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

Answer 2

The padarray function from the image processing toolbox can be used for this purpose:

B=padarray(A,[1,1])

Answer 3

A=ones(18,18);%// your actual image
[M,N] = size(A);
B = zeros(M+2,N+2);%// create matrix
B(2:end-1,2:end-1) = A; %// matrix with zero edge around.

This first gets the size of your image matrix, and creates a zero matrix with two additional columns and rows, after which you can set everything except the outer edges to the image matrix.

Example with a non-square matrix of size [4x6]:

B =

     0     0     0     0     0     0     0     0
     0     1     1     1     1     1     1     0
     0     1     1     1     1     1     1     0
     0     1     1     1     1     1     1     0
     0     1     1     1     1     1     1     0
     0     0     0     0     0     0     0     0

Answer 4

First make a matrix of 20 by 20 zeroes, Zimg, then insert your image matrix into the matrix of zeroes:

Zimg(2:end-1,2:end-1)=img;