wavedec2

Multilevel 2-D discrete wavelet transform

Syntax

[C,S] = wavedec2(X,N,wname)

[C,S] = wavedec2(X,N,LoD,HiD)

Description

[C,S] = wavedec2(X,N,wname) returns the wavelet decomposition of the matrix X at level N using the wavelet wname. The output decomposition structure consists of the wavelet decomposition vector C and the bookkeeping matrix S, which contains the number of coefficients by level and orientation.

Note

For gpuArray inputs, the supported modes are 'symh' ('sym') and 'per'. If the input is a gpuArray, the discrete wavelet transform extension mode used by wavedec2 defaults to 'symh' unless the current extension mode is 'per'. See the example Multilevel 2-D Discrete Wavelet Transform on a GPU.

[C,S] = wavedec2(X,N,LoD,HiD) returns the wavelet decomposition using the specified lowpass and highpass decomposition filters LoD and HiD, respectively. See wfilters for details.

Examples

Extract and Display Image Decomposition Level

This example shows how to extract and display images of wavelet decomposition level details.

Load an image. Perform a level 2 wavelet decomposition of the image using the haar wavelet.

load woman
[c,s]=wavedec2(X,2,'haar');

Extract the level 1 approximation and detail coefficients.

[H1,V1,D1] = detcoef2('all',c,s,1);
A1 = appcoef2(c,s,'haar',1);

Use wcodemat to rescale the coefficients based on their absolute values. Display the rescaled coefficients.

V1img = wcodemat(V1,255,'mat',1);
H1img = wcodemat(H1,255,'mat',1);
D1img = wcodemat(D1,255,'mat',1);
A1img = wcodemat(A1,255,'mat',1);

subplot(2,2,1)
imagesc(A1img)
colormap pink(255)
title('Approximation Coef. of Level 1')

subplot(2,2,2)
imagesc(H1img)
title('Horizontal Detail Coef. of Level 1')

subplot(2,2,3)
imagesc(V1img)
title('Vertical Detail Coef. of Level 1')

subplot(2,2,4)
imagesc(D1img)
title('Diagonal Detail Coef. of Level 1')

Extract the level 2 approximation and detail coefficients.

[H2,V2,D2] = detcoef2('all',c,s,2);
A2 = appcoef2(c,s,'haar',2);

Use wcodemat to rescale the coefficients based on their absolute values. Display the rescaled coefficients.

V2img = wcodemat(V2,255,'mat',1);
H2img = wcodemat(H2,255,'mat',1);
D2img = wcodemat(D2,255,'mat',1);
A2img = wcodemat(A2,255,'mat',1);

figure
subplot(2,2,1)
imagesc(A2img)
colormap pink(255)
title('Approximation Coef. of Level 2')

subplot(2,2,2)
imagesc(H2img)
title('Horizontal Detail Coef. of Level 2')

subplot(2,2,3)
imagesc(V2img)
title('Vertical Detail Coef. of Level 2')

subplot(2,2,4)
imagesc(D2img)
title('Diagonal Detail Coef. of Level 2')

2-D Wavelet Decomposition Structure

This example shows the structure of wavedec2 output matrices.

Load and display an image.

load woman
imagesc(X)
colormap(map)

Save the current discrete wavelet transform extension mode.

origMode = dwtmode('status','nodisplay');

Change to periodic boundary handling. The dwtmode function displays a message indicating that the DWT extension mode is changing.

dwtmode('per')

 
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!  WARNING: Change DWT Extension Mode  !
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
                                         
*****************************************
**  DWT Extension Mode: Periodization  **
*****************************************
                                         

Perform a level 3 decomposition of the image using the db1 (Haar) wavelet.

[c,s] = wavedec2(X,3,'db1');

Return the number of elements in the image X and coefficient vector c. Confirm the number of elements in each are equal.

numel(X)

ans = 
65536

numel(c)

ans = 
65536

Display the bookkeeping matrix s. The first row displays the dimensions of the coarse scale approximation of the image. The last row displays the dimensions of the original image. The intermediate rows display the dimensions of the detail coefficients at the three levels of the decomposition, proceeding from coarse to fine scale.

s

s = 5×2

    32    32
    32    32
    64    64
   128   128
   256   256

Reset discrete wavelet transform extension mode to its original mode.

dwtmode(origMode,'nodisplay')

Multilevel 2-D Discrete Wavelet Transform on a GPU

Refer to GPU Computing Requirements (Parallel Computing Toolbox) to see what GPUs are supported.

Load an image. Put the image on the GPU using gpuArray. Save the current extension mode.

load mask
imgg = gpuArray(X);
origMode = dwtmode('status','nodisp');

Use dwtmode to change the extension mode to zero-padding. Obtain the three-level DWT of the image on the GPU using the db4 wavelet.

dwtmode('zpd','nodisp')
[c,s] = wavedec2(imgg,3,'db4');

The current extension mode zpd is not supported for gpuArray input. Therefore, the DWT is instead performed using the sym extension mode. To confirm this, set the extension mode to sym and take DWT of noisdoppg, then compare with the previous result.

dwtmode('sym','nodisp')
[csym,ssym] = wavedec2(imgg,3,'db4');
[max(abs(c-csym)) max(abs(s-ssym))]

ans =

     0     0     0

Set the current extension mode to per and obtain the three-level DWT of imgg. The extension mode per is supported for gpuArray input. Confirm the result is different from the sym results.

dwtmode('per','nodisp')
[cper,sper] = wavedec2(imgg,3,'db4');
[length(csym) ; length(cper)]

ans = 2×1

       71542
       65536

ssym

ssym = 5×2

    38    38
    38    38
    69    69
   131   131
   256   256

sper

sper = 5×2

    32    32
    32    32
    64    64
   128   128
   256   256

Restore the extension mode to the original setting.

dwtmode(origMode,'nodisp')

Input Arguments

X — Input data

numeric array | logical array

Input data, specified as a numeric or logical array. X can be an M-by-N array representing an indexed image or an M-by-N-by-3 array representing a truecolor image. For more information on truecolor images, see Working with Image Types in MATLAB.

Data Types: double | single | uint8

N — Decomposition level

positive integer

Decomposition level, specified as a positive integer. wavedec2 does not enforce a maximum level restriction. Use wmaxlev to determine the maximum decomposition level possible of the matrix X using the wavelet wname. The maximum level is the last level for which at least one coefficient is correct.

Data Types: double

wname — Analyzing wavelet

character vector | string scalar

Analyzing wavelet, specified as a character vector or string scalar.

Note

wavedec2 supports only Type 1 (orthogonal) or Type 2 (biorthogonal) wavelets. See wfilters for a list of orthogonal and biorthogonal wavelets.

Data Types: char | string

LoD,HiD — Wavelet decomposition filters

even-length real-valued vectors

Wavelet decomposition filters associated with an orthogonal or biorthogonal wavelet, specified as even-length real-valued vectors. LoD is the lowpass decomposition filter, and HiD is the highpass decomposition filter. See wfilters for details.

Data Types: double | single

Output Arguments

C — Wavelet decomposition vector

real-valued vector

Wavelet decomposition vector. The vector C contains the approximation and detail coefficients organized by level. The bookkeeping matrix S is used to parse C.

The vector C is organized as A(N), H(N), V(N), D(N), H(N-1), V(N-1), D(N-1), …, H(1), V(1), D(1), where A, H, V, and D are each a row vector. Each vector is the column-wise storage of a matrix.

• A contains the approximation coefficients.

• H contains the horizontal detail coefficients.

• V contains the vertical detail coefficients.

• D contains the diagonal detail coefficients.

Data Types: double

S — Bookkeeping matrix

integer-valued matrix

Bookkeeping matrix. The matrix S contains the dimensions of the wavelet coefficients by level and is used to parse the wavelet decomposition vector C.

• S(1,:) = size of approximation coefficients(N).

• S(i,:) = size of detail coefficients(N-i+2) for i = 2, ...N+1 and S(N+2,:) = size(X).

The following diagram shows the relationship between C and S in the wavelet decomposition of a 512-by-512 matrix.

When X represents an indexed image, the output arrays cA, cH, cV, and cD are m-by-n matrices. When X represents a truecolor image, it is an m-by-n-by-3 array, where each m-by-n matrix represents a red, green, or blue color plane concatenated along the third dimension. The size of vector C and the size of matrix S depend on the type of analyzed image.

For a truecolor image, the decomposition vector C and the corresponding bookkeeping matrix S can be represented as shown.

Algorithms

For images, an algorithm similar to the one-dimensional case is possible for two-dimensional wavelets and scaling functions obtained from one-dimensional vectors by tensor product. This kind of two-dimensional DWT leads to a decomposition of approximation coefficients at level j in four components: the approximation at level j+1 and the details in three orientations (horizontal, vertical, and diagonal).

The chart describes the basic decomposition step for images:

where

• — Downsample columns: keep the even-indexed columns.

• — Downsample rows: keep the even-indexed rows.

• — Convolve with filter X the rows of the entry.

• — Convolve with filter X the columns of the entry.

and

Initialization: cA₀ = s.

So, for J = 2, the two-dimensional wavelet tree has the form

References

[1] Daubechies, Ingrid. Ten Lectures on Wavelets. CBMS-NSF Regional Conference Series in Applied Mathematics 61. Philadelphia, Pa: Society for Industrial and Applied Mathematics, 1992.

[2] Mallat, S.G. “A Theory for Multiresolution Signal Decomposition: The Wavelet Representation.” IEEE Transactions on Pattern Analysis and Machine Intelligence 11, no. 7 (July 1989): 674–93. https://doi.org/10.1109/34.192463.

[3] Meyer, Y. Wavelets and Operators. Translated by D. H. Salinger. Cambridge, UK: Cambridge University Press, 1995.

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using MATLAB® Coder™.

Usage notes and limitations:

• The input wname must be constant.

GPU Code Generation

Generate CUDA® code for NVIDIA® GPUs using GPU Coder™.

Usage notes and limitations:

• The input wname must be constant.

• The decomposition level, N must be a compile-time constant.

GPU Arrays

Accelerate code by running on a graphics processing unit (GPU) using Parallel Computing Toolbox™.

The wavedec2 function supports GPU array input with these usage notes and limitations:

• wavedec2 supports only the 'sym' and 'per' extension modes. See dwtmode.

For more information, see Run MATLAB Functions on a GPU (Parallel Computing Toolbox).

Version History

Introduced before R2006a