Implement a function for image correlation

Hi, I tried to implement a function, which can calculate the correlation of a matrix within a pattern operator function.

function[output] = correlation[input,pattern]
   % Calculate the half side lenghts of the operator pattern.
    h1 = (length(pattern)-1)/2;
    h2 = h1;

      % Implement the correlation. 
      [m,n] = size(input);
      for i = 1:m
          for j = 1:n
              output(i,j) = 0
              for u = -h1:h1
                  for v = -h2:h2
                      output(i,j) = output(i,j)+pattern(u,v)*input(i+u,j+v);
                  end
          end
      end
  end

I want to calculate the inner values and setting the outer values to zeros. But i was stuck by the negative index, Any suggestion?