# How can I change the intensities of a certain pixel in an image along with 3*3 surrounding neighbor pixels?

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Changoleon on 21 Sep 2016
Commented: Changoleon on 21 Sep 2016
Lets say I have a image(matrix) as following:
a =
1 1 1 1 1
1 1 1 1 1
1 2 2 2 1
1 2 0 2 1
1 2 2 2 1
1 1 1 1 1
1 1 1 1 1
Finding that 0 in the middle and changing it is easy. I can do this :
a(a==0)=4;
and the result will be this:
a =
1 1 1 1 1
1 1 1 1 1
1 2 2 2 1
1 2 4 2 1
1 2 2 2 1
1 1 1 1 1
1 1 1 1 1
Now I want a code that can find the zero, convert that zero to 4, and also convert the 3*3 neighbors of zero to 4 as well. So after the code that you're suggesting, my output should look something like this:
b =
1 1 1 1 1
1 1 1 1 1
1 4 4 4 1
1 4 4 4 1
1 4 4 4 1
1 1 1 1 1
1 1 1 1 1

Matt J on 21 Sep 2016
a( imdilate(a==0, true(3)) ) = 4;
Changoleon on 21 Sep 2016
That is very true and quick response, thank you

[x, y] = find( a == 0 );
a( x-1:x+1, y-1:y+1 ) = 4;
Obviously you would need to deal with when you are on the border and there is no x-1 or y+1 or whatever, but that is easy enough for you to adapt.
Matt J on 21 Sep 2016
Changoleon commented:
Fast and accurate answer. Thanks you. I just realized that I need to change my values circularly which is impossible to make a perfect circle inside a matrix ( since you can not cover half of an element). How to change it so it look like a hexagon or octagon?