Sum/Average every group of N elements-CodePudding

I have a 720 x 1280 matlab array. I need to sum every 5x5, 10x10, 20x20 and then 40x40 sub array within that and then take the average. This then needs to make 4 new arrays with size (720/5, 1280/5), (720/10, 1280/10),(720/20, 1280/20),(720/40, 1280/40), How do I do this with for loops?

CodePudding user response：

As @bla mentioned, traditionally this would not be done using for loops in MATLAB. Regardless, here is an example:

setup

data = rand(720, 1280);
windowSize = 5
out = nan(size(data)/windowSize);

Here we iterate over each block, where the limits of the block are defined by ((ii-1)*windowSize 1) and (ii*windowSize) (e.g. 1:5, 6:10, 11:15).

for ii = 1:size(data, 1)/windowSize
    for jj = 1:size(data, 2)/windowSize
         currentData = data(  ((ii-1)*windowSize 1):(ii*windowSize)  ,...
             ((jj-1)*windowSize 1):(jj*windowSize)  );
         out(ii,jj) = mean(currentData(:));
    end
end

Changing the window size can also be automated using a for loop, but I will leave this as a homework problem.

CodePudding user response：

For this kind of problems, I feel like converting the matrix into a cell array of blocks and then using cellfun on the blocks makes for a nice and readable code :

data = rand(720, 1280);
[sz1,sz2] = size(data);
wSize = 5;

idx1 = wSize*ones(1,sz1/wSize);
idx2 = wSize*ones(1,sz2/wSize);

% Convert input matrix to a cell array of wSize x wSize blocks
C = mat2cell(data,idx1,idx2);

% Apply the "Take the mean of all elements" function to each blocks
out = cellfun(@(X) mean(X(:)),C);

Note that cellfun, by default, directly concatenates the outputs into an array.