is obtained by retaining one out of every ${\displaystyle 2^{L}}$ samples: ${\displaystyle h_{A}^{(L)}(n,n_{i})=f_{h0}^{(L)}(n-n_{i}/2^{L})}$.

On the other hand, to reconstruct the signal x(n), we can consider a reference signal ${\displaystyle r_{L}(n)=\delta (n-n_{j})}$. If the detail signals ${\displaystyle d_{i}(n)}$ are equal to zero for ${\displaystyle 1\leq i\leq L}$, then the reference signal at the previous stage (${\displaystyle L-1}$ stage) is ${\displaystyle r_{L-1}(n)=g_{0}(n-2n_{j})}$, which is obtained by interpolating ${\displaystyle r_{L}(n)}$ and convoluting with ${\displaystyle g_{0}(n)}$. Similarly, the procedure is iterated to obtain the reference signal ${\displaystyle r(n)}$ at stage ${\displaystyle L-2,L-3,....,1}$. After L iterations, the synthesis impulse response is calculated: ${\displaystyle h_{s}^{(L)}(n,n_{i})=f_{g0}^{(L)}(n/2^{L}-n_{j})}$, which relates the reference signal ${\displaystyle r_{L}(n)}$ and the reconstructed signal.

To obtain the overall L level analysis/synthesis system, the analysis and synthesis responses are combined as below:

${\displaystyle h_{AS}^{(L)}(n,n_{i})=\sum _{k}f_{h0}^{(L)}(k-n_{i}/2^{L})f_{g0}^{(L)}(n/2^{L}-k)}$.

Finally, the peak to first sidelobe ratio and the average second sidelobe of the overall impulse response ${\displaystyle h_{AS}^{(L)}(n,n_{i})}$ can be used to evaluate the wavelet image compression performance.

Comparison with Fourier transform and time-frequency analysis[]

Transform	Representation	Input
Fourier transform	${\displaystyle {\hat {f}}(\xi )=\int _{-\infty }^{\infty }f(x)e^{-2\pi ix\xi }\,dx}$	${\displaystyle \xi }$ frequency
Time-frequency analysis	${\displaystyle X(t,f)}$	${\displaystyle t}$ time; ${\displaystyle f}$ frequency
Wavelet transform	${\displaystyle X(a,b)={\frac {1}{\sqrt {a}}}\int _{-\infty }^{\infty }{\overline {\Psi \left({\frac {t-b}{a}}\right)}}x(t)\,dt}$	${\displaystyle a}$ scaling; ${\displaystyle b}$ time shift factor

Wavelets have some slight benefits over Fourier transforms in reducing computations when examining specific frequencies. However, they are rarely more sensitive, and indeed, the common Morlet wavelet is mathematically identical to a short-time Fourier transform using a Gaussian window function.^[14] The exception is when searching for signals of a known, non-sinusoidal shape (e.g., heartbeats); in that case, using matched wavelets can outperform standard STFT/Morlet analyses.^[15]

Other practical applications[]

The wavelet transform can provide us with the frequency of the signals and the time associated to those frequencies, making it very convenient for its application in numerous fields. For instance, signal processing of accelerations for gait analysis,^[16] for fault detection,^[17] for design of low power pacemakers and also in ultra-wideband (UWB) wireless communications.^[18][19][20]

Synchro-squeezed transform[]

Synchro-squeezed transform can significantly enhance temporal and frequency resolution of time-frequency representation obtained using conventional wavelet transform.^[21][22]

See also[]

• Continuous wavelet transform
• Discrete wavelet transform
• Complex wavelet transform
• Constant-Q transform
• Stationary wavelet transform
• Dual wavelet
• Multiresolution analysis
• MrSID, the image format developed from original wavelet compression research at Los Alamos National Laboratory (LANL)
• ECW, a wavelet-based geospatial image format designed for speed and processing efficiency
• JPEG 2000, a wavelet-based image compression standard
• DjVu format uses wavelet-based IW44 algorithm for image compression
• scaleograms, a type of spectrogram generated using wavelets instead of a short-time Fourier transform
• Wavelet
• Haar wavelet
• Daubechies wavelet
• Binomial QMF (also known as Daubechies wavelet)
• Morlet wavelet
• Gabor wavelet
• Chirplet transform
• Time-frequency representation
• S transform
• Set partitioning in hierarchical trees
• Short-time Fourier transform
• Biorthogonal nearly coiflet basis, which shows that wavelet for image compression can also be nearly coiflet (nearly orthogonal).

References[]

^[1]

External links[]

Wikimedia Commons has media related to Wavelets.

• Amara Graps (June 1995). "An Introduction to Wavelets". IEEE Computational Science & Engineering.
• Robi Polikar (2001-01-12). "The Wavelet Tutorial".
• Concise Introduction to Wavelets by René Puschinger