Issue with creation of new header files in pset4

Question

//create new BITMAPINFOHEADER details
    BITMAPINFOHEADER bi_new;
    bi_new.biHeight = bi.biHeight * factor;
    bi_new.biWidth = bi.biWidth * factor;
    // determine padding for infile scanlines
    int padding = (4 - (bi.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;
    // determine padding for outfile scanlines
    int new_padding = (4 - (bi_new.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;
    bi_new.biSizeImage = (bi_new.biHeight * bi_new.biWidth * 9) + (new_padding * bi_new.biHeight);

    //create new BITMAPFILEHEADER details
    BITMAPFILEHEADER bf_new;
    bf_new.bfSize = bi_new.biSizeImage + sizeof(BITMAPINFOHEADER) + sizeof(BITMAPFILEHEADER);

    // write outfile's BITMAPFILEHEADER
    fwrite(&bf_new, sizeof(BITMAPFILEHEADER), 1, outptr);

    // write outfile's BITMAPINFOHEADER
    fwrite(&bi_new, sizeof(BITMAPINFOHEADER), 1, outptr);

I have written code for new header files of bit map. Please check this if it is correct, I guess something is going wrong. I didnt write code to resize the pixels yet. But, I have run my code, found that new bmp file is generated and I couldnt open the file. It says unsupported file format. I am not sure if this is expected behaviour, Please let me know if something's wrong in my code. Thank you!

Answer 1

You'd have to copy the other fields from bi to bi_new. Easiest way is to initialize bi_new using

BITMAPINFOHEADER bi_new = bi;

which copies the whole structure, you'd still have to change the fields related to the size.

Same applies to bf_new, which should be initialized to bf.

Your calculation for biSizeImage looks a bit off. It should be the number of bytes in a line (including padding) times the number of lines. Your first part is off by a factor of 3 (better use sizeof(RGBTriple) for clarity where that factor came from), and it would work only for positive heights. The number of lines may be negative, in which case the image is stored upside down, and the number of lines is the height's absolute value.