The BMP file format has been created by Microsoft and IBM. The BMP files are the way, Windows stores bit mapped images. The BMP image data is bit packed but every line must end on a dword boundary - if that’s not the case, it must be padded with zeroes. BMP files are stored bottom-up, that means that the first scan line is the bottom line.


BMP Format Contents

Palette content is used for lower than 24-bit images. In 24-bit image this content does not make any sense. Just ignore it. That means in 24-bit BMP image data starts from offset 0036h.

Sample C Source Code

Nowadays smaller than 24-bit BMP formats are used rarely. So here I wrote only 24-bit uncompressed BMP code. Enjoy it. Functions use dynamically allocated matrices.

/*

 *   Give a reference to this site if you like to use this code.

 *
 *                               Written by Chasan Chouse, 2004
 */

// Bitmap Structure
typedef struct {

    long    FileSz;
    long    Reserved;
    long    BmpOffset;
    long    BmpSzHead;
    long    X;
    long    Y;
    unsigned int    Planes;
    unsigned int    BitsPPixel;
    long    Comp; // compression
    long    BmpSzData;
    long    HRes;
    long    VRes;
    long    Colors;
    long    ImpColors;
} BMPHEADER;

#define UCHAR unsigned char
#define USHORT unsigned short
#define ULONG unsigned long

/*

 *   24-bit full color bitmap files

 *   Read Header and fill the structure.
 */

bool BMP_24_Get_Header(char FlName[], BMPHEADER *BmpHdr)
{
    char ch1, ch2;
    unsigned short  s1, s2;
    FILE *f;

    f =fopen(FlName,"rb");
    ch1 = fgetc(f);
    ch2 = fgetc(f);
    if( (ch1 != 'B') && (ch2 != 'M') )  //  Supports only Windows 3.1x, 95, NT  BMP format
        return (false);

    fread(&BmpHdr->FileSz, 4, 1, f);

    fread(&BmpHdr->Reserved, 4, 1, f);
    fread(&BmpHdr->BmpOffset, 4, 1, f);

    fread(&BmpHdr->BmpSzHead, 4, 1, f);

    fread(&BmpHdr->X, 4, 1, f);
    printf("\nWidth         : %ld", BmpHdr->X);

    fread(&BmpHdr->Y, 4, 1, f);
    printf("\nHeigth        : %ld", BmpHdr->Y);

    s1 = fgetc(f);
    s2 = fgetc(f);
    s2 <<=8;
    BmpHdr->Planes = 0;
    BmpHdr->Planes |= s1;
    BmpHdr->Planes |= s2;

    s1 = fgetc(f);
    s2 = fgetc(f);
    s2 <<=8;
    BmpHdr->BitsPPixel = 0;
    BmpHdr->BitsPPixel |= s1;
    BmpHdr->BitsPPixel |= s2;

    fread(&BmpHdr->Comp, 4, 1, f);

    fread(&BmpHdr->BmpSzData, 4, 1, f);

    fread(&BmpHdr->HRes, 4, 1, f);

    fread(&BmpHdr->VRes, 4, 1, f);

    fread(&BmpHdr->Colors, 4, 1, f);

    fread(&BmpHdr->ImpColors, 4, 1, f);

    fclose(f);
    return (true);
}
 

/* 
 *  - Read actual bitmap data. Note that bitmap lines are organized

 *    reversely. List line is placed in the first scan line.

 *  - Look at Dynamic Memory Allocation for double pointer(**RRed) Matrices.
 */
bool BMP_24_Get_Data(char FlName[],

                    BMPHEADER *BmpHdr,

                    UCHAR **RRed, 

                    UCHAR **GGreen,

                    UCHAR **BBlue)
{
    long i, j;
    unsigned long BmpPix, k;
    FILE *f;

    f=fopen(FlName,"rb");

    fseek(f,BmpHdr->BmpOffset, SEEK_SET);
    for(i=(long)BmpHdr->Y-1;i>=0;i--)
    {
        for (j=0;j<(long)BmpHdr->X;j++)
        {
            fread(&BmpPix, 3, 1, f);
            BmpPix &= 0x00FFFFFF;

            k = BmpPix & 0xFF0000;
            RRed[i][j] = k >> 16;

            k = BmpPix & 0x00FF00;
            GGreen[i][j] = k >> 8;

            k = BmpPix & 0x0000FF;
            BBlue[i][j] = k;
        }
    }

    fclose(f);
    return(true);
}
 

/* 
 *  - Write bitmap data. Note that bitmap lines are organized

 *    reversely. List line is placed in the first scan line.

 *  - Look at Dynamic Memory Allocation for double pointer(**RRed) Matrices.
 */
bool BMP_24_Put_Data(char FlNameFOut[],

                         BMPHEADER *BmpHdr,

                         UCHAR **RRed,

                         UCHAR **GGreen,

                         UCHAR **BBlue)
{
    long i, j, BD, nx, ny, BmpPix;
    char ch1, ch2;
    FILE *f2;

    nx = BmpHdr->X;
    ny = BmpHdr->Y;


    f2 = fopen(FlNameFOut,"wb");
    fseek(f2, 0, SEEK_SET);
    ch1 = 'B';
    ch2 = 'M';
    fputc(ch1, f2);
    fputc(ch2, f2);

    fseek(f2, 6, SEEK_SET);
    fwrite(&BmpHdr->Reserved, 4, 1, f2);

    fseek(f2, 14, SEEK_SET);
    fwrite(&BmpHdr->BmpSzHead, 4, 1, f2);

    fseek(f2, 18, SEEK_SET);
    fwrite(&BmpHdr->X, 4, 1, f2);

    fseek(f2, 22, SEEK_SET);
    fwrite(&BmpHdr->Y, 4, 1, f2);

    fseek(f2, 26, SEEK_SET);
    fwrite(&BmpHdr->Planes, 2, 1, f2);

    fseek(f2, 28, SEEK_SET);
    fwrite(&BmpHdr->BitsPPixel, 2, 1, f2);

    fseek(f2, 30, SEEK_SET);
    fwrite(&BmpHdr->Comp, 4, 1, f2);

    fseek(f2, 34, SEEK_SET);
    fwrite(&BmpHdr->BmpSzData, 4, 1, f2);

    fseek(f2, 38, SEEK_SET);
    fwrite(&BmpHdr->HRes, 4, 1, f2);

    fseek(f2, 42, SEEK_SET);
    fwrite(&BmpHdr->VRes, 4, 1, f2);

    fseek(f2, 46, SEEK_SET);
    fwrite(&BmpHdr->Colors, 4, 1, f2);

    fseek(f2, 2, SEEK_SET);
    fwrite(&BmpHdr->FileSz, 4, 1, f2);

//   Number Of important colors
    fseek(f2, 50, SEEK_SET);
    fwrite(&BmpHdr->ImpColors, 4, 1, f2);

//   Offset from beginning of file to the beginning of the bitmap data.
    fseek(f2, 10, SEEK_SET);
    fwrite(&BmpHdr->BmpOffset, 4, 1, f2);

    fseek(f2, BmpHdr->BmpOffset, SEEK_SET);

    for(i=ny-1;i>=0;i--)
    {
        for (j=0;j<nx;j++)
        {
            BD = RRed[i][j] & 0xFF;
            BmpPix = BD << 16;

            BD = GGreen[i][j] & 0xFF;
            BD <<= 8;
            BmpPix |= BD;

            BD = BBlue[i][j] & 0xFF;
            BmpPix |= BD;

            fwrite(&BmpPix, 3, 1, f2);
        }
    }

    fclose(f2);
    return (true);
}