function A = lapdir(g,b)
[n m] = size(g);
nn = sum(g(:)>0);
A=sparse(nn,nn);
for i = 1:n
    for j = 1:m
        if g(i,j) > 0
            if g(i,j) > b
                A(g(i,j),g(i,j)) = 4;
                A(g(i,j),g(i-1,j)) = -1;
                A(g(i,j),g(i+1,j)) = -1;
                A(g(i,j),g(i,j-1)) = -1;
                A(g(i,j),g(i,j+1)) = -1;
            else                
                A(g(i,j),g(i,j)) = 1;
            end
        end
    end
end