Making a video - quality issue

I have 100 matrices that I am drawing with the function imagesc(). I am making a movie from these images.

This is my code

   myObj=VideoWriter('test.avi');
   open(myObj);
   for i=1:100
         imagesc(F(:,:,i));
         drawnow
         frame=getframe(figure(1));
         writeVideo(myObj,frame);
     end
  close(myObj)

This works fine and it does produce wanted .avi file, but the quality is quite poor. Any suggestions?

Many thanks!