How to resize a matrix repeating the original values N times...

Hi all. Im trying to do some sort of UpSample on a matrix, what I need to do is repeat each value of my original matrix N times. Example with N=3

 A =

     1     2
     3     4
A_up =
     1     1     1     2     2     2
     1     1     1     2     2     2
     1     1     1     2     2     2
     3     3     3     4     4     4
     3     3     3     4     4     4
     3     3     3     4     4     4

This is going to be used on images of 2Mpx and right now im using matrix concatenation to do it, but it takes more then 10min for each image, and I have to be able to do this in more then 800 images... If someone have a hint of how to do it faster I would be very gratefull.

[Jan]

10 minutes sounds like you forgot the pre-allocation.

[Renan]

Thank you guys, that worked really well!!

Answer 1

[Jan]

A = [1, 2; ...
     3, 4];
B = kron(A, ones(3));

[Wesley]

Thanks so much for this answer! I was looking for a way to apply a correction factor after downsampling my data!

[Voss]

@Wesley: A more modern alternative is to use repelem (introduced in R2015a):

A = [1 2; 3 4]

A = 2×2

1 2 3 4

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A_up = repelem(A,3,3)

A_up = 6×6

1 1 1 2 2 2 1 1 1 2 2 2 1 1 1 2 2 2 3 3 3 4 4 4 3 3 3 4 4 4 3 3 3 4 4 4

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[Wesley]

@KONSTANTINOS VOSKAKISThat works even better for my application considering I just learned that the Kronecker product form matlab only works for 2D tensor rather than the 4D I work with!!!

Answer 2

[Azzi Abdelmalek]

N=3  
out=cell2mat(arrayfun(@(x) ones(N)*x,A,'un',false))

Or

A=[1 2 3;4 5 6;7 8 9]
n=3
m=size(A,1)*n
out=reshape(repmat(reshape(repmat(A(:)',n,1),m,[]),n,1),m,[])