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`timescale 1ns / 1ps

//////////////////////////////////////////////////////////////////////////////////

// Company: 

// Engineer: Yann Vernier

// 

// Create Date:    21:37:32 02/19/2011 

// Design Name: 

// Module Name:    vector2scanline 

// Project Name: PDP-1

// Target Devices: Spartan 3A

// Tool versions: 

// Description: Converts vector data (exposed points) into raster video

//

// Dependencies: 

//

//////////////////////////////////////////////////////////////////////////////////

module vector2scanline(

                       input clk,          // clock

                       input strobe,       // new exposed pixel trigger

                       input [9:0] x,      // column of exposed pixel

                       input [9:0] y,      // row of exposed pixel

                       // Video output interface

                       input [9:0] xout,   // current pixel column

                       input [9:0] yout,   // current pixel row

                       input newline,      // line buffer swap signal

                       input newframe,     // new frame signal

                       output [7:0] pixel  // output pixel intensity

                       );

   // TODO: figure out how to apply parameters to the port sizes above

   parameter X_WIDTH = 10;     // bit width of column coordinate

   parameter Y_WIDTH = 10;     // bit width of row coordinate

   parameter HIST_WIDTH = 10;  // log2 of maximum lit pixels (exposure buffer)

   parameter AGE_WIDTH = 8;    // width of exposure age counter

   // positions and age of lit pixels

   reg [X_WIDTH+Y_WIDTH+AGE_WIDTH-1:0] exposures [(2**HIST_WIDTH)-1:0];

   // output register of exposed pixels buffer

   reg [X_WIDTH+Y_WIDTH+AGE_WIDTH-1:0] expr;

   // data for next pixel to store in exposure buffer

   wire [X_WIDTH-1:0]                   expx;

   wire [Y_WIDTH-1:0]                   expy;

   wire [7:0]                           expi;

   // whether this pixel needs to be stored back in exposure buffer

   wire                                exposed;

   // addresses for exposure buffer read and write ports

   reg [HIST_WIDTH-1:0]         exprptr=0, expwptr=0;

   // scanline pixel buffers, store intensity

   // double-buffered; one gets wiped as it is displayed

   // the other gets filled in with current exposures

   reg [AGE_WIDTH-1:0]                  scanline0 [(2**X_WIDTH)-1:0];

   reg [AGE_WIDTH-1:0]                  scanline1 [(2**X_WIDTH)-1:0];

   // output intensity for current pixel

   reg [AGE_WIDTH-1:0]                  pixelout;

   // selection register for which scanline buffer is output/filled in

   reg                                 bufsel = 0;

   // address lines for scanline buffers

   wire [X_WIDTH-1:0]                   sl0w, sl1w;

   // RAM read out of exposure buffer

   always @(posedge clk) begin

      expr<=exposures[exprptr];

      if (!strobe) begin

         // do not skip current read position if strobe inserts a new pixel

         exprptr<=exprptr+1;

      end

   end

   // decode and mux: split fields from exposure buffer, or collect new at strobe

   assign expx = strobe?x:expr[X_WIDTH+Y_WIDTH+AGE_WIDTH-1:Y_WIDTH+AGE_WIDTH];

   assign expy = strobe?y:expr[Y_WIDTH+AGE_WIDTH-1:AGE_WIDTH];

   assign expi = strobe?(2**AGE_WIDTH)-1:expr[AGE_WIDTH-1:0];

   // detect whether pixel even needs to be stored back

   assign exposed = expi!=0;

   always @(posedge clk) begin

      // Feed incoming exposures into exposure buffer

      if (exposed) begin

         exposures[expwptr] <= {expx, expy, expy==yout?expi-1:expi};

         expwptr <= expwptr+1;

      end

   end

   // scanline buffers switch output or expose roles based on bufsel

   assign sl0w=bufsel?expx:xout;

   assign sl1w=bufsel?xout:expx;

   always @(posedge clk) begin

      // Read out front buffer

      pixelout <= bufsel?scanline1[sl1w]:scanline0[sl0w];

      // TODO: use a next line input port, this incrementer won't work for

      // line 0 (which is unused in display.vhd) and could be shared.

      if (expy==(y+1)) begin

         // Store exposures for current scanline and wipe front buffer

         scanline0[sl0w] <= bufsel?expi:0;

         scanline1[sl1w] <= bufsel?0:expi;

      end

      // swap buffers when signaled

      if (newline) begin

         bufsel <= ~bufsel;

      end

   end

   // output pixel intensity

   assign pixel = pixelout;

endmodule