how to display multi-level wavedec2 in MATLAB

Question

After computing the 4-level discrete wavelet transform of an image, how do I display this multi-level transformation in matlab? And how do I plot the histogram of the corresponding DWT coefficients?

Here is what I did so far:

I = imread('image.png');
N = 4;
[C,S] = wavedec2(im2double(I),N,'haar');
A = appcoef2(C,S,'haar',1);
[H1,V1,D1] = detcoef2('all',C,S,1);
[H2,V2,D2] = detcoef2('all',C,S,2);
[H3,V3,D3] = detcoef2('all',C,S,3);
[H4,V4,D4] = detcoef2('all',C,S,4);

Tommaso Belluzzo · Accepted Answer · 2017-11-28T07:54:54.843

1

This is what I would do it:

N = 4;

img = imread('image.png');
img = im2double(img);

[C,S] = wavedec2(img,N,'haar');

for i = 1:N
    lvl = ['Level ' num2str(i)];

    A = appcoef2(C,S,'haar',i);
    [H,V,D] = detcoef2('all',C,S,i);

    figure('Name',['Images (' lvl ')']);
    % Eventually, you can define a colormap for your images...
    % colormap(pink(255));
    subplot(2,2,1); imagesc(A);
    title('Approximation')
    subplot(2,2,2); imagesc(H);
    title('Horizontal Detail');
    subplot(2,2,3); imagesc(V);
    title('Vertical Detail');
    subplot(2,2,4); imagesc(D);
    title('Diagonal Detail'); 
    suptitle(lvl);

    % tweak the histogram bins as you prefer
    figure('Name',['Histograms (' lvl ')']);
    subplot(2,2,1); hist(A(:),32);
    subplot(2,2,2); hist(H(:),32);
    subplot(2,2,3); hist(V(:),32);
    subplot(2,2,4); hist(D(:),32);
    suptitle(lvl);
end

Actually, since I'm not very esperienced in digital image processing, it's up to you to tweak my example and make it fits your needs.

edited Nov 28 '17 at 07:54

answered Nov 27 '17 at 23:58

Tommaso Belluzzo

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Thank you, this is very helpful. – ABC2013 Nov 28 '17 at 04:29
Do you know if there is a way to combine all the coefficients for all the levels in order to get only one histogram? @Tommaso – ABC2013 Nov 28 '17 at 17:08
Within each loop, you save the coefficients of the current level into another variable declared before the loop... then you print the histograms when you have all of them. – Tommaso Belluzzo Nov 28 '17 at 17:11
by saving all the coefficients of the current level, you mean I add them together or multiply or what operation should I do? – ABC2013 Nov 29 '17 at 23:49
Append them to a variable (cell array). – Tommaso Belluzzo Nov 30 '17 at 07:08

how to display multi-level wavedec2 in MATLAB

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