Error in matrix assignment

I am attempting to create a MXNXP image stack, where M and N are the vertical and horizontal dimensions, and P is a third dimension that represents the number of images in the stack. For one particular set of images, I keep getting an error message that says "Unable to perform assignment because the size of the left side is 128-by-128 and the size of the right side is 256-by-256." Not sure what this means, but it looks like m and n are getting multiplied by 2.. Do not know why it would be doing this. This same code works for other sets of images. Here is the code:

function num_images = img_2_stack(path)
% path = '/path/to/image/directory'
% create image stack
dir_info = dir(fullfile(path, '*.png'));
num_images = numel(dir_info)
m = 128; % vertical pixels
n = 128; % horizontal pixels
img_stack = zeros(m,n,num_images);    % initialize image stack
    for i = 1:num_images
        fn = dir_info(i).name;
        img = imread(fullfile(path, fn));
        img_stack(:,:,i) = img;
    end
end

Answer 1

[Chunru]

The image from the file "img" has a size of 256-by-256. However, you have defined the img_stack with size of 128-by-128-by-#of_images. So that you can not do the assignment operation "img_stack(:,:,i) = img" due to different sizes. You can either change m, n to be 256 (assuming all images have size of 256-by-256) or resize the image to be 128-by-128. "doc imresize" for more info.