MacPlus_MiSTer/sys/scanlines.v

module scanlines #(parameter v2=0)
(
	input             clk,

	input       [1:0] scanlines,
	input      [23:0] din,
	input             hs_in,vs_in,
	input             de_in,

	output reg [23:0] dout,
	output reg        hs_out,vs_out,
	output reg        de_out
);

reg [1:0] scanline;
always @(posedge clk) begin
	reg old_hs, old_vs;

	old_hs <= hs_in;
	old_vs <= vs_in;
	
	if(old_hs && ~hs_in) begin
		if(v2) begin
			scanline <= scanline + 1'd1;
			if (scanline == scanlines) scanline <= 0;
		end
		else scanline <= scanline ^ scanlines;
	end
	if(old_vs && ~vs_in) scanline <= 0;
end

wire [7:0] r,g,b;
assign {r,g,b} = din;

reg [23:0] d;
always @(*) begin
	case(scanline)
		1: // reduce 25% = 1/2 + 1/4
			d = {{1'b0, r[7:1]} + {2'b00, r[7:2]},
			     {1'b0, g[7:1]} + {2'b00, g[7:2]},
				  {1'b0, b[7:1]} + {2'b00, b[7:2]}};

		2: // reduce 50% = 1/2
			d = {{1'b0, r[7:1]},
				  {1'b0, g[7:1]},
				  {1'b0, b[7:1]}};

		3: // reduce 75% = 1/4
			d = {{2'b00, r[7:2]},
			     {2'b00, g[7:2]},
				  {2'b00, b[7:2]}};

		default: d = {r,g,b};
	endcase
end

always @(posedge clk) begin
	reg [23:0] dout1, dout2;
	reg de1,de2,vs1,vs2,hs1,hs2;

	dout   <= dout2; dout2 <= dout1; dout1 <= d;     
	vs_out <= vs2;   vs2   <= vs1;   vs1   <= vs_in; 
	hs_out <= hs2;   hs2   <= hs1;   hs1   <= hs_in; 
	de_out <= de2;   de2   <= de1;   de1   <= de_in; 
end

endmodule