Writing bitmap in C

Question

This is my code for a bitmap image but it dosen't work and I have no idea why. I previously did something the same but I was reading an original bitmap and I had only to change its colors. The code was exactly the same thought and it did work that time but for reading an image from 0 it seems it didn't work. I searched a lot on the internet for answers and i think maybe it's a problem with data from headers or something about padding. Can you help me, please?

Header:

#pragma pack(1)

struct bmp_fileheader
{
unsigned char  fileMarker1; /* 'B' */
unsigned char  fileMarker2; /* 'M' */
unsigned int   bfSize; /* File's size */
unsigned short unused1;
unsigned short unused2;
unsigned int   imageDataOffset; /* Offset to the start of image data */
};

struct bmp_infoheader
{
unsigned int   biSize; /* Size of the info header - 40 bytes */
signed int     width; /* Width of the image */
signed int     height; /* Height of the image */
unsigned short planes;
unsigned short bitPix;
unsigned int   biCompression;
unsigned int   biSizeImage; /* Size of the image data */
int            biXPelsPerMeter;
int            biYPelsPerMeter;
unsigned int   biClrUsed;
unsigned int   biClrImportant;
};

#pragma pack()

---

Code:

#include <stdio.h>
#include <stdlib.h>
#include "bmp_header.h"

typedef struct {
unsigned char b;
unsigned char g;
unsigned char r;
}rgb;
int main() {

int i, j, k;
struct bmp_fileheader a;
struct bmp_infoheader b;
FILE * p = fopen("img.bmp", "wb");


a.fileMarker1 = 'B';
a.fileMarker2 = 'M';
a.unused1=0;
a.unused2=0;
a.bfSize = 54+3*40*40;
a.imageDataOffset=54;


b.biSize = 40;
b.width = 40;
b.width = 40;
b.planes = 1;
b.bitPix = 24;
b.biCompression = 0;
b.biSizeImage = 4*40*40;
b.biXPelsPerMeter = 0;
b.biYPelsPerMeter = 0;
b.biClrUsed = 0;
b.biClrImportant = 0;

rgb matrix[40][40];

for (i = 0; i < 10; i++) {
    for (j = 0; j < 40; j++) {
        matrix[i][j].b = 255;
        matrix[i][j].g = 255;
        matrix[i][j].r = 255;
    }
}

for (i = 10; i < 30; i++) {
    for (j = 0; j < 40; j++) {
        if (j < 10 || j > 29) {
            matrix[i][j].b = 255;
            matrix[i][j].g = 255;
            matrix[i][j].r = 255;
        } else {
            matrix[i][j].b = 255;
            matrix[i][j].g = 255;
            matrix[i][j].r = 0;
        }
    }
}

for (i = 30; i < 40; i++) {
    for (j = 0; j < 40; j++) {
        matrix[i][j].b = 255;
        matrix[i][j].g = 255;
        matrix[i][j].r = 255;
    }
}

fwrite( & a, sizeof(a), 1, p);
fwrite( & b, sizeof(b), 1, p);

int t = (4 - (40 * 3)%4)%4;//padding equation found on the internet
for(i = 0; i < 40; i++){
    for(j = 0; j < 40; j++)
        fwrite(&matrix[i][j], sizeof(matrix[i][j]), 1, p);

    for(k = 0; k < t; k++){
        putc(0,p); //padding
    }

}
fclose(p);
}

Answer 1

The scanline size is calculated as:

#define WIDTHBYTES(bits)    (((bits) + 31) / 32 * 4)

The biSizeImage field is computed as:

b.biSizeImage= WIDTHBYTES(xRes*24) * yRes;

The 24 is the number of bits per pixel.

You also set b.width twice. The second one should be height.

---

Also the infoheader fields must be calcualed as (adapt to your struct naming):

fheader.bfOffBits = sizeof(BITMAPFILEHEADER) + sizeof (BITMAPINFOHEADER) + 0/*no palette for 24 bits color*/;
fheader.bfSize = fheader.bfOffBits + iheader.biSizeImage;