Select pixels from a matrix given the centers ?

Suppose we have a given matrix (I) and a structure containing some centers of the matrix before.

For example:

In this case, I is a 9x9 matrix and we have the centers: a22, a63, a45, a26. Normally the number of centers is a square number.

Suppose we want to select all the pixels far from the centers of +1 position on the rows and +3 positions on the columns

The new matrix (A) will be:

My code is:

for c = 1:n
    for r = 1:n
        
        uno=Struttu.x(c + ( (r-1) * n));
        due=Struttu.y(c + ( (r-1) * n));
        
        A(r,c) = (I(  uno  +3  , due +1));
    end
end

where n*n is the total number of centers.

This code works fine, but is there a faster way to implement it?

Answer 1

[DGM]

For addressing scattered points, use sub2ind(). Here's an example.

% this is your array
M = reshape(0:99,10,10)

M = 10×10

0 10 20 30 40 50 60 70 80 90 1 11 21 31 41 51 61 71 81 91 2 12 22 32 42 52 62 72 82 92 3 13 23 33 43 53 63 73 83 93 4 14 24 34 44 54 64 74 84 94 5 15 25 35 45 55 65 75 85 95 6 16 26 36 46 56 66 76 86 96 7 17 27 37 47 57 67 77 87 97 8 18 28 38 48 58 68 78 88 98 9 19 29 39 49 59 69 79 89 99

% these are the "centers" [row col]
% note that indexing is not zero-based in MATLAB
A = [2 2;
    6 3;
    4 5;
    2 6]+1;

% get sizes
sz = size(M);

% these are the array values at those positions
Acenter = M(sub2ind(sz,A(:,1),A(:,2)))

Acenter = 4×1

22 36 54 62

% these are the array values at a given offset
% you'll have to decide how to handle cases where offset position
% is outside the array boundaries.  I'm just clamping them.
offset = [3 1]; % [row col]
samplerows = min(max(A(:,1) + offset(1),1),sz(1));
samplecols = min(max(A(:,2) + offset(2),1),sz(2));
Aoffset = M(sub2ind(sz,samplerows,samplecols))

Aoffset = 4×1

35 49 67 75

Answer 2

[Image Analyst]

Yes, you can use meshgrid. Code is in the FAQ: https://matlab.fandom.com/wiki/FAQ#How_do_I_create_a_circle?

Your x and y have to be column and row indexes of course.

Let me know if you can't figure it out.

[MementoMori]

I have read the examples and I have some ideas, but I don't understand how meshgrind can be useful

[Image Analyst]

So do you want a list of all pixels in an ellipse 6 pixels wide and 2 pixels tall? Or do you want a rectangular matrix of all those inside the box, like

inBox = m(row-1:row+1, col-3 : col+3); % Rectangular matrix

[MementoMori]

No, I want only the pixel that is in the position (+3,+1) with respect to the center

[Image Analyst]

centerRow = ceil(size(m, 1)/2);
centerCol = ceil(size(m, 2)/2);
youWant = m(centerRow + 3, centerCol + 1)

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