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wdencmp

De-noising or compression

Syntax

[XC,CXC,LXC,PERF0,PERFL2] = wdencmp('gbl',X,'wname',N,THR,SORH,KEEPAPP)
wdencmp('gbl',C,L,'wname',N,THR,SORH,KEEPAPP)
[XC,CXC,LXC,PERF0,PERFL2] = wdencmp('lvd',X,'wname',N,THR,SORH)
[XC,CXC,LXC,PERF0,PERFL2] = wdencmp('lvd',C,L,'wname',N,THR,SORH)
[XC,CXC,LXC,PERF0,PERFL2] = wdencmp('lvd',X,'wname',N,THR,SORH)
[XC,CXC,LXC,PERF0,PERFL2] = wdencmp('lvd',C,L,'wname',N,THR,SORH)

Description

wdencmp is a one- or two-dimensional de-noising and compression-oriented function.

wdencmp performs a de-noising or compression process of a signal or an image, using wavelets.

[XC,CXC,LXC,PERF0,PERFL2] = wdencmp('gbl',X,'wname',N,THR,SORH,KEEPAPP) returns a de-noised or compressed version XC of input signal X (one- or two-dimensional) obtained by wavelet coefficients thresholding using global positive threshold THR.

Additional output arguments [CXC,LXC] are the wavelet decomposition structure of XC (see wavedec or wavedec2 for more information). PERF0 and PERFL2 are L2 -norm recovery and compression score in percentage.

PERFL2 = 100 * (vector-norm of CXC / vector-norm of C)2 if [C,L] denotes the wavelet decomposition structure of X.

If X is a one-dimensional signal and 'wname' an orthogonal wavelet, PERFL2 is reduced to

100XC2X2

Wavelet decomposition is performed at level N and 'wname' is a character vector containing wavelet name (see wmaxlev and wfilters for more information). SORH ('s' or 'h') is for soft or hard thresholding (see wthresh for more information). If KEEPAPP = 1, approximation coefficients cannot be thresholded, otherwise it is possible.

wdencmp('gbl',C,L,'wname',N,THR,SORH,KEEPAPP) has the same output arguments, using the same options as above, but obtained directly from the input wavelet decomposition structure [C,L] of the signal to be de-noised or compressed, at level N and using 'wname' wavelet.

For the one-dimensional case and 'lvd' option, [XC,CXC,LXC,PERF0,PERFL2] = wdencmp('lvd',X,'wname',N,THR,SORH) or [XC,CXC,LXC,PERF0,PERFL2] = wdencmp('lvd',C,L,'wname',N,THR,SORH) have the same output arguments, using the same options as above, but allowing level-dependent thresholds contained in vector THR (THR must be of length N). In addition, the approximation is kept. Note that, with respect to wden (automatic de-noising), wdencmp allows more flexibility and you can implement your own de-noising strategy.

For the two-dimensional case and 'lvd' option, [XC,CXC,LXC,PERF0,PERFL2] = wdencmp('lvd',X,'wname',N,THR,SORH) or [XC,CXC,LXC,PERF0,PERFL2] = wdencmp('lvd',C,L,'wname',N,THR,SORH).

THR must be a matrix 3 by N containing the level-dependent thresholds in the three orientations, horizontal, diagonal, and vertical.

Like denoising, the compression procedure contains three steps:

  1. Decomposition.

  2. Detail coefficient thresholding. For each level from 1 to N, a threshold is selected and hard thresholding is applied to the detail coefficients.

  3. Reconstruction.

The difference with the denoising procedure is found in step 2.

Examples

collapse all

Denoise 1-D electricity consumption data using the Donoho-Johnstone global threshold.

Load the signal and select a segment for denoising.

load leleccum; indx = 2600:3100;
x = leleccum(indx);

Use ddencmp to determine the default global threshold and denoise the signal. Plot the original and denoised signals.

[thr,sorh,keepapp] = ddencmp('den','wv',x);
xd = wdencmp('gbl',x,'db3',2,thr,sorh,keepapp);
subplot(211)
plot(x); title('Original Signal');
subplot(212)
plot(xd); title('Denoised Signal');

Denoise 1-D electricity consumption data using the Donoho-Johnstone global threshold.

Load the signal and select a segment for denoising.

load leleccum; indx = 2600:3100;
x = leleccum(indx);

Use ddencmp to determine the default global threshold and denoise the signal. Plot the original and denoised signals.

[thr,sorh,keepapp] = ddencmp('den','wv',x);
xd = wdencmp('gbl',x,'db3',2,thr,sorh,keepapp);
subplot(211)
plot(x); title('Original Signal');
subplot(212)
plot(xd); title('Denoised Signal');

References

DeVore, R.A.; B. Jawerth, B.J. Lucier (1992), "Image compression through wavelet transform coding," IEEE Trans. on Inf. Theory, vol. 38, No 2, pp. 719–746.

Donoho, D.L. (1993), "Progress in wavelet analysis and WVD: a ten minute tour," in Progress in wavelet analysis and applications, Y. Meyer, S. Roques, pp. 109–128. Frontières Ed.

Donoho, D.L.; I.M. Johnstone (1994), "Ideal spatial adaptation by wavelet shrinkage," Biometrika, vol. 81, pp. 425–455.

Donoho, D.L.; I.M. Johnstone, G. Kerkyacharian, D. Picard (1995), "Wavelet shrinkage: asymptopia," Jour. Roy. Stat. Soc., series B, vol. 57 no. 2, pp. 301–369.

Donoho, D.L.; I.M. Johnstone, "Ideal de-noising in an orthonormal basis chosen from a library of bases," C.R.A.S. Paris, t. 319, Ser. I, pp. 1317–1322.

Donoho, D.L. (1995), "De-noising by soft-thresholding," IEEE Trans. on Inf. Theory, 41, 3, pp. 613–627.

Introduced before R2006a


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