c - Padding a Bitmap pixel array

Question

I'm making a program that creates a bitmap file in C. it's using 24-bit colour.

I'm writing the file in 3 stages, i first write the FileHeader, then the InfoHeader, and then the Pixel Data. I'm having trouble padding the pixel data so each row finishes on a word boundary.

The code below works sometimes, but only without the while loop (which adds the padding to the end of the line). For example, with a 12x12px image, I can scale it to 24x24, but not to 10x10 (the file is corrupt). When I put in the padding code below, the image becomes distorted, and sometimes gets corrupted too.

I can't seem to figure out what's going wrong, the code below should add padding to the end of each line until i hits a word boundary, and then starts the next line.

fwrite(&fh, 1, sizeof(FILEHEADER), n_img);
fwrite(&ih, 1, sizeof(INFOHEADER), n_img);
int i, j;
uint8_t pad = 0;
for (i = height-1; i >= 0; i--) {
    for (j = 0; j < width; j++)
        fwrite(n_pix+(i*width)+j, 1, sizeof(IMAGE), n_img);

    while(ftell(n_img)%4 != 0)
        fwrite(&pad, 1, 1, n_img);
}

Answer 1

You are not padding rows to word size, you are padding the current file position. And it doesn't work because the size of your headers add up to 54 -- not a multiple of 4.

Instead of using ftell to retrieve the 'current position', use maths. Make your pad an unsigned long, and insert before your loops:

int npad = (sizeof(IMAGE)*width) & 3;
if (npad)
  npad = 4-npad;

Then, instead of the while(ftell .. loop, write out the number of required bytes immediately:

fwrite (&pad, 1,npad, n_img);

npad will range from 0..3, that's why you have to make pad a 4-byte integer.