resize.c - shared from CS50 IDE

resize.c

 // read infile's BITMAPFILEHEADER
    BITMAPFILEHEADER bf;
    fread(&bf, sizeof(BITMAPFILEHEADER), 1, inptr);

    // read infile's BITMAPINFOHEADER
    BITMAPINFOHEADER bi;
    fread(&bi, sizeof(BITMAPINFOHEADER), 1, inptr);

    // ensure infile is (likely) a 24-bit uncompressed BMP 4.0
    if (bf.bfType != 0x4d42 || bf.bfOffBits != 54 || bi.biSize != 40 || 
        bi.biBitCount != 24 || bi.biCompression != 0)
    {
        fclose(outptr);
        fclose(inptr);
        fprintf(stderr, "Unsupported file format.\n");
        return 4;
    }
    
    
    // outfile's BITMAPINFOHEADER
    BITMAPINFOHEADER boi;
    boi.biWidth = bi.biWidth * size;
    boi.biHeight = bi.biHeight * size;
    
    // determine padding for scanlines
    int padding =  (4 - (bi.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;
    
    
    boi.biSizeImage = ( (sizeof(RGBTRIPLE) * boi.biWidth ) + padding ) * abs(boi.biHeight);
    
    // outfile's BITMAPFILEHEADER
    BITMAPFILEHEADER bof;
    bof.bfSize =  boi.biSizeImage + sizeof(BITMAPINFOHEADER) + sizeof(BITMAPFILEHEADER);
    
    // write outfile's BITMAPFILEHEADER
    fwrite(&bof, sizeof(BITMAPFILEHEADER), 1, outptr);

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

    int newpadding = (4 - (boi.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;

    // iterate over infile's scanlines
    for (int i = 0, biHeight = abs(bi.biHeight); i < biHeight; i++)
    {
        
        for (int s = 0; s < size; s++) {
            // iterate over pixels in scanline
            for (int j = 0; j < bi.biWidth; j++)
            {
                // temporary storage
                RGBTRIPLE triple;

                // read RGB triple from infile
                fread(&triple, sizeof(RGBTRIPLE), 1, inptr);

                // write RGB triple to outfile
                fwrite(&triple, sizeof(RGBTRIPLE), 1, outptr);
            }
        }
        // skip over padding, if any
        fseek(inptr, newpadding - padding, SEEK_CUR);

        // then add it back (to demonstrate how)
        for (int k = 0; k < newpadding - padding; k++)
        {
            fputc(0x00, outptr);
        }
    }