Enlarge matrix repeating its values

Question

I need to enlarge a matrix A to a matrix B with size n times the size of A. The values must be repeated, eg:

A size 2x3, n = 3, leads to B size 6x9:

Sample values:

A =  
1 2 3  
4 5 6

Results with:

B =  
1 1 1 2 2 2 3 3 3    
1 1 1 2 2 2 3 3 3  
1 1 1 2 2 2 3 3 3  
4 4 4 5 5 5 6 6 6  
4 4 4 5 5 5 6 6 6   
4 4 4 5 5 5 6 6 6

What is the fastest way to achieve that in Matlab?

one more option is the `repelem` - repeat elements in an Array. — GameOfThrows, Jun 02 '16 at 13:54

score 5 · Answer 1 · answered Jun 02 '16 at 13:49

5

There is also the Kronecker Tensor Product (kron) function:

n = 3;
B = kron(A,ones(n));

B =

     1     1     1     2     2     2     3     3     3
     1     1     1     2     2     2     3     3     3
     1     1     1     2     2     2     3     3     3
     4     4     4     5     5     5     6     6     6
     4     4     4     5     5     5     6     6     6
     4     4     4     5     5     5     6     6     6

answered Jun 02 '16 at 13:49

Benoit_11

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ah you beat me to it. Nice one! – GameOfThrows Jun 02 '16 at 13:52
These days you can just use the builtin [`repelem`](http://www.mathworks.com/help/matlab/ref/repelem.html) function – Dan Jun 02 '16 at 14:08
oh sweet I did not know about this one – Benoit_11 Jun 02 '16 at 15:04

Suever · Accepted Answer · 2016-06-02T13:58:09.600

2

If you have the Image Processing Toolbox you can easily do this using imresize with nearest neighbor interpolation.

A = [1 2 3; 4 5 6];

% Repeat each element 3 times in each direction
B = imresize(A, 3, 'nearest');

%   1     1     1     2     2     2     3     3     3
%   1     1     1     2     2     2     3     3     3
%   1     1     1     2     2     2     3     3     3
%   4     4     4     5     5     5     6     6     6
%   4     4     4     5     5     5     6     6     6
%   4     4     4     5     5     5     6     6     6

If you don't have the Image Processing Toolbox, you can use interp2 with nearest neighbor interpolation to do something similar.

scaleFactor = 3;


[xx,yy] = meshgrid(linspace(1, size(A, 2), size(A, 2) * scaleFactor), ...
                   linspace(1, size(A, 1), size(A, 1) * scaleFactor));


B = interp2(A, xx, yy, 'nearest');

edited Jun 02 '16 at 13:58

answered Jun 02 '16 at 13:44

Suever

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Check out [`repelem`](http://www.mathworks.com/help/matlab/ref/repelem.html) – Dan Jun 02 '16 at 14:09
@Dan, thanks I will. – Pedro77 Jun 02 '16 at 14:40
1

@Pedro77 As far as speed, I think that `kron` is going to be hard to beat. – Suever Jun 02 '16 at 14:46

Enlarge matrix repeating its values

2 Answers2