How I can know the size of a quantized image

I designed a simple uniform quantizer, it is a part of project, I used 'leena.jpg' image the original size is about 65.8 KB, after running this code:

img = imread('leena.jpg');

figure(1)

imshow(img)

[m,n] = size(img);

M = 8; K = 256/M;

for i = 1:m

      for j = 1:n

          quant_img(i,j) = floor(img(i,j)/K) * K + (K/2);

      end

end

figure(2)

imshow(quant_img)

-----------------

It is clear the image quality reduced, but when I save the two figures (the original and the quantized) as jpg format, I got 14.4 KB for the original and 16.5 KB for the quantized image! When I increase the number of levels (M = 32) the size decrease to 14.8 KB! Why the size of the image is decreasing with increasing the number of levels!! It should be the reverse, right?

Answer 1

[Image Analyst]

It's a moot question because you should not even be using jpg. Use PNG instead. It will be bigger but at least you won't have JPG artifacts because it's lossless compression, and pretty much the new standard that most people use.

[Osama Hussein]

Thank you, now I got smaller size for the quantized image (but it still confusing why there is error in JPG format!)..

I want to ask you, please, why the dimension of the two figures changes to 430*351 while the original image is 256*256?

[Image Analyst]

It doesn't. Run this code and tell me what you observe:

clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 20;

% img = imread('leena.jpg');
img = imread('cameraman.tif');
subplot(1,2,1);
imshow(img)
axis on;
title('Original Image', 'FontSize', fontSize);
[rows, columns, numberOfColorChannels] = size(img)

% Set up figure properties:
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0 0 1 1]);
% Get rid of tool bar and pulldown menus that are along top of figure.
set(gcf, 'Toolbar', 'none', 'Menu', 'none');
% Give a name to the title bar.
set(gcf, 'Name', 'Demo by ImageAnalyst', 'NumberTitle', 'Off')

message = sprintf('For quantized image\nRows = %d\nColumns = %d\nNumber of Color Channels = %d', rows, columns, numberOfColorChannels);
uiwait(helpdlg(message));

M = 8; 
K = 256/M;
for row = 1 : rows  
  for col = 1 : columns    
    quant_img(row,col) = floor(img(row,col)/K) * K + (K/2);    
  end  
end
subplot(1,2,2);
imshow(quant_img)
axis on;
[rows, columns, numberOfColorChannels] = size(quant_img)
title('Quantized Image', 'FontSize', fontSize);
message = sprintf('For quantized image\nRows = %d\nColumns = %d\nNumber of Color Channels = %d', rows, columns, numberOfColorChannels);
uiwait(helpdlg(message));

For the cameraman image it remains 256x256. Change it to your lena image and see what it says. It's not a color image is it? The numbers don't even make sense for a color image so I think there's something you're not sharing. There might be more code there that you've left out. Or you didn't clear variables and close figure windows so maybe something is still there from an earlier run where you had different values for the variables. Like maybe you didn't close the figures and put hold on, and then changed the image file to one with a different size.

[Osama Hussein]

Thank you very much.. yes it remains 256x256 now. Yes it is a gray scale image not color. Really I am grateful for you.

[Image Analyst]

So what turned out to be the problem? Nothing you did originally should have changed the size. Why did it not work at first?

[Osama Hussein]

When I used your program it works right with lena image and it remains the same size. With my program the size of the variable represent the image is 256x256 but when I save each figure as JPG or PNG the go to the folder location the size will be 430x351. I think I should check it within Matlab like you do.

[Image Analyst]

You're probably saving the figure window itself, rather than just the image. The figure will include things like gray background, tick marks, annotations in the graphical overlay, etc.

Use imwrite() rather than saveas().

[Osama Hussein]

Ah! Yes I used saveas(). I get it right now.. Thank you :)