function [] = show(mat)
   map = colormap;
   map(1,:) = [0 0 0];
   map(2,:) = [0.45 0.45 0.45];
   map(3,:) = [0.67 0.67 0.67];
   map(4,:) = [1 1 1];
   colormap(map);
   image(mat); 
   axis square;
