MATLAB Central Newsreader - Using wavelet decomposition function dwt2

Using wavelet decomposition function dwt2

Minh Tran — Sun, 08 Nov 2009 09:42:03 -0500

I'm having trouble with using dwt2 on my intensity image, specifically the LL output. From the examples I've seen elsewhere, the LL output shouldn't be very different from the original intensity image, yet mine is quite whitewashed/overexposed. Any suggestions about this problem ?

My code :
i = hsi_image(:,:,3);
[LL,LH,HL,HH] = dwt2(i,'db4');

Re: Using wavelet decomposition function dwt2

Wayne King — Sun, 08 Nov 2009 11:35:04 -0500

"Minh Tran" <overseer47@gmail.com> wrote in message <hd63pb$bgc$1@fred.mathworks.com>...
> I'm having trouble with using dwt2 on my intensity image, specifically the LL output. From the examples I've seen elsewhere, the LL output shouldn't be very different from the original intensity image, yet mine is quite whitewashed/overexposed. Any suggestions about this problem ?
>
> My code :
> i = hsi_image(:,:,3);
> [LL,LH,HL,HH] = dwt2(i,'db4');

Hi Minh, what command(s) are you using to actually view the images? It's hard for anyone to say without seeing those commands and knowing something about your image. Can you reproduce this behavior with one of the images that come with the wavelet toolbox, so we can all see?

>>load woman

for example

wayne

Re: Using wavelet decomposition function dwt2

Minh Tran — Sun, 08 Nov 2009 15:50:03 -0500

Thanks, Wayne. This is the image I'm working on and the LL output from dwt2:
http://yfrog.com/eldwtproblemp

And this is the results they got on wikimedia :
http://commons.wikimedia.org/wiki/File:Jpeg2000_2-level_wavelet_transform-lichtenstein.png

So apparently something's not right here. I tried grayscale conversion as the guy on wikimedia did instead of using RGB->HSI conversion and it's still the same. The problem shouldn't be the filter - I mean JPEG2000 is different from db4 but shouldn't be that much, as I tried many other filters too (haar, db, bior, etc.). The situation's the same with all other images - whitewashed/overexposed and of course almost useless LL output.

FYI, I'm using the colorspace converter by Pascal Getreuer (http://www.mathworks.com/matlabcentral/fileexchange/7744) to do RGB->HSI conversion. I run the dwt2 on the intensity image immediately after doing the conversion.

Re: Using wavelet decomposition function dwt2

Idil Selin — Wed, 23 Dec 2009 14:59:06 -0500

Hi Minh,
I was having the same problem. I am working with a grey scale image (a face of the YaleBkucuk database). I want to make 1 level wavelet transform and look at the result.
This was my code:
    [cA1,cH1,cV1,cD1]=dwt2(face, 'haar');
    subplot(2,2,1); imshow(cA1); %grey levels >256!!!
    subplot(2,2,2); imshow(cH1);
    subplot(2,2,3); imshow(cV1);
    subplot(2,2,4); imshow(cD1);
And the first picture was all white. I opened it and I found out that some grey levels were greater than 256. I solved the problems by doing like that:

subplot(2,2,1); imshow(cA1, []);

I hope it helps. Cheers, Idil