context_c.clearRect(0, 0, screen_width, screen_height);
  context_c.drawImage(canvas_b, -1, 0);
  // Patch emptiness with fresh tiles here.
  context_b.clearRect(0, 0, screen_width, screen_height);
  context_b.drawImage(canvas_c, 0, 0); // For use next frame.
  context_a.drawImage(canvas_c, 0, 0); // Draw to main canvas.