Question

I’m following this tutorial
http://www.gamedev.net/page/resources/_/technical/game-programming/how-to-load-a-bitmap-r1966 and I got the problem of wrong value for all the bitmap header/info that I load.

I’ve declared the structure to store the bitmap header, info

typedef struct BITMAPFILE_HEADER {
  WORD  bfType;
  DWORD   bfSize;
  WORD  bfReserved1;
  WORD  bfReserved2;
  DWORD   bfOffBits;
} BITMAPFILE_HEADER;

//Bitmap information header
//provides information specific to the image data
typedef struct BITMAPINFO_HEADER{
  DWORD  biSize;
  LONG   biWidth;
  LONG   biHeight;
  WORD   biPlanes;
  WORD   biBitCount;
  DWORD  biCompression;
  DWORD  biSizeImage;
  LONG   biXPelsPerMeter;
  LONG   biYPelsPerMeter;
  DWORD  biClrUsed;
  DWORD  biClrImportant;
} BITMAPINFO_HEADER;

//Colour palette
typedef struct RGB_QUAD {
  BYTE  rgbBlue;
  BYTE  rgbGreen;
  BYTE  rgbRed;
  BYTE  rgbReserved;
} RGB_QUAD;

After that I read the bitmap by using following codes:

    FILE *in;
    in = fopen("picture.bmp", "rb");

    if (in == NULL)
    {
        printf("Error opening file\n");
    }
    else
    {
        BITMAPFILE_HEADER bmfh;
        BITMAPINFO_HEADER bmih;

        fread(&bmfh, sizeof(BITMAPFILE_HEADER), 1, in);
        fread(&bmih, sizeof(BITMAPINFO_HEADER), 1, in);

        if (bmih.biBitCount != 24)
            printf("not 24");
    }

My picture is 24-bit but when I run this program, it shows “not 24”. I try to debug it in Visual Studio then I saw the bmih.biBitCount is 0. In addition, the image width and height is wrong as well, the only correct data that I found in the header is the bmfh.bfType which is 19778.

Anyone know what’s wrong with my codes?
Again, I just want to read the bmp but not displaying it.

PS:
Originally the picture is in JPEG format. I converted it to BMP format by using MS PAINT and re-save it in BMP format. I wonder does it affect the values?

Answer 1

I would imagine that you ran into an alignment issue, because the compiler can choose to align data in a structure as he sees fit. You can try to pack your struct, but I would suggest to read out every field and fill it in separately, it is more work, but it is clearer and avoids alignment issues.