Extracting a 3x3 matrix from an image

Question

I need to extract a 3x3 matrix from an image and store it separately, and it should continue like this till the end of image. For example, my image is the following matrix:

p = [ 1 2 3 4 5 6 7 8 9; ...
      1 2 3 4 5 6 7 8 9; ...
      1 2 3 4 5 6 7 8 9; ...
      1 2 3 4 5 6 7 8 9; ...
      1 2 3 4 5 6 7 8 9; ...
      1 2 3 4 5 6 7 8 9 ]

The output should be like this:

p1 = [1 2 3; 1 2 3; 1 2 3]
p2 = [4 5 6; 4 5 6; 4 5 6]
p3 = [7 8 9; 7 8 9; 7 8 9]

and so on....

Can you please suggest me code or a built-in function for this?

@ZoranPlesivčak MATLAB interprets matrix definitions just fine without ellipses. They are superfluous in this case. — Eitan T, Jun 17 '13 at 16:09
@EitanT I didn't know that... Maybe you should revert my change. — plesiv, Jun 17 '13 at 16:14
@ZoranPlesivčak It doesn't really matter. You can do it too, if you want. — Eitan T, Jun 17 '13 at 16:18
-1: I don't agree with the initial upvote. *Can you please suggest me code or a built-in function* shows lack of research effort. In addtion: Why do you have to extract the matrix and generate additional matrices? Why is indexing and performing computations in-place not an option? — Schorsch, Jun 17 '13 at 16:23
possible duplicate of [How to divide a matrix into equals parts?](http://stackoverflow.com/questions/1817174/), [divide the image into 3*3 blocks](http://stackoverflow.com/questions/9972684/), [Divide a matrix into submatrices in MATLAB](http://stackoverflow.com/questions/12554522/) and many more... — Eitan T, Jun 17 '13 at 16:30

plesiv · Answer 1 · 2013-06-19T11:26:27.947

1

Simplest way to extract submatrices:

p1 = img(1:3, 1:3);
p2 = img(4:6, 4:6);
p3 = img(7:9, 7:9);

Function for doing slicing.

function imgsl = img_slice( img, ry, rx )
    [ Y X ] = meshgrid(ry, rx);
    imgsl = reshape(img(sub2ind(size(img),Y(:),X(:))),[length(rx) length(ry)]).';

Use it as follows:

p1 = img_slice( p, 1:3, 1:3 );
p2 = img_slice( p, 4:6, 4:6 );
p3 = img_slice( p, 7:9, 7:9 );

edited Jun 19 '13 at 11:26

answered Jun 17 '13 at 15:57

plesiv

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score 0 · Answer 2 · edited Jun 17 '13 at 16:25

0

The image processing toolbox has a im2col function (link).

B = im2col(p, [3 3], 'distinct');

Then each column of B is a 3x3 block made into a vector, and you can use reshape(B(:, i), 3, 3) to recover the i-th 3x3 block. Note there are zeros padded to p.

edited Jun 17 '13 at 16:25

Eitan T

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answered Jun 17 '13 at 16:04

Da Kuang

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Extracting a 3x3 matrix from an image

2 Answers2