Image processing on FPGA using Verilog HDL

This FPGA project is aimed to show in details how to process an image using Verilog from reading an input bitmap image (.bmp) in Verilog, processing and writing the processed result to an output bitmap image in Verilog. The full Verilog code for reading image, image processing, and writing image is provided.

In this FPGA Verilog project, some simple processing operations are implemented in Verilog such as inversion, brightness control and threshold operations. The image processing operation is selected by a "parameter.v" file and then, the processed image data are written to a bitmap image output.bmp for verification purposes. The image reading Verilog code operates as a Verilog model of an image sensor/ camera, which can be really helpful for functional verifications in real-time FPGA image processing projects. The image writing part is also extremely useful for testing as well when you want to see the output image in BMP format. In this project, I added some simple image processing code into the reading part to make an example of image processing, but you can easily remove it to get raw image data. All the related questions asked by students are answered at the bottom of this article.

First of all, Verilog cannot read images directly. To read the .bmp image on in Verilog, the image is required to be converted from the bitmap format to the hexadecimal format. Below is a Matlab example code to convert a bitmap image to a .hex file. The input image size is 768x512 and the image .hex file includes R, G, B data of the bitmap image.

b=imread('kodim24.bmp'); % 24-bit BMP image RGB888 

k=1;
for i=512:-1:1 % image is written from the last row to the first row
for j=1:768
a(k)=b(i,j,1);
a(k+1)=b(i,j,2);
a(k+2)=b(i,j,3);
k=k+3;
end
end
fid = fopen('kodim24.hex', 'wt');
fprintf(fid, '%x\n', a);
disp('Text file write done');disp(' ');
fclose(fid);
% fpga4student.com FPGA projects, Verilog projects, VHDL projects

To read the image hexadecimal data file, Verilog uses this command: $readmemh or $readmemb if the image data is in a binary text file. After reading the image .hex file, the RGB image data are saved into memory and read out for processing.

Below is the Verilog code to the image reading and processing part:

/******************************************************************************/
/******************  Module for reading and processing image     **************/
/******************************************************************************/
`include "parameter.v"       // Include definition file
// fpga4student.com: FPGA projects for students

// FPGA project: Image processing in Verilog
module image_read
#(
  parameter     WIDTH  = 768,   // Image width
         HEIGHT  = 512,   // Image height
  INFILE  = "./img/kodim01.hex",  // image file
  START_UP_DELAY = 100, // Delay during start up time
  HSYNC_DELAY = 160,// Delay between HSYNC pulses 
  VALUE= 100, // value for Brightness operation
  THRESHOLD= 90, // Threshold value for Threshold operation
  SIGN=1         // Sign value using for brightness operation
  // SIGN = 0: Brightness subtraction           

  // SIGN = 1: Brightness addition
)
(
  input HCLK,          // clock     
  input HRESETn,         // Reset (active low)
  output VSYNC,        // Vertical synchronous pulse
 // This signal is often a way to indicate that one entire image is transmitted.
 // Just create and is not used, will be used once a video or many images are transmitted.
  output reg HSYNC,        // Horizontal synchronous pulse
 // An HSYNC indicates that one line of the image is transmitted.
 // Used to be a horizontal synchronous signals for writing bmp file.
  output reg [7:0]  DATA_R0,  // 8 bit Red data (even)
  output reg [7:0]  DATA_G0,  // 8 bit Green data (even)
  output reg [7:0]  DATA_B0,  // 8 bit Blue data (even)
  output reg [7:0]  DATA_R1,  // 8 bit Red  data (odd)
  output reg [7:0]  DATA_G1,  // 8 bit Green data (odd)
  output reg [7:0]  DATA_B1,  // 8 bit Blue data (odd)
  // Process and transmit 2 pixels in parallel to make the process faster, you can modify to transmit 1 pixels or more if needed
  output     ctrl_done     // Done flag
);   
//-------------------------------------------------
// Internal Signals
//-------------------------------------------------
parameter sizeOfWidth = 8;   // data width
parameter sizeOfLengthReal = 1179648;   // image data : 1179648 bytes: 512 * 768 *3 
// local parameters for FSM
localparam  ST_IDLE  = 2'b00,// idle state
   ST_VSYNC = 2'b01,// state for creating vsync 
   ST_HSYNC = 2'b10,// state for creating hsync 
   ST_DATA  = 2'b11;// state for data processing 
reg [1:0] cstate,   // current state
nstate;    // next state   
reg start;         // start signal: trigger Finite state machine beginning to operate
reg HRESETn_d;   // delayed reset signal: use to create start signal
reg   ctrl_vsync_run; // control signal for vsync counter  
reg [8:0] ctrl_vsync_cnt; // counter for vsync
reg   ctrl_hsync_run; // control signal for hsync counter
reg [8:0] ctrl_hsync_cnt; // counter  for hsync
reg   ctrl_data_run; // control signal for data processing
reg [7 : 0]   total_memory [0 : sizeOfLengthReal-1];// memory to store  8-bit data image
// temporary memory to save image data : size will be WIDTH*HEIGHT*3
integer temp_BMP   [0 : WIDTH*HEIGHT*3 - 1];   
integer org_R  [0 : WIDTH*HEIGHT - 1];  // temporary storage for R component
integer org_G  [0 : WIDTH*HEIGHT - 1]; // temporary storage for G component
integer org_B  [0 : WIDTH*HEIGHT - 1]; // temporary storage for B component
// counting variables
integer i, j;
// temporary signals for calculation: details in the paper.
integer tempR0,tempR1,tempG0,tempG1,tempB0,tempB1; // temporary variables in contrast and brightness operation

integer value,value1,value2,value4;// temporary variables in invert and threshold operation
reg [ 9:0] row; // row index of the image
reg [10:0] col; // column index of the image
reg [18:0] data_count; // data counting for entire pixels of the image
//-------------------------------------------------//
// -------- Reading data from input file ----------//
//-------------------------------------------------//
initial begin
    $readmemh(INFILE,total_memory,0,sizeOfLengthReal-1); // read file from INFILE
end
// use 3 intermediate signals RGB to save image data
always@(start) begin
    if(start == 1'b1) begin
        for(i=0; i<WIDTH*HEIGHT*3 ; i=i+1) begin
            temp_BMP[i] = total_memory[i+0][7:0]; 
        end
        
        for(i=0; i<HEIGHT; i=i+1) begin
            for(j=0; j<WIDTH; j=j+1) begin

     // Matlab code writes image from the last row to the first row

     // Verilog code does the same in reading to correctly save image pixels into 3 separate RGB mem
                org_R[WIDTH*i+j] = temp_BMP[WIDTH*3*(HEIGHT-i-1)+3*j+0]; // save Red component
                org_G[WIDTH*i+j] = temp_BMP[WIDTH*3*(HEIGHT-i-1)+3*j+1];// save Green component
                org_B[WIDTH*i+j] = temp_BMP[WIDTH*3*(HEIGHT-i-1)+3*j+2];// save Blue component
            end
        end
    end
end
//----------------------------------------------------//
// ---Begin to read image file once reset was high ---//
// ---by creating a starting pulse (start)------------//
//----------------------------------------------------//
always@(posedge HCLK, negedge HRESETn)
begin
    if(!HRESETn) begin
        start <= 0;
  HRESETn_d <= 0;
    end
    else begin           //          ______     
        HRESETn_d <= HRESETn;       //        |  |
  if(HRESETn == 1'b1 && HRESETn_d == 1'b0)  // __0___| 1 |___0____ : starting pulse
   start <= 1'b1;
  else
   start <= 1'b0;
    end
end
//-----------------------------------------------------------------------------------------------//
// Finite state machine for reading RGB888 data from memory and creating hsync and vsync pulses --//
//-----------------------------------------------------------------------------------------------//
always@(posedge HCLK, negedge HRESETn)
begin
    if(~HRESETn) begin
        cstate <= ST_IDLE;
    end
    else begin
        cstate <= nstate; // update next state 
    end
end
//-----------------------------------------//
//--------- State Transition --------------//
//-----------------------------------------//
// IDLE . VSYNC . HSYNC . DATA
always @(*) begin
 case(cstate)
  ST_IDLE: begin
   if(start)
    nstate = ST_VSYNC;
   else
    nstate = ST_IDLE;
  end   
  ST_VSYNC: begin
   if(ctrl_vsync_cnt == START_UP_DELAY) 
    nstate = ST_HSYNC;
   else
    nstate = ST_VSYNC;
  end
  ST_HSYNC: begin
   if(ctrl_hsync_cnt == HSYNC_DELAY) 
    nstate = ST_DATA;
   else
    nstate = ST_HSYNC;
  end  
  ST_DATA: begin
   if(ctrl_done)
    nstate = ST_IDLE;
   else begin
    if(col == WIDTH - 2)
     nstate = ST_HSYNC;
    else
     nstate = ST_DATA;
   end
  end
 endcase
end
// ------------------------------------------------------------------- //
// --- counting for time period of vsync, hsync, data processing ----  //
// ------------------------------------------------------------------- //
always @(*) begin
 ctrl_vsync_run = 0;
 ctrl_hsync_run = 0;
 ctrl_data_run  = 0;
 case(cstate)
  ST_VSYNC:  begin ctrl_vsync_run = 1; end  // trigger counting for vsync
  ST_HSYNC:  begin ctrl_hsync_run = 1; end // trigger counting for hsync
  ST_DATA:  begin ctrl_data_run  = 1; end // trigger counting for data processing
 endcase
end
// counters for vsync, hsync
always@(posedge HCLK, negedge HRESETn)
begin
    if(~HRESETn) begin
        ctrl_vsync_cnt <= 0;
  ctrl_hsync_cnt <= 0;
    end
    else begin
        if(ctrl_vsync_run)
   ctrl_vsync_cnt <= ctrl_vsync_cnt + 1; // counting for vsync
  else 
   ctrl_vsync_cnt <= 0;
   
        if(ctrl_hsync_run)
   ctrl_hsync_cnt <= ctrl_hsync_cnt + 1; // counting for hsync  
  else
   ctrl_hsync_cnt <= 0;
    end
end
// counting column and row index  for reading memory 
always@(posedge HCLK, negedge HRESETn)
begin
    if(~HRESETn) begin
        row <= 0;
  col <= 0;
    end
 else begin
  if(ctrl_data_run) begin
   if(col == WIDTH - 2) begin
    row <= row + 1;
   end
   if(col == WIDTH - 2) 
    col <= 0;
   else 
    col <= col + 2; // reading 2 pixels in parallel
  end
 end
end
//-------------------------------------------------//
//----------------Data counting---------- ---------//
//-------------------------------------------------//
always@(posedge HCLK, negedge HRESETn)
begin
    if(~HRESETn) begin
        data_count <= 0;
    end
    else begin
        if(ctrl_data_run)
   data_count <= data_count + 1;
    end
end
assign VSYNC = ctrl_vsync_run;
assign ctrl_done = (data_count == 196607)? 1'b1: 1'b0; // done flag
//-------------------------------------------------//
//-------------  Image processing   ---------------//
//-------------------------------------------------//
always @(*) begin
 
 HSYNC   = 1'b0;
 DATA_R0 = 0;
 DATA_G0 = 0;
 DATA_B0 = 0;                                       
 DATA_R1 = 0;
 DATA_G1 = 0;
 DATA_B1 = 0;                                         
 if(ctrl_data_run) begin
  
  HSYNC   = 1'b1;
  `ifdef BRIGHTNESS_OPERATION 
  /**************************************/  
  /*  BRIGHTNESS ADDITION OPERATION */
  /**************************************/
  if(SIGN == 1) begin
  // R0
  tempR0 = org_R[WIDTH * row + col   ] + VALUE;
  if (tempR0 > 255)
   DATA_R0 = 255;
  else
   DATA_R0 = org_R[WIDTH * row + col   ] + VALUE;
  // R1 
  tempR1 = org_R[WIDTH * row + col+1   ] + VALUE;
  if (tempR1 > 255)
   DATA_R1 = 255;
  else
   DATA_R1 = org_R[WIDTH * row + col+1   ] + VALUE; 
  // G0 
  tempG0 = org_G[WIDTH * row + col   ] + VALUE;
  if (tempG0 > 255)
   DATA_G0 = 255;
  else
   DATA_G0 = org_G[WIDTH * row + col   ] + VALUE;
  tempG1 = org_G[WIDTH * row + col+1   ] + VALUE;
  if (tempG1 > 255)
   DATA_G1 = 255;
  else
   DATA_G1 = org_G[WIDTH * row + col+1   ] + VALUE;  
  // B
  tempB0 = org_B[WIDTH * row + col   ] + VALUE;
  if (tempB0 > 255)
   DATA_B0 = 255;
  else
   DATA_B0 = org_B[WIDTH * row + col   ] + VALUE;
  tempB1 = org_B[WIDTH * row + col+1   ] + VALUE;
  if (tempB1 > 255)
   DATA_B1 = 255;
  else
   DATA_B1 = org_B[WIDTH * row + col+1   ] + VALUE;
 end
 else begin
 /**************************************/  
 /* BRIGHTNESS SUBTRACTION OPERATION */
 /**************************************/
  // R0
  tempR0 = org_R[WIDTH * row + col   ] - VALUE;
  if (tempR0 < 0)
   DATA_R0 = 0;
  else
   DATA_R0 = org_R[WIDTH * row + col   ] - VALUE;
  // R1 
  tempR1 = org_R[WIDTH * row + col+1   ] - VALUE;
  if (tempR1 < 0)
   DATA_R1 = 0;
  else
   DATA_R1 = org_R[WIDTH * row + col+1   ] - VALUE; 
  // G0 
  tempG0 = org_G[WIDTH * row + col   ] - VALUE;
  if (tempG0 < 0)
   DATA_G0 = 0;
  else
   DATA_G0 = org_G[WIDTH * row + col   ] - VALUE;
  tempG1 = org_G[WIDTH * row + col+1   ] - VALUE;
  if (tempG1 < 0)
   DATA_G1 = 0;
  else
   DATA_G1 = org_G[WIDTH * row + col+1   ] - VALUE;  
  // B
  tempB0 = org_B[WIDTH * row + col   ] - VALUE;
  if (tempB0 < 0)
   DATA_B0 = 0;
  else
   DATA_B0 = org_B[WIDTH * row + col   ] - VALUE;
  tempB1 = org_B[WIDTH * row + col+1   ] - VALUE;
  if (tempB1 < 0)
   DATA_B1 = 0;
  else
   DATA_B1 = org_B[WIDTH * row + col+1   ] - VALUE;
  end
  `endif
         /**************************************/  
  /*  INVERT_OPERATION       */
  /**************************************/
  `ifdef INVERT_OPERATION 
   value2 = (org_B[WIDTH * row + col  ] + org_R[WIDTH * row + col  ] +org_G[WIDTH * row + col  ])/3;
   DATA_R0=255-value2;
   DATA_G0=255-value2;
   DATA_B0=255-value2;
   value4 = (org_B[WIDTH * row + col+1  ] + org_R[WIDTH * row + col+1  ] +org_G[WIDTH * row + col+1  ])/3;
   DATA_R1=255-value4;
   DATA_G1=255-value4;
   DATA_B1=255-value4;  
  `endif
  /**************************************/  
  /********THRESHOLD OPERATION  *********/
  /**************************************/
  `ifdef THRESHOLD_OPERATION
  value = (org_R[WIDTH * row + col   ]+org_G[WIDTH * row + col   ]+org_B[WIDTH * row + col   ])/3;
  if(value > THRESHOLD) begin
   DATA_R0=255;
   DATA_G0=255;
   DATA_B0=255;
  end
  else begin
   DATA_R0=0;
   DATA_G0=0;
   DATA_B0=0;
  end
  value1 = (org_R[WIDTH * row + col+1   ]+org_G[WIDTH * row + col+1   ]+org_B[WIDTH * row + col+1   ])/3;
  if(value1 > THRESHOLD) begin
   DATA_R1=255;
   DATA_G1=255;
   DATA_B1=255;
  end
  else begin
   DATA_R1=0;
   DATA_G1=0;
   DATA_B1=0;
  end  
  `endif
 end
end

endmodule

The image processing operation is selected in the following "parameter.v" file. To change the processing operation, just switch the comment line.

/***************************************/
 /****************** Definition file ********/ 
/************** **********************************************/ 
`define INPUTFILENAME "your_image.hex" // Input file name 
`define OUTPUTFILENAME "output.bmp" // Output file name 
// Choose the operation of code by delete 
// in the beginning of the selected line 
//`define BRIGHTNESS_OPERATION 
 `define INVERT_OPERATION 
//`define THRESHOLD_OPERATION
// fpga4student.com FPGA projects, Verilog projects, VHDL projects

The "parameter.v" file is also to define paths and names of the input and output file. After processing the image, it is needed to write the processed data to an output image for verifications. 

The following Verilog code is to write the processed image data to a bitmap image for verification:

/****************** Module for writing .bmp image *************/ 
/***********************************************************/ 
// fpga4student.com FPGA projects, Verilog projects, VHDL projects
// Verilog project: Image processing in Verilog
module image_write #(parameter 
WIDTH = 768, // Image width 
HEIGHT = 512, // Image height 
INFILE = "output.bmp", // Output image 
BMP_HEADER_NUM = 54 // Header for bmp image 
) 
( 
input HCLK, // Clock input 
HRESETn, // Reset active low 
input hsync, // Hsync pulse 
input [7:0] DATA_WRITE_R0, // Red 8-bit data (odd) 
input [7:0] DATA_WRITE_G0, // Green 8-bit data (odd) 
input [7:0] DATA_WRITE_B0, // Blue 8-bit data (odd) 
input [7:0] DATA_WRITE_R1, // Red 8-bit data (even) 
input [7:0] DATA_WRITE_G1, // Green 8-bit data (even) 
input [7:0] DATA_WRITE_B1, // Blue 8-bit data (even) 
output reg Write_Done 
); 
// fpga4student.com FPGA projects, Verilog projects, VHDL projects
//-----------------------------------// 
//-------Header data for bmp image-----// 
//-------------------------------------// 
// Windows BMP files begin with a 54-byte header
initial  begin 
BMP_header[ 0] = 66;BMP_header[28] =24; 
BMP_header[ 1] = 77;BMP_header[29] = 0; 
BMP_header[ 2] = 54;BMP_header[30] = 0; 
BMP_header[ 3] = 0;BMP_header[31] = 0;
BMP_header[ 4] = 18;BMP_header[32] = 0;
BMP_header[ 5] = 0;BMP_header[33] = 0; 
BMP_header[ 6] = 0;BMP_header[34] = 0; 
BMP_header[ 7] = 0;BMP_header[35] = 0; 
BMP_header[ 8] = 0;BMP_header[36] = 0; 
BMP_header[ 9] = 0;BMP_header[37] = 0; 
BMP_header[10] = 54;BMP_header[38] = 0; 
BMP_header[11] = 0;BMP_header[39] = 0; 
BMP_header[12] = 0;BMP_header[40] = 0; 
BMP_header[13] = 0;BMP_header[41] = 0; 
BMP_header[14] = 40;BMP_header[42] = 0; 
BMP_header[15] = 0;BMP_header[43] = 0; 
BMP_header[16] = 0;BMP_header[44] = 0; 
BMP_header[17] = 0;BMP_header[45] = 0; 
BMP_header[18] = 0;BMP_header[46] = 0; 
BMP_header[19] = 3;BMP_header[47] = 0;
BMP_header[20] = 0;BMP_header[48] = 0;
BMP_header[21] = 0;BMP_header[49] = 0; 
BMP_header[22] = 0;BMP_header[50] = 0; 
BMP_header[23] = 2;BMP_header[51] = 0; 
BMP_header[24] = 0;BMP_header[52] = 0; 
BMP_header[25] = 0;BMP_header[53] = 0; 
BMP_header[26] = 1; BMP_header[27] = 0; 
end
//---------------------------------------------------------//
//--------------Write .bmp file  ----------------------//
//----------------------------------------------------------//
initial begin
    fd = $fopen(INFILE, "wb+");
end
always@(Write_Done) begin // once the processing was done, bmp image will be created
    if(Write_Done == 1'b1) begin
        for(i=0; i<BMP_HEADER_NUM; i=i+1) begin
            $fwrite(fd, "%c", BMP_header[i][7:0]); // write the header
        end
        
        for(i=0; i<WIDTH*HEIGHT*3; i=i+6) begin
  // write R0B0G0 and R1B1G1 (6 bytes) in a loop
            $fwrite(fd, "%c", out_BMP[i  ][7:0]);
            $fwrite(fd, "%c", out_BMP[i+1][7:0]);
            $fwrite(fd, "%c", out_BMP[i+2][7:0]);
            $fwrite(fd, "%c", out_BMP[i+3][7:0]);
            $fwrite(fd, "%c", out_BMP[i+4][7:0]);
            $fwrite(fd, "%c", out_BMP[i+5][7:0]);
        end
    end
end

The header data for the bitmap image is very important and it is published here. If there is no header data, the written image could not be correctly displayed. In Verilog HDL, $fwrite command is used to write data to file.

Next, let's write a test bench Verilog code to verify the image processing operations.

`timescale 1ns/1ps /**************************************************/ 
/******* Testbench for simulation ****************/
/*********************************************/ 
 // fpga4student.com FPGA projects, Verilog projects, VHDL projects
// Verilog project: Image processing in Verilog
`include "parameter.v" // include definition file module tb_simulation; 
//------------------ // Internal Signals 
//------------------------------------------------- 
reg HCLK, HRESETn; 
wire vsync; 
wire hsync;
wire [ 7 : 0] data_R0; 
wire [ 7 : 0] data_G0; 
wire [ 7 : 0] data_B0; 
wire [ 7 : 0] data_R1; 
wire [ 7 : 0] data_G1; 
wire [ 7 : 0] data_B1; 
wire enc_done; 
image_read #(.INFILE(`INPUTFILENAME)) 
u_image_read 
( .HCLK (HCLK ), 
.HRESETn (HRESETn ),
 .VSYNC (vsync ), 
.HSYNC (hsync ), 
.DATA_R0 (data_R0 ),
 .DATA_G0 (data_G0 ), 
.DATA_B0 (data_B0 ), 
.DATA_R1 (data_R1 ), 
.DATA_G1 (data_G1 ), 
.DATA_B1 (data_B1 ), 
.ctrl_done (enc_done) 
); 
image_write #(.INFILE(`OUTPUTFILENAME)) 
u_image_write ( 
.HCLK(HCLK), 
.HRESETn(HRESETn),
 .hsync(hsync), 
.DATA_WRITE_R0(data_R0),
 .DATA_WRITE_G0(data_G0),
 .DATA_WRITE_B0(data_B0), 
.DATA_WRITE_R1(data_R1), 
.DATA_WRITE_G1(data_G1), 
.DATA_WRITE_B1(data_B1),
 .Write_Done()
 ); 
//------------- // Test Vectors 
//------------------------------------- 
initial 
begin 
HCLK = 0; 
forever #10 HCLK = ~HCLK; 
end 
initial 
begin 
HRESETn = 0; 
#25 HRESETn = 1; 
end endmodule

Finally, we have everything to run a simulation to verify the image processing code. Let's use the following image as the input bitmap file:

Input bitmap image

Run the simulation for 6ms, close the simulation and open the output image for checking the result. Followings are the output images which are processed by the selected operations in parameter.v:

Output bitmap image after inverting

Output bitmap image after threshold operation

Output bitmap image after subtracting brightness

Since the reading code is to model an image sensor/camera for simulation purposes, it is recommended not to synthesize the code. If you really want to synthesize the processing code and run this directly on FPGA, you need to replace these image arrays (total_memory, temp_BMP, org_R, org_B, org_G) in the code by block memory (RAMs) and design address generators to read image data from the block memory.

After receiving so many questions related to this project, followings are the answers to your questions:

1. Explanation and tutorial on how to run the simulation:

2. The full Verilog code for this image processing project can be downloaded here. Run the simulation about 6ms and close the simulation, then you will be able to see the output image.
3. The reading part operates as a Verilog model of an image sensor/camera (output RGB data, HSYNC, VSYNC, HCLK). The Verilog image reading code is extremely useful for functional verification in real-time FPGA image/video projects. 
4. In this project, I added the image processing part to make an example of image enhancement. You can easily remove the processing part to get only raw image data in the case that you want to use the image sensor model only for verifying your image processing design.
5. The image saving into three separate RGB mems: Because Matlab code writes the image hexadecimal file from the last row to the first row, the RGB saving codes (org_R, org_B, org_G) do the same in reading the temp_BMP memory to save RGB data correctly. You can change it accordingly if you want to do it differently.
6. You might find the following explanation for the BMP header useful if you want to change the image size:
Image size = 768*512*3= 1179648 bytes
BMP header = 54 bytes
BMP File size = Image size + BMP Header = 1179702 Bytes 
Convert it to hexadecimal numbers: 1179702 in Decimal = 120036 in Hexadecimal
Then 4-byte size of BMP file: 00H, 12 in Hexa = 18 Decimal, 00H, 36 in Hexa = 54 Decimal
That's how we get the following values:
BMP_header[ 2] = 54;
BMP_header[ 3] = 0 ;
BMP_header[ 4] = 18;
BMP_header[ 5] = 0 ;
Image width = 768 => In hexadecimal: 0x0300. The 4 bytes of the image width are 0, 3, 0, 0. That's how you get the following values:
BMP_header[18] = 0;
BMP_header[19] = 3;
BMP_header[20] = 0;
BMP_header[21] = 0; 
Image height = 512 => In hexadecimal: 0x0200. The 4 bytes of the image width are 0, 2, 0, 0. That's how we get the following values:
BMP_header[22] = 0;
BMP_header[23] = 2;
BMP_header[24] = 0;
BMP_header[25] = 0;
7. You should not synthesize this code because it is not designed for running on FPGA, but rather for functional verification purposes.  If you really want to synthesize this code (read and process) and load the image into FPGA for processing directly on FPGA, replace all the temp. variables (org_R, org_B, org_G, tmp_BMP = total_memory) by block RAMs and generate addresses to read the image data (remove the always @(start) and all the "for loops" - these are for simulation purposes). There are two ways: 1. write a RAM code and initialize the image data into the memory using $readmemh; 2. generate a block memory using either Xilinx Core Generator or Altera MegaFunction and load the image data into the initial values of memory (.coe file for Xilinx Core Gen. and .mif for Altera MegaFunction), then read the image data from memory and process it (FSM Design). 
8. In this project, two even and old pixels are read at the same time for speeding up the processing, but you can change the number of pixels being read depending on your design.   
9. The writing Verilog code is also very helpful for testing purposes as you can see the output in BMP format.
10. If you want to do real-time image processing, you can check this for the camera interface code: Basys 3 FPGA OV7670 Camera

Recommended Verilog projects:

1. What is an FPGA? How Verilog works on FPGA

2. Verilog code for FIFO memory
3. Verilog code for 16-bit single-cycle MIPS processor
4. Programmable Digital Delay Timer in Verilog HDL
5. Verilog code for basic logic components in digital circuits
6. Verilog code for 32-bit Unsigned Divider
7. Verilog code for Fixed-Point Matrix Multiplication
8. Plate License Recognition in Verilog HDL
9. Verilog code for Carry-Look-Ahead Multiplier
10. Verilog code for a Microcontroller
11. Verilog code for 4x4 Multiplier
12. Verilog code for Car Parking System
13. Image processing on FPGA using Verilog HDL
14. How to load a text file into FPGA using Verilog HDL
15. Verilog code for Traffic Light Controller
16. Verilog code for Alarm Clock on FPGA
17. Verilog code for comparator design
18. Verilog code for D Flip Flop
19. Verilog code for Full Adder
20. Verilog code for counter with testbench
21. Verilog code for 16-bit RISC Processor
22. Verilog code for button debouncing on FPGA
23. How to write Verilog Testbench for bidirectional/ inout ports

24. Tic Tac Toe Game in Verilog and LogiSim
25. 32-bit 5-stage Pipelined MIPS Processor in Verilog (Part-1)
26. 32-bit 5-stage Pipelined MIPS Processor in Verilog (Part-2)
27. 32-bit 5-stage Pipelined MIPS Processor in Verilog (Part-3)

28. Verilog code for Decoder
29. Verilog code for Multiplexers

30.  N-bit Adder Design in Verilog
31. Verilog vs VHDL: Explain by Examples
32. Verilog code for Clock divider on FPGA
33. How to generate a clock enable signal in Verilog
34. Verilog code for PWM Generator
35. Verilog coding vs Software Programming
36. Verilog code for Moore FSM Sequence Detector
37. Verilog code for 7-segment display controller on Basys 3 FPGA