process(clk,valid,x,y,Data) is begin
    if(rising_edge(clk)) then
      if(x<2) then prevDat<=256-Data; else prevDat <= Dat; end if;
      Dat <= 256 - Data;
      if(valid='1') then
        if((((y>=prevDat)and(y<=Dat))or((y>=Dat)and(y<=prevDat)))and(x<512)) then
          R <= '1';  G <= '1';  B <= '1';
        else
          R <= '0';  G <= '0';  B <= '0';
        end if;
      else
        R <= '0';  G <= '0';  B <= '0';
      end if;
    end if;
  end process;