create 3*3 matrix around a given pixel

I have a centroid plotted on a blank image using the command img(a(2),a(1))=1;

I would like to create a 3*3 matrix of ones around this pixel as centroid.

Can someone tell me how i can achieve this.

Thankyou

Answer 1

[Jos (10584)]

One solution:

img = zeros(6,7)
a = [4 3]
img(a(2),a(1)) = 1
B = [1 1 1 ; 1 0 1 ; 1 1 1];
img2 = conv2(img, B ,'same')

Answer 2

[Mahendra Amrao]

img = zeros(6,7)

a = [4 3]

img(a(2),a(1)) = 1

B = [1 1 1 ; 1 0 1 ; 1 1 1];

img2 = conv2(img, B ,'same')