How can I make a joint histogram of 3D matrices?

Hello everybody,

I'm trying to make a joint histogram of 3D matrices. Does anybody have any idea how to make it. I applied this code to a 2D matrix, but the output is a black image.

function h=joint_histogram(image_1,image_2)

rows=size(image_1,1);
cols=size(image_1,2);
N=256;

h=zeros(N,N);

for i=1:rows;    %  col 
  for j=1:cols;   %   rows
    h(image_1(i,j)+1,image_2(i,j)+1)= h(image_1(i,j)+1,image_2(i,j)+1)+1;
  end
end