Image Stacking for noise reduction using MATLAB [closed]

Question

I'm new to Image processing and started using MATLAB for Astrophotography processing. I'm trying to process 10 corrupted images (Same Image but mixed with different noise) of Planet Saturn using MATLAB. I learned that by stacking the 10 images together leads to a noise reduced picture with high PSNR and tried the below coding to make it work.

But the output looks like a unclear saturated image with no noise reduction.

Can you please look at the code and point me where i went wrong.

Thank You

%% We are going to stack the 10 corrupted images and finally calculate the PSNR SSIM
clearvars;% Clear all the variables
close all;

load('planetdata.mat'); %to load the corrupted Image set (4-D uint8)
Clean = imread('Clean Image of Saturn.jpg');%Clean Image of Saturn.600x800x3 uint8
planet1(: , :, :)   = planetdata(1, :, :, :);%One corrupted Image as reff

% Set the number of images to stack is 10
stack_number = 10;

% Lets use Clean image as reference of dimensions required
im_x = size(Clean, 1);
im_y = size(Clean, 2);
im_z = size(Clean, 3);

% Lets Generate a blank image for image stacking
resultIM = uint8(zeros(im_x, im_y, im_z));

% Iterate through the images to stack
for i = 1:1:stack_number

% Read in the target object image
 CorruptIM(: , :, :)   = planetdata(i, :, :, :);

% Perform image stacking using the target object image
 resultIM = resultIM + CorruptIM;

end

% resultIM = resultIM / stack_number; 

%% Lets Display Results
workspace;  % to Make sure the work space panel is showing.
fontSize = 15;
figure;
subplot(1, 3, 1);
imshow(Clean);
title('Clean Image', 'FontSize', fontSize);
% Enlarge figure to full screen.
set(gcf, 'Position', get(0,'Screensize')); 
% Give a name to the title bar.
set(gcf,'name','Stacking','numbertitle','off')

% Display one corrupt image as reference
subplot(1, 3, 2);
imshow(planet1);
title('Corrupt Image 1 : Ref', 'FontSize', fontSize);

% Display Stacked image
subplot(1, 3, 3);
imshow(resultIM);
title('Stacked Image', 'FontSize', fontSize);

%% PSNR AND SSIM Calculation
%Lets Find PSNR for For Resultant Image

[row,col]   = size(Clean);
size_host   = row*col;
o_double    = double(Clean);
w_double    = double(resultIM);

s=0;
for j = 1:size_host % the size of the original image

s = s+(w_double(j) - o_double(j))^2 ; 
end

mes     =s/size_host;
psnr    =10*log10((255)^2/mes);
fprintf('The PSNR value for Stacked Image is %0.4f.\n',psnr);


%Lets Find SSIM for resultant Image
[ssimval, ssimmap] = ssim(uint8(resultIM),Clean);
fprintf('The SSIM value for Stacked Image is %0.4f.\n',ssimval);

Answer 1

Image stacking is a process by which you can reduce noise, but it doesn't work by adding the images together additively, but rather averaging them. The reason that stacking works is that signal from the same photo taken multiple times will be the same, but random noise will be different each time.

If you average the images, the noise will tend to be reduces as it is not the same between images.

Answer 2

Your noise may not be zero mean, but the larger issue is that you are adding to a uint8 array. These values go up to 255, and will overflow likely by the second image.

You should do your intermediate steps in floating point, and then later cast to uint8 and perhaps normalize if necessary.

Answer 3

Yes, this answer should be in SO, along with the question. Here goes:

Your "noise" is almost certainly not zero-mean. You are adding all the noise values in all frames, which naturally leads to an average value for each of R,G,B that is the same, i.e. a white (or grey) pixel. Your final image is unhelpfully autoscaled by imshow . Since you failed to provide the code you used to synthesize noise, I might be wrong, but please review your noise-only files and try again.

edit: adding Sean's info:

A more immediate issue, however, is that because resultIM is of type uint8 it clips values over 255. By adding 10 different copies of CorruptIM, it looks like a large number of pixels are being turned white (255)