Please correct my code : rgb2gray image conversion using VHDL code

Please correct my code : rgb2gray image conversion using VHDL code. I need in help in VHDL coding. This code is not working.

entity colorfilter is Port ( RI : in STD_LOGIC_VECTOR (7 downto 0); GI : in STD_LOGIC_VECTOR (7 downto 0); BI : in STD_LOGIC_VECTOR (7 downto 0); RTarget : in STD_LOGIC_VECTOR (7 downto 0); GTarget : in STD_LOGIC_VECTOR (7 downto 0); BTarget : in STD_LOGIC_VECTOR (7 downto 0); O : out STD_LOGIC); end colorfilter;

 architecture Behavioral of colorfilter is
  signal sqrtsum : std_logic_vector(17 downto 0);
  signal threshold : std_logic_vector(17 downto 0) : ="000000001100000000"
  begin
  process(RI, BI, GI)
  varible Rdiff, Gdiff, Bdiff : std_logic_vector(17 downto 0);
  variable sum : std_logic_vector(17 downto 0);
  variable R2, G2, B2 : std_logic_vector(15 downto 0);
  begin
 if (RI > RTarget) then
  Rdiff := RI-Rtarget;
  else
 Rdiff := Rtarget-RI;
  end if;
  if (GI > GTarget) then
 Gdiff := GI - GTarget;
colorfilter.vhd  
 else
  end if;
 if (BI > BTarget) then
  Bdiff := BI - BTarget;
  else
 Bdiff := BTarget - BI;
  end if;
  R2 := Rdiff*Rdiff;
  G2 := Gdiff*Gdiff;
  B2 := Bdiff*Bdiff;
 sqrtsum <= ("00"&R2) + ("00"&G2) + ("00"&B2);
  if (sqrtsum > threshold) then
  O <= '0';
 else
  O <= '1';
  end if;
  end process;
  end Behavioral;