resize.c adequately resizes 1x1 and 2x2 images to 4x4, fails resizing 1x1images to 2x2,3x3 or 5x5

resize.c

/**
 * resize.c
 *
 * Computer Science 50
 * Problem Set 4
 *
 * modification of copy.c
 * Resizes a .bmp image by a factor of n
 */

#include <stdio.h>
#include <stdlib.h>

#include "bmp.h"

int main(int argc, char* argv[])
{
    // ensure proper usage
    if (argc != 4)
    {
        printf("Usage: ./resize n infile outfile\n");
        return 1;
    }

    // remember resize factor, filenames
    int n = atoi(argv[1]);
    char* infile = argv[2];
    char* outfile = argv[3];

    // open input file 
    FILE* inptr = fopen(infile, "r");
    if (inptr == NULL)
    {
        printf("Could not open %s.\n", infile);
        return 2;
    }

    // open output file
    FILE* outptr = fopen(outfile, "w");
    if (outptr == NULL)
    {
        fclose(inptr);
        fprintf(stderr, "Could not create %s.\n", outfile);
        return 3;
    }
    
    // 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;
    }

    // determine infile padding for scanlines
    int infpadding =  (4 - (bi.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;
    
    // create copy of headers so as to modify copies for outfile
    BITMAPFILEHEADER outbf = bf;
    BITMAPINFOHEADER outbi = bi;

    // modify BITMAPINFOHEADER elements Size, Width, Height
    outbi.biWidth = bi.biWidth * n;
    outbi.biHeight = bi.biHeight * n;
    outbi.biSizeImage = (outbi.biWidth * sizeof(RGBTRIPLE)) * abs(outbi.biHeight);

    // determine outfile padding for scanlines
    int outfpadding =  (4 - (outbi.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;

    // Modify BITMAPFILEHEADER elements accounting for padding
    outbf.bfSize = (((outbi.biWidth * sizeof(RGBTRIPLE)) + outfpadding) * abs(outbi.biHeight)) + 54;
    // old: outbf.bfSize = (bf.bfSize - 54) * n * n + 54;

    // start writing outfile
    // write outfile's BITMAPFILEHEADER
    fwrite(&outbf, sizeof(BITMAPFILEHEADER), 1, outptr);

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

    // iterate over infile's scanlines
    for (int i = 0, infbiHeight = abs(bi.biHeight); i < infbiHeight; i++)
    {
        // copy each scanline n times
        for (int l = 0, infbiWidth = bi.biWidth; l < n; l++)
        {
            // return to beginning of scanline after second iteration
            if (l > 0)
            {
            fseek(inptr, -(infbiWidth * 3), SEEK_CUR);
            }
            
            // iterate over pixels in scanline
            for (int j = 0; j < infbiWidth; j++)
            {
                // temporary storage
                RGBTRIPLE triple;
    
                // read RGB triple from infile
                fread(&triple, sizeof(RGBTRIPLE), 1, inptr);
    
                // write RGB triple to outfile n times
                for (int t = 0; t < n; t++)
                {
                    fwrite(&triple, sizeof(RGBTRIPLE), 1, outptr);
                }
            }
            
            // add outfile-size padding to each line of outfile
            for (int k = 0; k < outfpadding; k++)
            {
                fputc(0x00, outptr);
            }
        }
        // skip over padding in infile, if any, after copying scanline n times
        fseek(inptr, infpadding, SEEK_CUR);
    }

    // close infile
    fclose(inptr);

    // close outfile
    fclose(outptr);

    // that's all folks
    return 0;
}