How to do background subtraction between two images

I got many numerical frames. Please help me to revise the code to subtract the background so that I can work on the region which I am Interested (spray evolution). I also attached two pictures for your refenences.

My code:

backgroundImage = imread('animation-1_0000.png');

% Ensure the background image is grayscale

if size(backgroundImage, 3) == 3

backgroundImage = rgb2gray(backgroundImage);

end

% Check if the file exists

if isfile(currentFileName)

% Read the current image

thisImage = imread(currentFileName);

% Convert to grayscale if necessary

if size(thisImage, 3) == 3

thisImage = rgb2gray(thisImage);

end

% Debug: Print sizes before resizing

fprintf('Background image size: [%d, %d]\n', size(backgroundImage));

fprintf('Current image size: [%d, %d]\n', size(thisImage));

% Resize to match the background image size

if ~isequal(size(thisImage), size(backgroundImage))

thisImage = imresize(thisImage, size(backgroundImage));

end

% Perform subtraction

subtractionImage = backgroundImage - thisImage;

% Save the subtraction result

saveFileName = sprintf('%s/Sub/%04d.bmp', savefolder, i);

imwrite(subtractionImage, saveFileName);

% Black and White Image Processing

% Create a binary mask based on threshold

mask1 = abs(Wh) > 10; % Threshold above the noise level (10 gray levels)

% Rotate the mask image

J = imrotate(mask1, 90);

% Save the rotated mask

saveFileName = sprintf('%s/Wh/animation-1_%04d.png', savefolder, i);

imwrite(J, saveFileName);

end

Answer 1

[Ayush]

Hi Khai,

I understand that you need to subtract the background effectively. Few improvements to the current logic can be done and integrated with your specific region of interest, which is the spray evolution.

Here is how you can enhance the process:

1. The current approach directly subtracts the background from each frame, but it may create negative values or lose fine details. To avoid this, we can take the absolute value of the subtraction or apply a threshold to highlight significant differences (such as spray movement).

2. After subtraction, we can focus on the region of interest (e.g., the spray). You might want to either define a fixed region or use image segmentation to extract dynamic areas where the spray evolves.

3. To better isolate the spray, we can apply thresholding to the difference image, followed by morphological operations (like erosion or dilation) to remove noise and fill gaps in the spray's structure.

4. We will add your mask-processing logic for further refinement, ensuring only relevant areas (spray) are focused upon.

Here is pseudo code for your reference:

backgroundImage = imread('animation-1_0000.png'); 
% Ensure the background image is grayscale
if size(backgroundImage, 3) == 3
    backgroundImage = rgb2gray(backgroundImage);
end

if isfile(currentFileName)
    % Read the current image
    thisImage = imread(currentFileName);
    
    
if size(thisImage, 3) == 3
        thisImage = rgb2gray(thisImage);
    end
    
    
if ~isequal(size(thisImage), size(backgroundImage))
        thisImage = imresize(thisImage, size(backgroundImage));
    end
    
    % Perform background subtraction (absolute difference to avoid negative values)
    subtractionImage = abs(double(backgroundImage) - double(thisImage));
    
    % Threshold to remove small differences (noise removal)
    threshold = 10;  
    subtractionImage(subtractionImage < threshold) = 0;
    
    % Apply binary thresholding to focus on significant differences (e.g., spray)
    binaryImage = subtractionImage > threshold; % Create a binary mask
    
    % Apply morphological operations to clean up the binary mask (optional)
    binaryImage = imclose(binaryImage, strel('disk', 5)); % Fill small gaps
    binaryImage = imopen(binaryImage, strel('disk', 3)); % Remove small noise
    
    saveFileName = sprintf('%s/Sub/%04d.bmp', savefolder, i);
    imwrite(subtractionImage, saveFileName);
    
    % Further processing on the mask
    % Rotate the mask for consistency with your logic
    rotatedMask = imrotate(binaryImage, 90);
    
   
 saveFileName = sprintf('%s/Wh/animation-1_%04d.png', savefolder, i);
    imwrite(rotatedMask, saveFileName);
    
end

For more information on “imread”, you can refer here: https://www.mathworks.com/help/matlab/ref/imread.html

For more information on “imwrite”, you can refer here: https://www.mathworks.com/help/matlab/ref/imwrite.html

For more information on “rgb2gray”, you can refer here: https://www.mathworks.com/help/matlab/ref/rgb2gray.html

Hope it helps!