How to use blkdiag in 3D matrix

Hi all,

I would like to know how I can use the blkdiag function for the case of a 3D matrix. In specific, what I mean is the following:

Lets say I have the two following 3D matrices

A(:,:,1) =
     1     1     1
     1     1     1
     1     1     1
A(:,:,2) =
     2     2     2
     2     2     2
     2     2     2
   %================  
B(:,:,1) =
     3     3     3
     3     3     3
     3     3     3
     
B(:,:,2) = 
     4     4     4
     4     4     4
     4     4     4

And I would like to convert those to some matrix D such that blkdiag(A,B) would give the following:

D(:,:,1) = 
     1     1     1     0     0     0
     1     1     1     0     0     0
     1     1     1     0     0     0
     0     0     0     3     3     3
     0     0     0     3     3     3
     0     0     0     3     3     3
D(:,:,2) =
     2     2     2     0     0     0
     2     2     2     0     0     0
     2     2     2     0     0     0
     0     0     0     4     4     4
     0     0     0     4     4     4
     0     0     0     4     4     4

However I find that when trying to use the blkdiag function in such context I get an error. Any other thoughts for a work around?

Thanks for your help in advance.

Answer 1

[Ameer Hamza]

Try this

A = cat(3, 1*ones(3), 2*ones(3));
B = cat(3, 3*ones(3), 4*ones(3));

C = {A, B}; % combine all matrices in a cell array for easy processing
[rows, cols, pages] = cellfun(@size, C);

M = zeros(sum(rows), sum(cols), pages(1));
for i = 1:pages(1)
    C_ = cellfun(@(x) {x(:,:,i)}, C);
    M(:,:,i) = blkdiag(C_{:});
end

Answer 2

[madhan ravi]

A slight variation:

M = cell(size(B, 3), 1);
for k = 1 : size(A, 3)
    M{k} = blkdiag(A(:, : , k), B(:, :, k));   
end
M = cat(3, M{:})