Wrong values of PSNR for two nearly identical images

Hello,

my original image is Lena.gif (256x256) which i attached and for this image i did a first level Wavelet (haar) decomposition with Wavelet analyzer toolbox, i attached approximation named LL1.

When i calculate PSNR with function in Matlab "psnr" i got values 27.68, which is not logically. Also, i computed PSNR with my own code:

I = imread('lena.gif');
  I = imread('LL1.gif');
  I = uint8(I);
  L1 = uint8(L1);
  % I = im2double(I);
  % L1 = im2double(L1)
  mse = sum((I(:)-L1(:)).^2)/numel(L1);
  psnr1 = 10*log10(255*255/mse);
  fprintf('\n MYYYY The PSNR is: %0.2f', psnr1);

and results are also not logically. It is know that if PSNR<=30 then image having low quality, else if PSNR>=30 && PSNR<=60 then image having Medium quality else image having good quality.

What is wrong with PSNR function?

[Walter Roberson]

mse = sum((double(I(:))-double(L1(:))).^2)/numel(L1);

Answer 1

[Walter Roberson]

montage({'lena.gif','L1.gif'})

When you compare them side by side you can see that the wavelet processed one is low quality.

[mavicV]

I agree that the wavelet image processed is lower quality then original, but it is certainly not so low that the PNSR values would be below the bad quality limit (27.68).

[Walter Roberson]

I have to disagree. See http://homepages.inf.ed.ac.uk/rbf/CVonline/LOCAL_COPIES/VELDHUIZEN/node18.html and notice that the image with 30 is still quite recognizable

[mavicV]

Yes, it is recognizable, but look at quality at that images with 30 PSNR and my image with 27.68 PSNR, my look almost like original, so PSNR is to low for that and should be over 40 like your example. Look example that i attached and compare PSNR with your example.

[Walter Roberson]

You are using too small of a figure to see the differences. Use the montage() command I showed, and display with a minimum figure size of 720 x 1440