How to reduce a matrix size

I want to reduce the size of a matrix (M,N) by a fourth by averaging the values of neighboring cells.

So for instance the cells (1,1), (1,2), (2,1) and (2,2) in the old matrix will now be represented by their average which will be stored in (1,1) of the new matrix.

How can I code this?

Thanks!

Answer 1

[Jan]

X = rand(20, 30);
R = reshape(X, 2, 10, 2, 15);
S = sum(sum(R, 1), 3) * 0.25;
Y = reshape(S, 10, 15);

This can be wriiten in one line also. If more speed is needed you can use the faster C-Mex FEX: BlockMean.

Answer 2

[Christian Zitzmann]

The question is very old and propably already solved, but I've got a very nice solution as well.

X = reshape(1:400,20,20);
Y = imresize(X,[3,3]);

[Martin Grden]

Nice, it helped me.