Help with manually creating a BMP image
 

Hello, im a noob with C++ at the moment and Im trying to create a BMP image from pixel data stored in a array. Everything seems to work except that when i look at the file generated in a hex editor i get an extra '00 00' in between the BM identifier and the bytes reserved for the file size. It is as though the member short BM within the bmp_header struct is taking 4 bytes where i think i it is suppose to take 2 bytes. Could someone please explain why this is happening. Thanks in advance. (there are probably much more efficient ways of doing this but this is the way i want 2 do it at the moment)


Template is from http://en.wikipedia.org/wiki/BMP_file_format (the 2 by 2 pixel example)

#include <fstream>

#include <iostream>

using namespace std;



#define w_in_pix 2

#define h_in_pix 2

short padding = 0x0000;





struct bmp_header

{

       short BM;

       long size_of_file;

       long reserve;

       long offset_of_pixle_data;

       long size_of_header;

       long width;

       long hight;

       short num_of_colour_plane;

       short num_of_bit_per_pix;

       long compression;

       long size_of_pix_data;

       long h_resolution;

       long v_resolution;

       long num_of_colour_in_palette;

       long important_colours;

       

} 

HEADER;



// pix array ///////////////////////////////////////////////////////////////////////////

// L to R //

char BGR [12] = {

0xFF,0x00,0x00,   0x00,0xFF,0x00,

0x00,0x00,0xFF,   0xFF,0xFF,0xFF};

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



int main()

{

    HEADER.BM = 0x4d42;

    HEADER.size_of_file =  sizeof(HEADER) + sizeof(BGR) + sizeof(padding) * h_in_pix;

    HEADER.reserve = 0000; 

    HEADER.offset_of_pixle_data = 54;

    HEADER.size_of_header = 40;

    HEADER.width = w_in_pix;

    HEADER.hight = h_in_pix;

    HEADER.num_of_colour_plane = 1;

    HEADER.num_of_bit_per_pix = 24;

    HEADER.compression = 0;

    HEADER.size_of_pix_data = sizeof(BGR) + sizeof(padding) * h_in_pix;

    HEADER.h_resolution = 2835;

    HEADER.v_resolution = 2835;

    HEADER.num_of_colour_in_palette = 0;

    HEADER.important_colours = 0;

    

    // write BMP Header ////////////////////////////////////////////////////////////////

    ofstream file;

    file.open ("test.bmp", ios::out | ios::trunc | ios::binary);

    file.write ((char*)(&HEADER), sizeof(HEADER));

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

    // write BMP data //////////////////////////////////////////////////////////////////

    //file.write ((char*)(&BGR[0]),1);

     

    

    int position = w_in_pix * h_in_pix * 3;

    while (position != 0)

    {

          for (int n = 0; n != w_in_pix * 3; n++)

          {

              file.write ((char*)(&BGR[position - w_in_pix * 3 + n]), 1);

          }

          file.write ((char*)(&padding), 2);

          position = position - w_in_pix * 3;

    } 

    

    

}

Re: Help with manually creating a BMP image
 

The short answer is padding and alignment.

> short BM;
> long size_of_file;
To ensure efficient access to size_of_file, the compiler inserts a pair of padding bytes after BM.

The common way to "fix" this is to use something like

#pragma pack(1)

, but there is no standard way to tell a compiler how to do this (or indeed how well your request for packing the struct will be met).

Also look up "endian-ess"

Unfortunately, the only good long-term answer (which is pretty horrible) is to read/write bytes one at a time, and assign them to the struct in the correct order.

Re: Help with manually creating a BMP image
 

Thanks 4 your help. Ill experiment with what you told me today

Re: Help with manually creating a BMP image
 

Yet another common and portable approach: use ordinar structures to create BMP info then pack them into the final char buffer (use memcpy(buffer+offset,&member,sizeof member), move offset forward and so on). Of course, it's possible to write data to a file stream directly (no need in current write pos forwarding).
Add some syntax sugar, for example:

struct bmp_header

{

       short BM;

       long size_of_file;

       long reserve;

       long offset_of_pixle_data;

       long size_of_header;

       long width;

       long hight;

       short num_of_colour_plane;

       short num_of_bit_per_pix;

       long compression;

       long size_of_pix_data;

       long h_resolution;

       long v_resolution;

       long num_of_colour_in_palette;

       long important_colours;

    void* pack(void* pbuf) {

        char* p = (char*)pbuf;

        int i = 0;

    # define PACK(member) {memcpy(p+i,&member,sizeof member);i+=sizeof member;}

        PACK(BM)

        PACK(size_of_file)

        ...

        PACK(important_colours)

    # undef PACK

        return pbuf;

    }

       

}

HEADER;

...

   HEADER.pack(buffer)

A nasty thing but it works ;)

Re: Help with manually creating a BMP image
 

Can i suggest you store the value of BM outside of the structure and store it in a char array of size 2 then write each one to the file before calling write() on the structure.

Chris

Re: Help with manually creating a BMP image
 

I did what freaky_chris suggested and created the value of BM outside of the bmp_header struct and it seems to work. Final code is now

#include <fstream>

#include <iostream>

using namespace std;



#define w_in_pix 2

#define h_in_pix 2

short padding = 0x0000;

short BM = 0x4d42;



struct bmp_header

{

       

       long size_of_file;

       long reserve;

       long offset_of_pixle_data;

       long size_of_header;

       long width;

       long hight;

       short num_of_colour_plane;

       short num_of_bit_per_pix;

       long compression;

       long size_of_pix_data;

       long h_resolution;

       long v_resolution;

       long num_of_colour_in_palette;

       long important_colours;

       

} 

HEADER;



// pix array ///////////////////////////////////////////////////////////////////////////

// L to R //

char BGR [12] = {

0xFF,0x00,0x00,   0x00,0xFF,0x00,

0x00,0x00,0xFF,   0xFF,0xFF,0xFF};

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



int main()

{

    HEADER.size_of_file =  sizeof(HEADER) + sizeof(BGR) + sizeof(padding) * h_in_pix + 2;

    HEADER.reserve = 0000; 

    HEADER.offset_of_pixle_data = 54;

    HEADER.size_of_header = 40;

    HEADER.width = w_in_pix;

    HEADER.hight = h_in_pix;

    HEADER.num_of_colour_plane = 1;

    HEADER.num_of_bit_per_pix = 24;

    HEADER.compression = 0;

    HEADER.size_of_pix_data = sizeof(BGR) + sizeof(padding) * h_in_pix;

    HEADER.h_resolution = 2835;

    HEADER.v_resolution = 2835;

    HEADER.num_of_colour_in_palette = 0;

    HEADER.important_colours = 0;

    

    // write BMP Header ////////////////////////////////////////////////////////////////

    ofstream file;

    file.open ("test.bmp", ios::out | ios::trunc | ios::binary);

    file.write ((char*)(&BM), 2);

    file.write ((char*)(&HEADER), sizeof(HEADER));

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

    // write BMP data //////////////////////////////////////////////////////////////////

    //file.write ((char*)(&BGR[0]),1);

     

    

    int position = w_in_pix * h_in_pix * 3;

    while (position != 0)

    {

          for (int n = 0; n != w_in_pix * 3; n++)

          {

              file.write ((char*)(&BGR[position - w_in_pix * 3 + n]), 1);

          }

          file.write ((char*)(&padding), 2);

          position = position - w_in_pix * 3;

    } 

    

    

}

Re: Help with manually creating a BMP image
 

Thanks 4 all of ur help