Selecting specific region around the known indices of a matrix.

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Hi,
I have a path around which I want to select 3 points above and 3 points below to acheive something similar to the picture. Basically I want to select the points that lie within 3 radius points. I have attached the MAT file which includes the indices locations around which I want to select points. It also has the matrix from which the figure can be ploted. Orginal matrix has the same dimensions. untitled.png

Accepted Answer

Thiago Henrique Gomes Lobato
Edited: Thiago Henrique Gomes Lobato on 3 Nov 2019
You can dilate the image with a disk kernel and then take the difference to get the indexes:
A = load('matlab.mat');
h = fspecial('disk',3 ); % Disk element for dilatation
ADilatated = imdilate(A.crack,h); %
PointsAround = ADilatated-A.crack; % Difference to get the new points
IndexAroud = find(PointsAround~=0); % This is the array with the indexes around it
figure
subplot(1,3,1)
imshow(A.crack)
title('Original Image')
subplot(1,3,2)
imshow(ADilatated)
title('Dilated Image')
subplot(1,3,3)
imshow(PointsAround)
title('Points in a 3-pixel radius')
In case you don't want a disk (radius) but rather a horizontal/vertical line you can just change the kernel, some examples can be founded here https://de.mathworks.com/help/images/ref/imdilate.html
  7 Comments
Thiago Henrique Gomes Lobato
Not in fspecial as far as I know, but you can achieve a similar effect using a radius 4 and then setting the first row from h to zero, like the example I gave you in the last comment. If I were to convert my example with your notation what I did would be a [3,1].

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