N=255;     
[j,i]=meshgrid(0:N,0:N);
r=sqrt((i/N).^2+(j/N).^2); 
  % r:   is distance from left-top corner
  %      ranges from 0 to sqrt(2)

fi=1./(sqrt(2)+0.3-r)*pi*30;
IM=round((cos(fi)+1)/2*255);
figure(1);imshow(IM,[]);