MATLAB: [M, N, ~] = size(im) why we use

digital image processing

[M, N, ~] = size(im) why we use

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They just want to know the number of rows and columns and don't care how many higher dimensions there are.

For images that may possibly be gray scale or color, I do this:

[rows, columns, numberOfColorChannels] = size(yourImage)

Note how I used more descriptive variable names that your code did. For grayscale, numberOfColorChannels = 1. For RGB, numberOfColorChannels = 3.

For a thorough explanation see Steve's blog: https://blogs.mathworks.com/steve/2011/03/22/too-much-information-about-the-size-function/

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MATLAB: How to use Two-Dimensional True Compression on arbitrarly sized images

Since wcompress works only on images of sizes with power of 2, you could either

resize the images to a particular size like (256, 512 etc)
pad the images with zeros

You can use padding if you do not want any loss of information in the images, as resizing might change the original information in the images.

im = imread('path/to/image');
% Resize
new_im = imresize(im, [256, 256]);
% Padding
[m, n] = size(im,1:2);
% m,n should be less than 256, else use a number greater than 256
new_m = (256-m)/2; 
new_n = (256-n)/2; 
new_im = padarray(im, [new_m, new_n]);

For more information on imresize and padarray, you can refer to the following links:

https://www.mathworks.com/help/images/ref/imresize.html

https://www.mathworks.com/help/images/ref/padarray.html

MATLAB: Making two images of equal dimensions in MATLAB

imresize() perhaps.

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MATLAB: How to use Two-Dimensional True Compression on arbitrarly sized images

MATLAB: Making two images of equal dimensions in MATLAB

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