cwtft2

2-D continuous wavelet transform

Syntax

cwtstruct = cwtft2(x)

cwtstruct = cwtft2(x,'plot')

cwtstruct = cwtft2(x,Name,Value)

Description

cwtstruct = cwtft2(x) returns the 2-D continuous wavelet transform (CWT) of the 2-D matrix, x. cwtft2 uses a Fourier transform-based algorithm in which the 2-D Fourier transforms of the input data and analyzing wavelet are multiplied together and inverted.

cwtstruct = cwtft2(x,'plot') plots the data and the 2-D CWT.

cwtstruct = cwtft2(x,Name,Value) uses additional options specified by one or more Name,Value pair arguments.

Examples

Compare Isotropic and Anisotropic Wavelets

Shows how an isotropic wavelet does not discern the orientation of features while an anisotropic wavelet does. The example uses the Mexican hat isotropic wavelet and the directional (anisotropic) Cauchy wavelet.

Load and view the hexagon image.

Im = imread('hexagon.jpg');
imagesc(Im); colormap(jet);

Obtain the scale-one 2-D CWT with both the Mexican hat and Cauchy wavelets. Specify a vector of angles going from 0 to 15?/8 in ?/8 increments.

cwtcauchy = cwtft2(Im,'wavelet','cauchy','scales',1,...
    'angles',0:pi/8:2*pi-pi/8);
cwtmexh = cwtft2(Im,'wavelet','mexh','scales',1,...
    'angles',0:pi/8:2*pi-pi/8);

Visualize the scale-one 2-D CWT coefficient magnitudes at each angle.

angz = {'0', 'pi/8', 'pi/4', '3pi/8', 'pi/2', '5pi/8', '3pi/4', ...
    '7pi/8','pi', '9pi/8', '5pi/4', '11pi/8', '3pi/2', ...
    '13pi/8' '7pi/4', '15pi/8'};
for angn = 1:length(angz)
    subplot(211)
    imagesc(abs(cwtmexh.cfs(:,:,1,1,angn)));
    title(['Mexican hat at ' angz(angn) 'radians']);
    subplot(212)
    imagesc(abs(cwtcauchy.cfs(:,:,1,1,angn)));
    title(['Cauchy wavelet at ' angz(angn) 'radians']);
    pause(1);
end

Plot 2-D CWT

Load an image of a woman, obtain the 2-D CWT using the Morlet wavelet, and plot the CWT coefficients.

load woman;
cwtmorl = cwtft2(X,'scales',1:4,'angles',0:pi/2:3*pi/2,'plot');

2-D CWT with Morlet Wavelet

Obtain the 2-D CWT of the star image using the default Morlet wavelet, scales 2^(0:5), and an angle of 0.

Im = imread('star.jpg');
cwtout = cwtft2(Im);

Input Arguments

x — Input data

array

Input data, specified as a 2-D matrix or 3-D array. If the input data is a 3-D array, the input matrix is a truecolor image.

Example: X = imread('stars.jpg');

Data Types: double | uint8

Name-Value Arguments

Specify optional pairs of arguments as Name1=Value1,...,NameN=ValueN, where Name is the argument name and Value is the corresponding value. Name-value arguments must appear after other arguments, but the order of the pairs does not matter.

Before R2021a, use commas to separate each name and value, and enclose Name in quotes.

Example: 'wavelet','paul','scales',2^(0:5) specifies to use the Paul wavelet and a vector of scales.

angles — Angles

0 (default) | scalar | vector

Angles in radians, specified as a comma-separated pair consisting of 'angles' and either a scalar or a vector.

Example: 'angles',[0 pi/2 pi]

norm — Normalization

'L2' (default) | 'L1' | 'L0'

Normalization used in the 2-D CWT, specified as a comma-separated pair consisting of 'norm' and one of these character vectors:

• 'L2' — The Fourier transform of the analyzing wavelet at a given scale is multiplied by the corresponding scale. 'L2' is the default normalization.

• 'L1' — The Fourier transform of the analyzing wavelet is multiplied by 1 at all scales.

• 'L0' — The Fourier transform of the analyzing wavelet at a given scale is multiplied by the square of the corresponding scale.

Example: 'norm','L1'

scales — Scales

2^(0:5) (default) | scalar | vector

Scales, specified as a comma-separated pair consisting of 'scales' and either a positive real-valued scalar or a vector of positive real numbers.

Example: 'scales',2^(1:6)

wavelet — Analyzing wavelet

'morl' (default) | character vector | string scalar | structure | cell array

Analyzing wavelet, specified as a comma-separated pair consisting of 'wavelet' and a character vector, a string scalar, a structure, or a cell array. cwtftinfo2 provides a comprehensive list of supported wavelets and associated parameters.

If you specify 'wavelet' as a structure, the structure must contain two fields:

• name — the character vector or string scalar corresponding to a supported wavelet.

• param — a cell array with the parameters of the wavelet.

If you specify 'wavelet' as a cell array, wav, the cell array must contain two elements:

• wav{1} — the character vector or string scalar corresponding to a supported wavelet.

• wav{2} — a cell array with the parameters of the wavelet.

Example: 'wavelet',{'morl',{6,1,1}}

Example: 'wavelet',struct('name','paul','param',{'p',2})

Output Arguments

cwtstruct — 2-D CWT

structure

The 2-D CWT, returned as a structure with the following fields:

wav — Analyzing wavelet and parameters

structure

Analyzing wavelet and parameters, returned as a structure with the following fields:

• wname — name

• param — parameters

wav_norm — Normalization constants

matrix

Normalization constants, returned as a M-by-N matrix where M is the number of scales and N is the number of angles.

cfs — CWT coefficients

array

CWT coefficients, returned as an N-D array. The row and column dimensions of the array equal the row and column dimensions of the input data. The third page of the array is equal to 1 or 3 depending on whether the input data is a grayscale or truecolor image. The fourth page of the array is equal to the number of scales and the fifth page of the array is equal to the number of angles.

scales — Scales

vector

Scales for the 2-D CWT, returned as a row vector.

angles — Angles

vector

Angles for the 2-D CWT, returned as a row vector.

meanSIG — Mean

scalar

Mean of the input data, returned as a scalar

Version History

Introduced in R2013b