cwtft2

2-D continuous wavelet transform

Syntax

cwtstruct = cwtft2(X)

cwtstruct = cwtft2(X,"plot")

cwtstruct = cwtft2(___,Name,Value)

Description

cwtstruct = cwtft2(X) returns the 2-D continuous wavelet transform (CWT) of X.

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

cwtstruct = cwtft2(___,Name,Value) specifies options using one or more name-value arguments in addition to the input arguments in previous syntaxes.

Examples

2-D CWT with Morlet Wavelet

Load and display the star image.

img = imread("star.jpg");
image(img)

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

cwtout = cwtft2(img);
sca = 1;
imagesc(abs(cwtout.cfs(:,:,1,1,sca)))

Plot 2-D CWT

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

load woman
cwtmorl = cwtft2(X,"plot");

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 Marr isotropic wavelet and the directional (anisotropic) Cauchy wavelet.

Load and view the hexagon image.

img = imread("hexagon.jpg");
imagesc(img)

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

cwtAngles = 0:pi/8:2*pi-pi/8;
cwtcauchy = cwtft2(img,wavelet="cauchy",scales=1, ...
    angles=cwtAngles);
cwtmarr = cwtft2(img,wavelet="marr",scales=1, ...
    angles=cwtAngles);

There are 16 angles. Visualize the scale-one 2-D CWT coefficient magnitudes at any two consecutive angles. Confirm that using the Marr isotropic wavelet does not discern the orientation of features, but the Cauchy wavelet does.

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"};

indexAngle1 = 7;
indexAngle2 = 8;

tiledlayout(2,2)
for k=[indexAngle1 indexAngle2]
    nexttile
    imagesc(abs(cwtmarr.cfs(:,:,1,1,k)));
    title(["Marr Wavelet at " angz(k) "radians"]);
    nexttile
    imagesc(abs(cwtcauchy.cfs(:,:,1,1,k)));
    title(["Cauchy Wavelet at " angz(k) "radians"]);
end

Visualize the scale-one 2-D CWT coefficient magnitudes obtained using the Marr isotropic wavelet at any two angles. Confirm the wavelet does not discern the orientation of features.

indexAngle1 = 2;
indexAngle2 = 7;

tiledlayout(1,2)
for k=[indexAngle1 indexAngle2]
    nexttile
    imagesc(abs(cwtmarr.cfs(:,:,1,1,k)));
    title(["Marr Wavelet at " angz(k) "radians"]);
end

Input Arguments

X — Input data

array

Input data, specified as a numeric 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.

Data Types: double | single | 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.

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

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 used in the 2-D CWT, specified as 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 one of these:

• "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 positive real-valued scalar or a vector of positive real numbers.

Example: scales=2.^(1:6)

wavelet — Analyzing wavelet

"morlet" (default) | character vector | string scalar | structure | cell array

Analyzing wavelet, specified as 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 — character vector or string scalar corresponding to a supported wavelet.

• param — cell array containing optional parameters, which depend on the wavelet. If you do not wish to specify optional parameters, use an empty cell array.

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

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

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

Example: "wavelet",{"morlet",{6,1,1}} specifies the Morlet wavelet as a cell array.

Example: "wavelet",struct("name","paul","param",{{2}}) specifies the Paul wavelet as a structure array.

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 — Wavelet name

• param — Wavelet parameters

wav_norm — Normalization constants

matrix

Normalization constants, returned as an 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.

• 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