I meet some problems with VGA display using DE1SOC

I wrote a short verilog code to test VGA display. Vga display needs red, blue, green, vscan and hscan signal. And DE1SOC use another DAC to convert red, blue, green digital signals to analog signals. DAC needs dac_clock, scan_n and blank_n signals.

​

The code is shown below. I use a pll to convert clk_50m to clk_25m and clk_100m. Clk_50m is originate from the clock in board.

module vgatest(
input wire clk_50m,
input wire rst_n,
 
output reg [7:0] red,
output reg [7:0] green,
output reg [7:0] blue,
output wire hsync,
output wire vsync,
output wire dac_clk,
output wire dac_sync,
output wire dac_blank
);
//-------------------------------------------------//
//          640*480 60Hz VGA
//-------------------------------------------------//
/*
back porch  48    33
sync pulse  96     2
front porch 16    10
*/
parameter H_SYNC_END   = 96;     
parameter V_SYNC_END   = 2;      
parameter H_SYNC_TOTAL = 800;    
parameter V_SYNC_TOTAL = 525;    
parameter H_SHOW_START = 10'd144;    
parameter V_SHOW_START = 10'd35;     
 
integer i,j;
 
reg [9:0] x_cnt;//0~800，96+48=144~640~16
reg [9:0] y_cnt;//0~525，2+33=35~480~10
 
wire [9:0] x_pos;
wire [9:0] y_pos;
 
wire [5:0] x_mo;
wire [5:0] y_mo;
wire [3:0] x_remainder;
wire [3:0] y_remainder;
wire clk_25m;
wire clk_100m;
wire clk_200m;
reg [3:0] direction_reg;
reg [7:0] red_reg;
reg [7:0] green_reg;
reg [7:0] blue_reg;
 
assign x_pos =(x_cnt <=10'd783 && x_cnt>=H_SHOW_START)?(x_cnt - H_SHOW_START) :10'd0 ;////0~639 
assign y_pos =(y_cnt <=10'd514 && y_cnt>=V_SHOW_START)?(y_cnt - V_SHOW_START):9'd0;////0~399 
assign x_mo=x_pos%10;//0~63
assign y_mo=y_pos%10;//0~39
assign x_remainder=x_pos/10;//0~9
assign y_remainder=y_pos/10;//0~9
 
	
 
pll_clock M1(
		.refclk(clk_50m),   //  refclk.clk
		.rst(~rst_n),      //   reset.reset
		.outclk_0(clk_25m), // outclk0.clk
		.outclk_1(clk_100m),  // outclk1.clk
		.outclk_2(clk_200m)
);
 
assign dac_clk=clk_100m;
assign dac_sync=	1'b0;
assign dac_blank	= 1'b1;
 
//horizontal scan
always@(posedge clk_25m or negedge rst_n)    
	if(~rst_n) x_cnt <= 10'd0;
	else if (x_cnt == (H_SYNC_TOTAL-1)) x_cnt <= 10'd0;
	else  x_cnt <= x_cnt + 1'b1;
 
//vertical scan
always@(posedge clk_25m or negedge rst_n)    
	if(~rst_n) y_cnt <= 10'd0;
	else if (y_cnt == (V_SYNC_TOTAL-1)) y_cnt <= 10'd0;
	else if (x_cnt == (H_SYNC_TOTAL-1)) y_cnt <= y_cnt + 1'b1;
 
//H_SYNC signal
assign hsync=(x_cnt<H_SYNC_END)?1'b0:1'b1;
//V_SYNC signal
assign vsync=(y_cnt<V_SYNC_END)?1'b0:1'b1;
 
always@(posedge clk_25m, negedge rst_n)
	begin
//		red={8{1'b1}};
		if(~rst_n)
			begin
				red=8'b0;
				green=8'b0;
				blue=8'b0;
			end
 
		else if (x_pos>=10'd1 && x_pos<=10'd10)
			begin
				red<= 8'b11111111;
				green<= 8'b0;
				blue<= 8'b0;
			end
		else if (x_pos>=10'd10 && x_pos<=10'd40)
			begin
				red<= 8'b0;
				green<= {8{1'b1}};
				blue<= 8'b0;
			end
		else if (x_pos>=10'd635 && x_pos<=10'd639)
			begin
				red<= 8'b11111111;
				green<= 8'b0;
				blue<= 8'b0;
			end
 
		else
			begin
				red=8'b0;
				green=8'b0;
				blue=8'b11111111;
			end
	end
endmodule

The monitor is almosts black. In line 120, I make the default color be blue rather than black. That is my first confusion.

When I change the line 97 from

else if (x_pos>=10'd1 && x_pos<=10'd10)

to

else if (x_pos>=10'd0 && x_pos<=10'd10)

The result confuse me again.

​

Blue part is shown correctly, but the red part is missing.

I tried to solve this problem these days, but I don't know why. Now that the monitor display vga signal, I believe vscan and hscan is correct. The problem may among r,g,b signal. I really don't know the reason.