Thread: write bmp

Hi,
I'm trying to output a plain red bitmap using only the C++ stl (so I can't use windows.h and it's functions). How would I go about doing this? At the moment, I've imported a (working) bitmap header file. I read this file in, and then output it to a new file using streams. The new file then ends up being a 700 x 700 bitmap file, like it should be. However, when I open it in windows picture viewer, it says drawing failed. I assume this is because I need to append some more information on to the end of the header. What would I append to the end of it in order to make it plain red?
Thanks

Thanks for the reply. What would be the best way to be able to access (read and write) to each pixel? How would I go about getting a pixel, and storing it in an array?

Here's what I've got so far:

Code:

#include <fstream>
#include <iostream>
#include <vector>



using namespace std;


struct rgb { //declares pixel structure

char b;
char g;
char r;


};


rgb *pixel = new rgb[700 * 700]; 

int main()
{
    char fileName[54];
    char buffer[255];
    

    ofstream fout("bitmap.bmp");
    
    ifstream fin("bmpH.hdr");
    char ch;
    
    while(fin.get(ch))     
    {
                           fout << ch;
    
                           
     }
          
for (int j=(699); j >=0 ; j--){ //iterate through all the pixels
for (int l=0; l<700; l++){

int i = j+(((700-1)-l)*700); // flip image since it's upside down in a bitmap




//paint every pixel yellow
pixel[i].b = 0x00; 
pixel[i].g = 0xFF;
pixel[i].r = 0xFF;

 fout << pixel[i].b;
 fout << pixel[i].g;
 fout << pixel[i].r;





}
}


//paint ONE pixel black (Doesn't work) 
int i = (200)+(200 * 700);

pixel[i].b = 0x00; 
pixel[i].g = 0x00;
pixel[i].r = 0x00;
 cout << pixel;
 
 fout << pixel[i].b;
 fout << pixel[i].g;
 fout << pixel[i].r;
 


    
         


    fout.close();
    fin.close();
    
  
    
   system("PAUSE");
   remove("bitmap.bmp");
    return 0;
    
}

It paints every pixel yellow, so I get a nice background. However, I can't seem to write the one black pixel. What am I doing wrong?
Thank you for the help.

A bitmap file doesn't just start off with the image data, It contains a header, attributes, and then it chunks the red, green, and blue data in bytes that have to be interpreted as such.

I've added the header file in, which I assume contains the attributes as well? I tried appending the information of the pixels (by setting fout as ios::app), but it didn't make a difference. Sorry, I'm really new to this and I've spent a long time looking at this and it just feels like I'm going in circles.

Are you looking to create an actual bitmap file, or just a replication of one? By Header I mean each bitmap has a header (54 bytes I believe) the first two bytes being the characters BM to identify it as a bitmap. Plus it has information regarding the width, height, etc. To correctly output a bitmap you have to know how it looks as a file, wotsit has specifications on a lot of files for the purpose of programming for them.

Thanks for the information. I've been given a header file (54 bytes). I am supposed to create a new bitmap file, using the header file given. Then, I just need to create different shapes (like triangles) on to the bitmap. However, all I need to try and figure out is how to write one pixel, and I should be able to to do the rest of the maths.
Thanks again.

First of all, the block for the bitmap data should be created like this:

Code:

char image_data[width*length*bytes_per_pixel];
//White background
memset(image_data, 0xFF, width*length*bytes_per_pixel);

Assuming you're using 24-bit color, each pixel uses a whole byte for each primary color. BMP files are stored in BGR format, so, if I wanted to make the pixel at (x, y) red, I would do this:

Code:

enum {
BLUE_OFF = 0,
GREEN_OFF,
RED_OFF
};

image_data[x*width + y + BLUE_OFF] = 0x00;
image_data[x*width + y + GREEN_OFF] = 0x00;
image_data[x*width + y + RED_OFF] = 0xFF;

If you're using 16-bit color, a little more trickery is required. And if you're in 8-bit color, you need a palette.

Try putting the pixel declaration and the memset call inside main. Also, you might try allocating pixel dynamically.

Thanks, UMR_Student. I put the memset in the main, and it ran with no errors and I can have the background any colour I want. I noticed that if I have anything after the two FOR.....NEXT loops, they don't get recognised. I tried copy and pasting the another for loop and put it black. I was expecting that it would overwrite what was currently there, but it ignored it and just had the colours from the first loop.
By allocating dynamically, due you mean creating it on the heap using New()?

> By allocating dynamically, due you mean creating it on the heap using New()?

Yes.