How can I extract all matrix element neighbors ?

I want to built a 3x3 block for a given matrix point with that point in the center of the block. This is my code

    % code
function frmBlock = fetchNeighbors(frame, row, column)
    %Create a 3x3 matrix contains the neighbors of the point(x, y)
    %[n, m] = size(frame);
    frmBlock = zeros(3, 3);

    x = floor(row);
    y = floor(column);

        frmBlock(1) = frame(x-1, y-1);
        frmBlock(2) = frame(x, y-1);
        frmBlock(3) = frame(x+1, y+1);
        frmBlock(4) = frame(x-1, y);
        frmBlock(5) = frame(x, y);
        frmBlock(6) = frame(x+1, y);
        frmBlock(7) = frame(x-1, y+1);
        frmBlock(8) = frame(x, y+1);
        frmBlock(9) = frame(x+1, y-1);

   end

Whene I run this code I got an error saying:

• Error using fetchNeighbors (line 12)Index exceeds matrix dimensions.*

Can someone help ?

Answer 1

[Thorsten]

You have to ensure that the indices are always valid, i.e., between 1 and the maximum number of rows / columns. For example, what is the output for row = 1, col = 1? In your implementation, you try to get frame(0, 0) in this case, which does not work.

You can use something like

frame(min(1, x - 1), min(1, y - 1))

frame(max(size(frame, 1), x +1, max(size(frame, 2), y + 1))

and accordingly for all instances where you refer to x/y +/- 1.

Please not that the first index into a matrix is the row that runs from top to bottom. Using variable named 'x' is bit misleading.

[sendja450]

When the index doesn't exist I want to put a zero in instead. But I don't know how to do that.

[sendja450]

As you can see, I create I 3x3 matrix initialized by 0. What I want to do is to fill that matrix with all the neighbors of the coordinate in input(row, column). If I can't get the neighbors for some reason I do nothing(i.e let that position in the 3x3 block as 0).

[sendja450]

Thanks for your answer @Thorsten, I got an answer in stackoverflow and this is the link to it. http://stackoverflow.com/questions/43523685/given-a-pixel-coordinates-how-can-i-extract-all-his-neighbors