How can I put margins in an image?

Question

I have a binary image of 18x18 pixels and I want to put margins around this image with the purpose of obtaining an image 20x20 pixels.

The image is binary and

Answer 1

Let's get hackish:

%// Data:
A = magic(3);                 %// example original image (matrix)
N = 1;                        %// margin size

%// Add margins:
A(end+N, end+N) = 0;          %// "missing" values are implicitly filled with 0
A = A(end:-1:1, end:-1:1);    %// now flip the image up-down and left-right ...
A(end+N, end+N) = 0;          %// ... do the same for the other half ...
A = A(end:-1:1, end:-1:1);    %// ... and flip back