function rotatecolor(R,a,n)
    fi = linspace(0,2*pi-2*pi/n,n);
    x = R*cos(fi);
    y = R*sin(fi);
    h = zeros(n);
    for i=1:n
      h(i) =  square(x(i),y(i),a);
    endfor
    drawnow()
    circulate(h, n);  
  endfunction
   
  function circulate(h, N)
    n = length(h);
    i = 1; k = 0;
    while k < N
      c = get(h(i), "facecolor");
      set(h(i),"facecolor","red");
      drawnow()
      fname = sprintf("frame%03d.png", i);
      print(fname, "-dpng","-S300,300");
      set(h(i),"facecolor",c);
      i = mod(i+1, n+1);
      if i == 0
        i = 1;
      endif
      k = k+1;
    end
  endfunction
  
  function h = square(x,y,a)
    a2 = a/2;
    x = [x-a2,x+a2,x+a2, x-a2, x-a2];
    y = [y-a2,y-a2,y+a2, y+a2, y-a2];
    h = patch(x,y,"yellow");
  endfunction