improve speed --- image matrix replace

i have a code which has three "for". it's so slow. 
if nx=2000,ny=2000. it takes me two minutes to finish replacement.
xfig(nx,ny,3) is a image matrix.
please help me!!

w0=xfig;
  nx=length(xfig(:,1,1));%coordinate x
  ny=length(xfig(1,:,1));%coordinate y
  w1=w0;
  for k=1:4
       for x=1:nx
          for y=1:ny
              w1(x,y,:)=w0(mod1(x+y,nx),mod1(x+2*y,ny),:);
          end;
      end;
  %t=1:1:nx;v=1:1:ny;
  %w1(t,v,:)=w0(mod1(t+v,nx),mod1(t+2*v,ny),:);
  w0=w1;
  end;

% if mod(8,4)=0 then it isn't 0, instead of 4
function T=mod1(x,y)
if mod(x,y)==0 
    T=y;
else
    T=mod(x,y);
end
end

[Sean de Wolski]

Have you used the profiler to run your code and identify bottlenecks?

Answer 1

[David Sanchez]

1st: initialize w1

w1 = zeros(nx,ny,3);

The two inner loops do not make use of k, the outer loop index. Why do you have them there? Take them out.

       for x=1:nx
          for y=1:ny
              w1(x,y,:)=w0(mod1(x+y,nx),mod1(x+2*y,ny),:);
          end;
      end;

what's this for?

for k=1:4
%t=1:1:nx;v=1:1:ny;
%w1(t,v,:)=w0(mod1(t+v,nx),mod1(t+2*v,ny),:);
w0=w1;
end;

[Bob]

the outer loops k=1:4 is also important,it means iteration of the w1. I must iterate w1 for more than 50 times. so the speed must improve. now if we run the code

     for x=1:nx
          for y=1:ny
              w1(x,y,:)=w0(mod1(x+y,nx),mod1(x+2*y,ny),:);
          end;
      end;

20 seconds is necessary. it's so bad. please help me!! i have already had no idea!!