Reduce total logic elements?

Hi all, at first, sorry for my bad english.

I'm currently working on Bomberman project. I have some problems and huge total logic elements is the worst.

For example, i would like to display a static map, consist of stone, brick, wall and grass (RGB 12bit), and this map uses 21% total logic elements of DE1. So how can i reduce this number to 10% or smaller?

Below is my VHDL code:

Library ieee;
Use ieee.std_logic_1164.all;
Use ieee.numeric_std.all;
Entity display_map1 is
	Port (	clk				: in std_logic;
				video_on			: in std_logic;
				brick_on			: in std_logic;
				p_row, p_col	: in std_logic_vector(9 downto 0);
				stone_on_out	: out std_logic;					
				rgb				: out std_logic_vector(11 downto 0));
End display_map1;
Architecture Behave of display_map1 is
	Type ROM_type is array (0 to 31, 0 to 31) of std_logic_vector(11 downto 0);
	Constant Stone_ROM: ROM_type := 
		(
--	32x32x12 bit ROM for Stone image
		);
	Constant Grass_ROM: ROM_type := 
		(
-- 	32x32x12 bit ROM for Grass image
		);
	Constant Wall_ROM: ROM_type	:=
		(
--	32x32x12 bit ROM for Wall image
		);
	Constant Brick_ROM: ROM_type := 
		(
--	32x32x12 bit ROM for Brick image
		);
	Signal wall_on, stone_on, grass_on, inner_map	: std_logic;
	Signal wall_rgb, stone_rgb, grass_rgb	: std_logic_vector(11 downto 0);
	Signal x, i, j, x_1, y_1, i_1, j_1	: integer;
	Signal rgb_reg	: std_logic_vector(11 downto 0);
	Signal p_row_o, p_col_o	: unsigned(9 downto 0);
	
	Begin
		x <= to_integer(unsigned(p_col(9 downto 5)));
		i <= to_integer(unsigned(p_col(4 downto 0)));
		j <= to_integer(unsigned(p_row(4 downto 0)));
		p_col_o <=	unsigned(p_col) - "0001100000" when (p_col >= "0001100000") else
						"1111011111";
		p_row_o <= 	unsigned(p_row) - "0000010000" when (p_row >= "0000010000") else
						"1111011111";
		x_1 <= to_integer(p_col_o(9 downto 5));
		y_1 <= to_integer(p_row_o(9 downto 5));
		i_1 <= to_integer(p_col_o(4 downto 0));
		j_1 <= to_integer(p_row_o(4 downto 0));	
		
		-- Inner map
		inner_map <=	'1' when (x_1 < 11) and (y_1 < 19) else
							'0';
							
		-- Display stone
		stone_on <=	'1' when (inner_map = '1') and (p_col_o(5) = '1') and (p_row_o(5) = '1') else
						'0';
		stone_on_out <= stone_on;
		stone_rgb <= Stone_ROM(i_1, j_1);
		-- Display wall				 
		wall_on <= 	'1' when (inner_map = '0') and (x > 1) else
						'0';
		wall_rgb <= Wall_ROM(i, j);
		-- Display grass when don't have wall or stone
		grass_on <= '1' when (inner_map = '1') and (stone_on = '0') else
						'0';
		grass_rgb <= Grass_ROM(i_1, j_1);
		
		-- Display color when video_on = '1'
		rgb <= 	rgb_reg when (video_on = '1') else
					(others => '0');
		rgb_reg <=	wall_rgb when (wall_on = '1') else 
					Brick_ROM(i_1, j_1) when (brick_on = '1') else
					stone_rgb when (stone_on = '1') else
					grass_rgb when (grass_on = '1') else
					(others => '1');
End Behave;

I think the worst part of this code is

		rgb_reg <=	wall_rgb when (wall_on = '1') else 
					Brick_ROM(i_1, j_1) when (brick_on = '1') else
					stone_rgb when (stone_on = '1') else
					grass_rgb when (grass_on = '1') else
					(others => '1');

This part create a lot of MUX.