CS50 Task Resize More I am unable to reduce the BMP file correctly

Question

In this task(course CS50, PSET3, Resize (More cofortable), the part that enlarges the picture (f> = 1) works correctly. But the part of the problem that should reduce the picture at f <1 does not work correctly. I may have problems with skipping reading and writing horizontally and vertically. Please help me understand the reason. thanks

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

#include "bmp.h"

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

  // read f
  float f = atof (argv[1]);

  if (f <= 0 || f > 100)
  {
     fprintf(stderr, "Usage: ./resize f infile outfile\n");
     return 2;
  }

  // remember filenames
  char *infile = argv[2];
  char *outfile = argv[3];

  // open input file
  FILE *inptr = fopen(infile, "r");
  if (inptr == NULL)
  {
      fprintf(stderr, "Could not open %s.\n", infile);
      return 3;
  }

  // open output file
  FILE *outptr = fopen(outfile, "w");
  if (outptr == NULL)
  {
      fclose(inptr);
      fprintf(stderr, "Could not create %s.\n", outfile);
      return 4;
  }

  // read infile's BITMAPFILEHEADER
  BITMAPFILEHEADER bf, bf1;
  fread(&bf, sizeof(BITMAPFILEHEADER), 1, inptr);
  bf1 = bf;

  // read infile's BITMAPINFOHEADER
  BITMAPINFOHEADER bi, bi1;
  fread(&bi, sizeof(BITMAPINFOHEADER), 1, inptr);
  bi1 = bi;

  bi1.biWidth = floor (bi.biWidth*f);
  if (bi.biHeight>=0)
  {
  bi1.biHeight = ceil (bi.biHeight*f);
  }
  else
  {
      bi1.biHeight = floor (bi.biHeight*f);
  }

  // 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 5;
  }

   // determine padding for scanlines
  int padding = (4 - (bi.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;
  int padding1 = (4 - (bi1.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;

  // The new size, in bytes, of the image
  bi1.biSizeImage = (bi1.biWidth * sizeof(RGBTRIPLE) + padding1) * abs (bi1.biHeight);

  // The new size, in bytes, of the bitmap file
  bf1.bfSize = bi1.biSizeImage + sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER);

  // write outfile's BITMAPFILEHEADER
  fwrite(&bf1, sizeof(BITMAPFILEHEADER), 1, outptr);

  // write outfile's BITMAPINFOHEADER
  fwrite(&bi1, sizeof(BITMAPINFOHEADER), 1, outptr);


  if (f >= 1)
  {
  int n = floor (f);
  // iterate over infile's scanlines
  for (int i = 0, biHeight = abs(bi.biHeight); i < biHeight; i++)
  {
      // iterate over pixels in scanline
      for (int j = 0; j < n; j++)
      {
          for (int k = 0; k < bi.biWidth; k++)
          {

          // temporary storage
          RGBTRIPLE triple;

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

          // write RGB triple to outfile
          for (int l = 0; l < n; l++)
              {
                 fwrite(&triple, sizeof(RGBTRIPLE), 1, outptr);
              }
          }

          // add padding
          for (int m = 0; m < padding1; m++)
          {
              fputc (0x00, outptr);
          }
          if (j < n - 1)
          {
              fseek (inptr, -bi.biWidth * sizeof(RGBTRIPLE), SEEK_CUR);
          }
      }

      // skip over padding, if any
      fseek(inptr, padding, SEEK_CUR);
  }
  }
  else
  {
  int Dh = abs(bi.biHeight)/(abs(bi.biHeight) - abs(bi1.biHeight));
  int Dw = bi.biWidth/(bi.biWidth - bi1.biWidth);
  // iterate over infile's scanlines
    for (int i = 0, biHeight = abs(bi.biHeight); i < biHeight; i++)
    {
      if (i % Dh != 0)
      {
          
      // iterate over pixels in scanline
        for (int k = 0; k < bi.biWidth; k++)
          {

            // temporary storage
            RGBTRIPLE triple;
            
            if (k % Dw != 0)
            {
            // read RGB triple from infile
            fread(&triple, sizeof(RGBTRIPLE), 1, inptr);

            // write RGB triple to outfile
           
             fwrite(&triple, sizeof(RGBTRIPLE), 1, outptr);
             
            }
          }
             
             // add padding
             for (int m = 0; m < padding1; m++)
                {
                   fputc (0x00, outptr);
                }
             
              // skip over padding, if any
              fseek(inptr, padding, SEEK_CUR);
             
      }
    }
  }

  // close infile
  fclose(inptr);

  // close outfile
  fclose(outptr);

  // success
  return 0;
}