I don't understand how the new image will implement new data i imput in like new_biWidth or new_biheigh. I added this new variables in bmp.h and I asigned to them same tipes of data like the old ones. But how does my new image know how to incorporate in header new variables instead of using the old ones. Is it better to just modify the old variables and use for example just bi.biweight *= n;? About padding, i don't exactly know how to implement it inheader to calculate the size of the image. Do I need to modify it before calculating other stuff, like the new image width? Or can i use the old formula like i used in the code.

for the moment when i try to run the program I recive the following

error:Unsupported file format

Why does it behave like that? Is it becasue I mess up by creating the new variables in the header ?

// Copies a BMP file

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

#include "bmp.h"

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

// 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 2;

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

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

// read infile's BITMAPINFOHEADER
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)
    fprintf(stderr, "Unsupported file format.\n");
    return 4;

// Updteing the header
bi.new_biWidth = bi.biWidth * n;
bi.new_biHeight = bi.biHeight * n;
int padding = (4 - (bi.new_biWidth * sizeof(RGBTRIPLE)) % 4) % 4;
bi.biSizeImage = ((sizeof(RGBTRIPLE) * bi.new_biWidth) + padding) * abs(bi.new_biHeight);
bf.bfSize = bi.biSizeImage + sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER);

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

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

// 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 < 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, padding, SEEK_CUR);

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

// close infile

// close outfile

// success
return 0;
  • There are no variables called new_biWidth and new_biHeight in the BITMAPFILEHEADER struct, so you can't that bi.new_biWidth = bi.biWidth * n;. You need to modify the old variables like bi.biWidth, but also save them to use them later Commented Aug 23, 2019 at 13:36
  • I created those variables in bmp.h
    – Criticalll
    Commented Aug 23, 2019 at 14:09
  • You shouldn't modify the struct because it's standard for all BMPs BITMAPINFOHEADER structure. So when you changed it, it became a new format and you got error:Unsupported file format Commented Aug 23, 2019 at 14:49
  • if i create another variables new_bi and new_bf and i assign them to bi and bf respectively how my code knows when to use in header the new variables i created instead of old ones. This is what I can't understand.
    – Criticalll
    Commented Aug 23, 2019 at 15:20
  • You don't need new_bi or new_bf. The code reads the header information for original BMP and saving them in bf and bi using fread(&bf, sizeof(BITMAPFILEHEADER), 1, inptr); and fread(&bi, sizeof(BITMAPINFOHEADER), 1, inptr);. Now you need to change the attributes of these headers using for example bi.biweight *= n; but also you need to save the old values in a variable to use them later in the for loop for example. Now after changing the attributes, you will write the new header in the new BMP using fwrite(&bi, sizeof(BITMAPINFOHEADER), 1, outptr); Commented Aug 23, 2019 at 16:09


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