Where am I going wrong with this resize?

Question

My output is a grey square with a white rectangle at the top and bottom. What is it fwriting?

  // iterate over infile's scanlines (for each row)
for (int i = 0; i < bi.biHeight; i++)
{
    //iterate over scale factor for rows
    for (int n = 0; n < scale_factor; n++)
    {

       // iterate over pixels (for each pixel/column)
        for (int j = 0; j < bi.biWidth; j++)
        {
            // temporary storage
            RGBTRIPLE triple;

            // read RGB triple from infile
            fread(&triple, sizeof(RGBTRIPLE), 1, inptr);     //are you sure it is inptr

            //iterate over scale factor (for the scale factor - columns)
            for (int m = 0; m < scale_factor; m++)
            {
            // write RGB triple to outfile
            fwrite(&triple, sizeof(RGBTRIPLE), 1, outptr);

            }                                                           //closes (for the scale factor - columns)
        }               //closes (for every pixel)
    // then add padding to output file.
    for (int k = 0; k < outfile_padding; k++)
    {
        fputc(0x00, outptr);                    //I think this is getting hit too many times
    }                                           // close for outfile padding



                     //skip padding on input file
    }                                                               //close for scale_factor loop
           fseek(inptr, infile_padding, SEEK_CUR);
}                                                               //closes for each row loop

// close infile
fclose(inptr);

// close outfile
fclose(outptr);

// success
return 0;

}

Answer 1

For each input line, you currently read bi.biWidth * scale_factor pixels from input. Obviously, the input line does not have that many (unless the factor is 1). One approach is to read the pixels to some array once, and use that one for writing multiple times. Another approach is to use fseek for jumping back the number of pixels just read (you could use SEEK_SET with an absolute position, or SEEK_CUR with a negative number), so you can read the same input line again.