//The image buffers 
        // are not displayed directly, they should be copied to or 
        // combined somehow with the rbuf_window(). rbuf_window() is
        // the only buffer that can be actually displayed.
        rendering_buffer& rbuf_window()          { return m_rbuf_window; } 
        rendering_buffer& rbuf_img(unsigned idx) { return m_rbuf_img[idx]; } 
        
//=======
  
        void copy_img_to_window(unsigned idx)
        {
            if(idx < max_images && rbuf_img(idx).buf())
            {
                rbuf_window().copy_from(rbuf_img(idx));
            }
        }
        
        //--------------------------------------------------------------------
        void copy_window_to_img(unsigned idx)
        {
            if(idx < max_images)
            {
                create_img(idx, rbuf_window().width(), rbuf_window().height());
                rbuf_img(idx).copy_from(rbuf_window());
            }
        }
       
        //--------------------------------------------------------------------
        void copy_img_to_img(unsigned idx_to, unsigned idx_from)
        {
            if(idx_from < max_images && 
               idx_to < max_images && 
               rbuf_img(idx_from).buf())
            {
                create_img(idx_to, 
                           rbuf_img(idx_from).width(), 
                           rbuf_img(idx_from).height());
                rbuf_img(idx_to).copy_from(rbuf_img(idx_from));
            }
        }