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

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

thanks in advance, Sina

Answer 1

[Matt J]

 a(  imdilate(a==0, true(3))  )  = 4;

[Matt J]

How to change it so it look like a hexagon or octagon?

Instead of true(3), give a hexagon or octagon shaped strel to imdilate.

[Image Analyst]

Use the strel() function to generate different shaped kernels.

[Changoleon]

That is very true and quick response, thank you

Answer 2

[Adam]

[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]

Changoleon commented:

Adam,

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?